| libpng.txt - a description on how to use and modify libpng |
| |
| libpng 1.0 beta 3 - version 0.89 |
| Updated and distributed by Andreas Dilger <adilger@enel.ucalgary.ca>, |
| based on: |
| |
| libpng 1.0 beta 2 - version 0.88 |
| For conditions of distribution and use, see copyright notice in png.h |
| Copyright (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc. |
| May 24, 1996 |
| Updated/rewritten per request in the libpng FAQ |
| Copyright (c) 1995 Frank J. T. Wojcik |
| December 18, 1995 && January 20, 1996 |
| |
| I. Introduction |
| |
| This file describes how to use and modify the PNG reference library |
| (known as libpng) for your own use. There are five sections to this |
| file: introduction, structures, reading, writing, and modification and |
| configuration notes for various special platforms. In addition to this |
| file, example.c is a good starting point for using the library, as |
| it is heavily commented and should include everything most people |
| will need. |
| |
| Libpng was written as a companion to the PNG specification, as a way |
| to reduce the amount of time and effort it takes to support the PNG |
| file format in application programs. Most users will not have to |
| modify the library significantly; advanced users may want to modify it |
| more. All attempts were made to make it as complete as possible, |
| while keeping the code easy to understand. Currently, this library |
| only supports C. Support for other languages is being considered. |
| |
| Libpng has been designed to handle multiple sessions at one time, |
| to be easily modifiable, to be portable to the vast majority of |
| machines (ANSI, K&R, 16 bit, 32 bit) available, and to be easy to |
| use. The ultimate goal of libpng is to promote the acceptance of |
| the PNG file format in whatever way possible. While there is still |
| work to be done (see the pngtodo.txt file), libpng should cover the |
| majority of the needs of it's users. |
| |
| Libpng uses zlib for its compression and decompression of PNG files. |
| The zlib compression utility is a general purpose utility that is |
| useful for more than PNG files, and can be used without libpng. |
| See the documentation delivered with zlib for more details. |
| |
| Libpng is thread safe, provided the threads are using different |
| instances of the structures. Each thread should have its own |
| png_struct and png_info instances, and thus its own image. |
| Libpng does not protect itself against two threads using the |
| same instance of a structure. |
| |
| |
| |
| II. Structures |
| |
| There are two main structures that are important to libpng, png_struct |
| and png_info. The first, png_struct, is an internal structure that |
| will not, for the most part, be used by a user except as the first |
| variable passed to every png function call. |
| |
| The png_info structure is designed to provide information about the |
| png file. All of its fields are intended to be examined or modified |
| by the user. See png.h for a good description of the png_info fields. |
| png.h is also an invaluable reference for programming with libpng. |
| |
| And while I'm on the topic, make sure you include the png header file: |
| |
| #include <png.h> |
| |
| |
| |
| III. Reading |
| |
| Reading PNG files: |
| |
| We'll now walk you through the possible functions to call when reading |
| in a PNG file, briefly explaining the syntax and purpose of each one. |
| See example.c and png.h for more detail. While Progressive reading |
| is covered in the next section, you will still need some of the |
| functions discussed in this section to read a PNG file. |
| |
| You will want to do the I/O initialization(*) before you get into libpng, |
| so if it doesn't work, you don't have much to undo. Of course, you |
| will also want to insure that you are, in fact, dealing with a PNG |
| file. Libpng provides a simple check to see if a file is a PNG file. |
| To use it, pass in the first 1 to 8 bytes of the file, and it will |
| return true or false (1 or 0) depending on whether the bytes could be |
| part of a PNG file. Of course, the more bytes you pass in, the |
| greater the accuracy of the prediction. If you pass in more then |
| eight bytes, libpng will only look at the first eight bytes. However, |
| because libpng automatically checks the file header, this is not often |
| necessary, and you should pass a newly opened file pointer to libpng |
| when reading a file. |
| |
| (*): If you are not using the standard I/O functions, you will need |
| to replace them with custom functions. See the discussion under |
| Customizing libpng. |
| |
| FILE *fp = fopen(file_name, "rb"); |
| if (!fp) |
| { |
| return; |
| } |
| fread(header, 1, number, fp); |
| is_png = png_check_sig(header, number); |
| if (!is_png) |
| { |
| return; |
| } |
| fclose(fp); |
| |
| Next, png_struct and png_info need to be allocated and initialized. |
| In order to ensure that the size of these structures is correct even |
| with a dynamically linked libpng, there are functions to initialize |
| and allocate the structures. We also pass the library version, and |
| optionally pointers to error handling functions (these can be NULL |
| if the default error handlers are to be used). See the section on |
| Changes to Libpng below regarding the old initialization functions. |
| |
| png_structp png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, |
| (void *)user_error_ptr, user_error_fn, user_warning_fn); |
| if (!png_ptr) |
| return; |
| png_infop info_ptr = png_create_info_struct(png_ptr); |
| if (!info_ptr) |
| { |
| png_destroy_read_struct(&png_ptr, (png_infopp)NULL, (png_infopp)NULL); |
| return; |
| } |
| png_infop end_info = png_create_info_struct(png_ptr); |
| if (!end_info) |
| { |
| png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL); |
| return; |
| } |
| |
| The error handling routines passed to png_create_read_struct() |
| are only necessary if you are not using the libpng supplied |
| error handling functions. When libpng encounters an error, |
| it expects to longjmp back to your routine. Therefore, you |
| will need to call setjmp and pass the jmpbuf field of your |
| png_struct. If you read the file from different routines, you |
| will need to update the jmpbuf field every time you enter a new |
| routine that will call a png_ function. See your documentation |
| of setjmp/longjmp for your compiler for more information on |
| setjmp/longjmp. See the discussion on libpng error handling |
| in the Customizing Libpng section below for more information on |
| the libpng error handling. If an error occurs, and libpng |
| longjmp's back to your setjmp, you will want to call |
| png_destroy_read_struct() to free any memory. |
| |
| if (setjmp(png_ptr->jmpbuf)) |
| { |
| png_destroy_read_struct(&png_ptr, &info_ptr, &end_info); |
| fclose(fp); |
| return; |
| } |
| |
| Now you need to set up the input code. The default for libpng is to |
| use the C function fread(). If you use this, you will need to pass a |
| valid FILE * in the function png_init_io(). Be sure that the file is |
| opened in binary mode. Again, if you wish to handle reading data in |
| another way, see the discussion on libpng I/O handling in the Customizing |
| Libpng section below. |
| |
| png_init_io(png_ptr, fp); |
| |
| You are now ready to read all the file information up to the actual |
| image data. You do this with a call to png_read_info(). |
| |
| png_read_info(png_ptr, info_ptr); |
| |
| The png_info structure is now filled in with all the data necessary |
| to read the file. Some of the more important parts of the info_ptr are: |
| |
| width - holds the width of the file |
| height - holds the height of the file |
| bit_depth - holds the bit depth of one of the image channels |
| color_type - describes the channels and what they mean |
| (see the PNG_COLOR_TYPE_ macros for more information) |
| channels - number of channels of info for the color type |
| pixel_depth - bits per pixel, the result of multiplying the |
| bit_depth times the channels |
| rowbytes - number of bytes needed to hold a row |
| interlace_type - currently 0 for none, 1 for interlaced |
| valid - this details which optional chunks were found in the |
| file to see if a chunk was present, AND '&' valid with |
| the appropriate PNG_INFO_<chunk name> define. |
| |
| These are also important, but their validity depends on whether a |
| corresponding chunk exists. Use valid (see above) to ensure that what |
| you're doing with these values makes sense. |
| |
| palette - the palette for the file (PNG_INFO_PLTE) |
| num_palette - number of entries in the palette |
| gamma - the gamma the file is written at (PNG_INFO_gAMA) |
| sig_bit - the number of significant bits (PNG_INFO_sBIT) |
| for the gray, red, green, and blue channels, whichever |
| are appropriate for the given color type. |
| trans_values - transparent pixel for non-paletted images (PNG_INFO_tRNS) |
| trans - array of transparent entries for paletted images |
| num_trans - number of transparent entries |
| hist - histogram of palette (PNG_INFO_hIST) |
| mod_time - time image was last modified (PNG_VALID_tIME) |
| background - background color (PNG_VALID_bKGD) |
| text - text comments in the file. |
| num_text - number of comments |
| |
| for more information, see the png_info definition in png.h and the |
| PNG specification for chunk contents. Be careful with trusting |
| rowbytes, as some of the transformations could increase the space |
| needed to hold a row (expand, RGBX, XRGB, gray_to_rgb, etc.). |
| See png_update_info(), below. |
| |
| A quick word about text and num_text. PNG stores comments in |
| keyword/text pairs, one pair per chunk. While there are suggested |
| keywords, there is no requirement to restrict the use to these |
| strings. There is a requirement to have at least one character for a |
| keyword. It is strongly suggested that keywords be sensible to humans |
| (that's the point), so don't use abbreviations. See the png |
| specification for more details. There is also no requirement to have |
| text after the keyword. |
| |
| Keywords should be limited to 80 characters without leading or trailing |
| spaces, but non-consecutive spaces are allowed within the keyword. It is |
| possible to have the same keyword any number of times. The text field |
| is an array of png_text structures, each holding pointer to a keyword |
| and a pointer to a text string. Only the text string may be null. |
| The keyword/text pairs are put into the array in the order that |
| they are received. However, some or all of the text chunks may be |
| after the image, so, to make sure you have read all the text chunks, |
| don't mess with these until after you read the stuff after the image. |
| This will be mentioned again below in the discussion that goes with |
| png_read_end(). |
| |
| After you've read the file information, you can set up the library to |
| handle any special transformations of the image data. The various |
| ways to transform the data will be described in the order that they |
| should occur. This is important, as some of these change the color |
| type and/or bit depth of the data, and some others only work on |
| certain color types and bit depths. Even though each transformation |
| checks to see if it has data that it can do somthing with, you should |
| make sure to only enable a transformation if it will be valid for the |
| data. For example, don't swap red and blue on grayscale data. |
| |
| Data will be decoded into the supplied row buffers packed into bytes |
| unless the library has been told to transform it into another format. |
| For example, 4 bit/pixel paletted or grayscale data will be returned |
| 2 pixels/byte with the leftmost pixel in the high-order bits of the |
| byte, unless png_set_packing() is called. 8-bit RGB data will be stored |
| in RGBRGBRGB format unless png_set_filler() is called to insert filler |
| bytes, either before or after each RGB triplet. 16-bit RGB data will |
| be returned RRGGBBRRGGBB, with the most significant byte of the color |
| value first, unless png_set_strip_16() is called to transform it to |
| regular RGBRGB triplets. |
| |
| The following code transforms grayscale images of less than 8 to 8 bits, |
| changes paletted images to RGB, and adds a full alpha channel if there is |
| transparency information in a tRNS chunk. This is most useful on |
| grayscale images with bit depths of 2 or 4 or if there is a multiple-image |
| viewing application that wishes to treat all images in the same way. |
| |
| if (info_ptr->color_type == PNG_COLOR_TYPE_PALETTE && |
| info_ptr->bit_depth < 8) |
| png_set_expand(png_ptr); |
| |
| if (info_ptr->color_type == PNG_COLOR_TYPE_GRAY && |
| info_ptr->bit_depth < 8) |
| png_set_expand(png_ptr); |
| |
| if (info_ptr->valid & PNG_INFO_tRNS) |
| png_set_expand(png_ptr); |
| |
| PNG can have files with 16 bits per channel. If you only can handle |
| 8 bits per channel, this will strip the pixels down to 8 bit. |
| |
| if (info_ptr->bit_depth == 16) |
| png_set_strip_16(png_ptr); |
| |
| PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as |
| they can, resulting in, for example, 8 pixels per byte for 1 bit |
| files. This code expands to 1 pixel per byte without changing the |
| values of the pixels: |
| |
| if (info_ptr->bit_depth < 8) |
| png_set_packing(png_ptr); |
| |
| PNG files have possible bit depths of 1, 2, 4, 8, and 16. It is then |
| required that values be "scaled" or "shifted" up to the bit depth used |
| in the file (ie from 5 bits/sample in the range [0,31] to 8 bits/sample |
| in the range [0, 255]). However, they also provide a way to describe |
| the true bit depth of the image. See the PNG specification for details. |
| This call reduces the pixels back down to the true bit depth: |
| |
| if (info_ptr->valid & PNG_INFO_sBIT) |
| png_set_shift(png_ptr, &(info_ptr->sig_bit)); |
| |
| PNG files store 3 color pixels in red, green, blue order. This code |
| changes the storage of the pixels to blue, green, red: |
| |
| if (info_ptr->color_type == PNG_COLOR_TYPE_RGB || |
| info_ptr->color_type == PNG_COLOR_TYPE_RGB_ALPHA) |
| png_set_bgr(png_ptr); |
| |
| PNG files store RGB pixels packed into 3 bytes. This code expands them |
| into 4 bytes for windowing systems that need them in this format: |
| |
| if (info_ptr->bit_depth == 8 && |
| info_ptr->color_type == PNG_COLOR_TYPE_RGB) |
| png_set_filler(png_ptr, filler_byte, PNG_FILLER_BEFORE); |
| |
| where filler_byte is the number to fill with, and the location is |
| either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether |
| you want the filler before the RGB or after. |
| |
| For some uses, you may want a gray-scale image to be represented as |
| RGB. This code will do that conversion: |
| |
| if (info_ptr->color_type == PNG_COLOR_TYPE_GRAY || |
| info_ptr->color_type == PNG_COLOR_TYPE_GRAY_ALPHA) |
| png_set_gray_to_rgb(png_ptr); |
| |
| The following code handles alpha and transparency by replacing it with |
| a background value. If there was a valid bKGD in the file, you can use |
| it if you want. However, you can replace it with your own if you want |
| also. If there wasn't one in the file, you must supply a color. If |
| libpng is doing gamma correction, you will need to tell libpng where |
| the background came from so it can do the appropriate gamma |
| correction. If you have a grayscale and you are using png_set_expand() |
| to change to a higher bit-depth you must indicate if the background gray |
| needs to be expanded to the new bit-depth. Similarly, if you are reading |
| a paletted image, you must indicate if you have supplied the background |
| index that needs to be expanded to RGB values. You can always specify |
| RGB color values directly when setting your background for paletted images. |
| |
| png_color_16 my_background; |
| |
| if (info_ptr->valid & PNG_INFO_bKGD) |
| png_set_backgrond(png_ptr, &(info_ptr->background), |
| PNG_BACKGROUND_GAMMA_FILE, 1, 1.0); |
| else |
| png_set_background(png_ptr, &my_background, |
| PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0); |
| |
| The following code handles gamma transformations of the data. Pass |
| both the file gamma and the desired screen gamma. If the file does |
| not have a gamma value, you can pass one anyway if you wish. Note |
| that file gammas are inverted from screen gammas. See the discussions |
| on gamma in the PNG specification for more information. It is |
| strongly recommended that viewers support gamma correction. |
| |
| if (info_ptr->valid & PNG_INFO_gAMA) |
| png_set_gamma(png_ptr, screen_gamma, info_ptr->gamma); |
| else |
| png_set_gamma(png_ptr, screen_gamma, 0.45); |
| |
| If you need to reduce an RGB file to a paletted file, or if a paletted |
| file has more entries then will fit on your screen, png_set_dither() |
| will do that. Note that this is a simple match dither that merely |
| finds the closest color available. This should work fairly well with |
| optimized palettes, and fairly badly with linear color cubes. If you |
| pass a palette that is larger then maximum_colors, the file will |
| reduce the number of colors in the palette so it will fit into |
| maximum_colors. If there is a histogram, it will use it to make |
| more intelligent choices when reducing the palette. If there is no |
| histogram, it may not do as good a job. |
| |
| if (info_ptr->color_type & PNG_COLOR_MASK_COLOR) |
| { |
| if (info_ptr->valid & PNG_INFO_PLTE) |
| { |
| png_set_dither(png_ptr, info_ptr->palette, |
| info_ptr->num_palette, max_screen_colors, |
| info_ptr->histogram, 1); |
| } |
| else |
| { |
| png_color std_color_cube[MAX_SCREEN_COLORS] = |
| { ... colors ... }; |
| |
| png_set_dither(png_ptr, std_color_cube, MAX_SCREEN_COLORS, |
| MAX_SCREEN_COLORS, NULL,0); |
| } |
| } |
| |
| PNG files describe monochrome as black being zero and white being one. |
| The following code will reverse this (make black be one and white be |
| zero): |
| |
| if (info_ptr->bit_depth == 1 && |
| info_ptr->color_type == PNG_COLOR_GRAY) |
| png_set_invert_mono(png_ptr); |
| |
| PNG files store 16 bit pixels in network byte order (big-endian, |
| ie. most significant bits first). This code chages the storage to the |
| other way (little-endian, ie. least significant bits first, eg. the |
| way PCs store them): |
| |
| if (info_ptr->bit_depth == 16) |
| png_set_swap(png_ptr); |
| |
| The last thing to handle is interlacing; this is covered in detail below, |
| but you must call the function here. |
| |
| if (info_ptr->interlace_type) |
| number_passes = png_set_interlace_handling(png_ptr); |
| |
| After setting the transformations, libpng can update your png_info |
| structure to reflect any transformations you've requested with this |
| call. This is most useful to update the info structures rowbytes |
| field, so you can use it to allocate your image memory. This function |
| will also update your palette with the correct display gamma and |
| background if these have been given with the calls above. |
| |
| png_read_update_info(png_ptr, info_ptr); |
| |
| After you call png_read_update_info(), you can allocate any |
| memory you need to hold the image. The row data is simply |
| raw byte data for all forms of images. As the actual allocation |
| varies among applications, no example will be given. If you |
| are allocating one large chunk, you will need to build an |
| array of pointers to each row, as it will be needed for some |
| of the functions below. |
| |
| After you've allocated memory, you can read the image data. |
| The simplest way to do this is in one function call. If you are |
| allocating enough memory to hold the whole image, you can just |
| call png_read_image() and libpng will read in all the image data |
| and put it in the memory area supplied. You will need to pass in |
| an array of pointers to each row. |
| |
| This function automatically handles interlacing, so you don't need |
| to call png_set_interlace_handling() or call this function multiple |
| times, or any of that other stuff necessary with png_read_rows(). |
| |
| png_read_image(png_ptr, row_pointers); |
| |
| where row_pointers is: |
| |
| png_bytep row_pointers[height]; |
| |
| You can point to void or char or whatever you use for pixels. |
| |
| If you don't want to read int the whole image at once, you can |
| use png_read_rows() instead. If there is no interlacing (check |
| info_ptr->interlace_type), this is simple: |
| |
| png_read_rows(png_ptr, row_pointers, NULL, number_of_rows); |
| |
| where row_pointers is the same as in the png_read_image() call. |
| |
| If you are doing this just one row at a time, you can do this with |
| row_pointers: |
| |
| png_bytep row_pointers = row; |
| png_read_row(png_ptr, &row_pointers, NULL); |
| |
| If the file is interlaced (info_ptr->interlace_type != 0), things get |
| somewhat harder. The only currently (as of 6/96 -- PNG |
| Specification version 1.0) defined interlacing scheme for PNG files |
| (info_ptr->interlace_type == 1) is a someewhat complicated 2D interlace |
| scheme, known as Adam7, that breaks down an image into seven smaller |
| images of varying size, based on an 8x8 grid. |
| |
| libpng can fill out those images or it can give them to you "as is". |
| If you want them filled out, there are two ways to do that. The one |
| mentioned in the PNG specification is to expand each pixel to cover |
| those pixels that have not been read yet. This results in a blocky |
| image for the first pass, which gradually smoothes out as more pixels |
| are read. The other method is the "sparkle" method, where pixels are |
| draw only in their final locations, with the rest of the image remaining |
| whatever colors they were initialized to before the start of the read. |
| The first method usually looks better, but tends to be slower, as there |
| are more pixels to put in the rows. |
| |
| If you don't want libpng to handle the interlacing details, just call |
| png_read_rows() seven times to read in all seven images. Each of the |
| images are valid images by themselves, or they can be combined on an |
| 8x8 grid to form a single image (although if you intend to combine them |
| you would be far better off using the libpng interlace handling). |
| |
| The first pass will return an image 1/8 as wide as the entire image |
| (every 8th column starting in column 0) and 1/8 as high as the original |
| (every 8th row starting in row 0), the second will be 1/8 as wide |
| (starting in column 4) and 1/8 as high (also starting in row 0). The |
| third pass will be 1/4 as wide (every 4th pixel starting in row 0) and |
| 1/8 as high (every 8th row starting in row 4), and the fourth pass will |
| be 1/4 as wide and 1/4 as high (every 4th column starting in column 2, |
| and every 4th row starting in row 0). The fifth pass will return an |
| image 1/2 as wide, and 1/4 as high (starting at column 0 and row 2), |
| while the sixth pass will be 1/2 as wide and 1/2 as high as the original |
| (starting in column 1 and row 0). The seventh and final pass will be as |
| wide as the original, and 1/2 as high, containing all of the odd |
| numbered scanlines. Phew! |
| |
| If you want libpng to expand the images, call this before calling |
| png_start_read_image() or png_read_update_info(): |
| |
| if (info_ptr->interlace_type) |
| number_passes = png_set_interlace_handling(png_ptr); |
| |
| This will return the number of passes needed. Currently, this |
| is seven, but may change if another interlace type is added. |
| This function can be called even if the file is not interlaced, |
| where it will return one pass. |
| |
| If you are not going to display the image after each pass, but are |
| going to wait until the entire image is read in, use the sparkle |
| effect. This effect is faster and the end result of either method |
| is exactly the same. If you are planning on displaying the image |
| after each pass, the rectangle effect is generally considered the |
| better looking one. |
| |
| If you only want the "sparkle" effect, just call png_read_rows() as |
| normal, with the third parameter NULL. Make sure you make pass over |
| the image number_passes times, and you don't change the data in the |
| rows between calls. You can change the locations of the data, just |
| not the data. Each pass only writes the pixels appropriate for that |
| pass, and assumes the data from previous passes is still valid. |
| |
| png_read_rows(png_ptr, row_pointers, NULL, number_of_rows); |
| |
| If you only want the first effect (the rectangles), do the same as |
| before except pass the row buffer in the third parameter, and leave |
| the second parameter NULL. |
| |
| png_read_rows(png_ptr, NULL, row_pointers, number_of_rows); |
| |
| After you are finished reading the image, you can finish reading |
| the file. If you are interested in comments or time, which may be |
| stored either before or after the image data, you should pass the |
| info_ptr pointer from the png_read_info() call, or you can pass a |
| separate png_info struct if you want to keep the comments from |
| before and after the image separate. If you are not interested, you |
| can pass NULL. |
| |
| png_read_end(png_ptr, end_info); |
| |
| When you are done, you can free all memory allocated by libpng like this: |
| |
| png_destroy_read_struct(&png_ptr, &info_ptr, &end_info); |
| |
| For a more compact example of reading a PNG image, see the file example.c. |
| |
| |
| Reading PNG files progressively: |
| |
| The progressive reader is slightly different then the non-progressive |
| reader. Instead of calling png_read_info(), png_read_rows(), and |
| png_read_end(), you make one call to png_process_data(), which calls |
| callbacks when it has the info, a row, or the end of the image. You |
| set up these callbacks with png_set_progressive_read_fn(). You don't |
| have to worry about the input/output functions of libpng, as you are |
| giving the library the data directly in png_process_data(). I will |
| assume that you have read the section on reading PNG files above, |
| so I will only highlight the differences (although I will show |
| all of the code). |
| |
| png_structp png_ptr; |
| png_infop info_ptr; |
| |
| /* An example code fragment of how you would initialize the progressive |
| reader in your application. */ |
| int |
| initialize_png_reader() |
| { |
| png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, |
| (void *)user_error_ptr, user_error_fn, user_warning_fn); |
| if (!png_ptr) |
| return -1; |
| info_ptr = png_create_info_struct(png_ptr); |
| if (!info_ptr) |
| { |
| png_destroy_read_struct(&png_ptr, (png_infopp)NULL, (png_infopp)NULL); |
| return -1; |
| } |
| |
| if (setjmp(png_ptr->jmpbuf)) |
| { |
| png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL); |
| return -1; |
| } |
| |
| /* This one's new. You can provide functions to be called |
| when the header info is valid, when each row is completed, |
| and when the image is finished. If you aren't using all |
| functions, you can specify a NULL parameter. You can use |
| any struct as the user_ptr (cast to a void pointer for the |
| function call), and retrieve the pointer from inside the |
| callbacks using the function png_get_progressive_ptr(png_ptr); |
| which will return a void pointer, which you have to cast |
| appropriately. |
| */ |
| png_set_progressive_read_fn(png_ptr, (void *)user_ptr, |
| info_callback, row_callback, end_callback); |
| |
| return 0; |
| } |
| |
| /* A code fragment that you call as you recieve blocks of data */ |
| int |
| process_data(png_bytep buffer, png_uint_32 length) |
| { |
| if (setjmp(png_ptr->jmpbuf)) |
| { |
| png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL); |
| return -1; |
| } |
| |
| /* This one's new also. Simply give it a chunk of data |
| from the file stream (in order, of course). On machines |
| with segmented memory models machines, don't give it any |
| more than 64K. The library seems to run fine with sizes |
| of 4K. Although you can give it much less if necessary |
| (I assume you can give it chunks of 1 byte, I haven't |
| tried less then 256 bytes yet). When this function returns, |
| you may want to display any rows that were generated in the |
| row callback if you don't already do so there. |
| */ |
| png_process_data(png_ptr, info_ptr, buffer, length); |
| return 0; |
| } |
| |
| /* This function is called (as set by png_set_progressive_fn() above) |
| when enough data has been supplied so all of the header has been read. |
| */ |
| void |
| info_callback(png_structp png_ptr, png_infop info) |
| { |
| /* Do any setup here, including setting any of the transformations |
| mentioned in the Reading PNG files section. For now, you _must_ |
| call either png_start_read_image() or png_read_update_info() |
| after all the transformations are set (even if you don't set |
| any). You may start getting rows before png_process_data() |
| returns, so this is your last chance to prepare for that. |
| */ |
| } |
| |
| /* This function is called when each row of image data is complete */ |
| void |
| row_callback(png_structp png_ptr, png_bytep new_row, |
| png_uint_32 row_num, int pass) |
| { |
| /* If the image is interlaced, and you turned on the interlace |
| handler, this function will be called for every row in every pass. |
| Some of these rows will not be changed from the previous pass. |
| When the row is not changed, the new_row variable will be NULL. |
| The rows and passes are called in order, so you don't really |
| need the row_num and pass, but I'm supplying them because it |
| may make your life easier. |
| |
| For the non-NULL rows of interlaced images, you must call |
| png_progressive_combine_row() passing in the row and the |
| old row. You can call this function for NULL rows (it will |
| just return) and for non-interlaced images (it just does the |
| memcpy for you) if it will make the code easier. Thus, you |
| can just do this for all cases: |
| */ |
| |
| png_progressive_combine_row(png_ptr, old_row, new_row); |
| |
| /* where old_row is what was displayed for previous rows. Note |
| that the first pass (pass == 0, really) will completely cover |
| the old row, so the rows do not have to be initialized. After |
| the first pass (and only for interlaced images), you will have |
| to pass the current row, and the function will combine the |
| old row and the new row. |
| */ |
| } |
| |
| void |
| end_callback(png_structp png_ptr, png_infop info) |
| { |
| /* This function is called after the whole image has been read, |
| including any chunks after the image (up to and including |
| the IEND). You will usually have the same info chunk as you |
| had in the header, although some data may have been added |
| to the comments and time fields. |
| |
| Most people won't do much here, perhaps setting a flag that |
| marks the image as finished. |
| */ |
| } |
| |
| |
| |
| IV. Writing |
| |
| Much of this is very similar to reading. However, everything of |
| importance is repeated here, so you won't have to constantly look |
| back up in the reading section to understand writing. |
| |
| You will want to do the I/O initialization before you get into libpng, |
| so if it doesn't work, you don't have anything to undo. If you are not |
| using the standard I/O functions, you will need to replace them with |
| custom writing functions. See the discussion under Customizing libpng. |
| |
| FILE *fp = fopen(file_name, "wb"); |
| if (!fp) |
| { |
| return; |
| } |
| |
| Next, png_struct and png_info need to be allocated and initialized. |
| As these can be both relatively large, you may not want to store these |
| on the stack, unless you have stack space to spare. Of course, you |
| will want to check if they return NULL. |
| |
| png_structp png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, |
| (void *)user_error_ptr, user_error_fn, user_warning_fn); |
| if (!png_ptr) |
| return; |
| |
| png_infop info_ptr = png_create_info_struct(png_ptr); |
| if (!info_ptr) |
| { |
| png_destroy_write_struct(&png_ptr, (png_infopp)NULL); |
| return; |
| } |
| |
| After you have these structures, you will need to set up the |
| error handling. When libpng encounters an error, it expects to |
| longjmp() back to your routine. Therefore, you will need to call |
| setjmp and pass the jmpbuf field of your png_struct. If you |
| write the file from different routines, you will need to update |
| the jmpbuf field every time you enter a new routine that will |
| call a png_ function. See your documentation of setjmp/longjmp |
| for your compiler for more information on setjmp/longjmp. See |
| the discussion on libpng error handling in the Customizing Libpng |
| section below for more information on the libpng error handling. |
| |
| if (setjmp(png_ptr->jmpbuf)) |
| { |
| png_destroy_write_struct(&png_ptr, &info_ptr); |
| fclose(fp); |
| return; |
| } |
| |
| Now you need to set up the input code. The default for libpng is to |
| use the C function fwrite(). If you use this, you will need to pass a |
| valid FILE * in the function png_init_io(). Be sure that the file is |
| opened in binary mode. Again, if you wish to handle writing data in |
| another way, see the discussion on libpng I/O handling in the Customizing |
| Libpng section below. |
| |
| png_init_io(png_ptr, fp); |
| |
| You now have the option of modifying how the compression library will |
| run. The following functions are mainly for testing, but may be useful |
| in some cases, like if you need to write png files extremely fast and |
| are willing to give up some compression, or if you want to get the |
| maximum possible compression at the expense of slower writing. If you |
| have no special needs in this area, let the library do what it wants by |
| not calling this function at all, as it has been tuned to deliver a good |
| speed/compression ratio. The second parameter to png_set_filter() is |
| the filter method, for which the only valid value is '0' (as of the |
| 06/96 PNG specification. The third parameter is a flag that indicates |
| which filter type(s) are to be tested for each scanline. See the |
| Compression Library for details on the specific filter types. |
| |
| |
| /* turn on or off filtering, and/or choose specific filters */ |
| png_set_filter(png_ptr, 0, |
| PNG_FILTER_NONE | PNG_FILTER_SUB | PNG_FILTER_PAETH); |
| |
| The png_set_compression_???() functions interface to the zlib compression |
| library, and should mostly be ignored unless you really know what you are |
| doing. The only generally useful call is png_set_compression_level() |
| which changes how much time zlib spends on trying to compress the image |
| data. See the Compression Library for details on the compression levels. |
| |
| /* set the zlib compression level */ |
| png_set_compression_level(png_ptr, Z_BEST_COMPRESSION); |
| |
| /* set other zlib parameters */ |
| png_set_compression_mem_level(png_ptr, 8); |
| png_set_compression_strategy(png_ptr, Z_DEFAULT_STRATEGY); |
| png_set_compression_window_bits(png_ptr, 15); |
| png_set_compression_method(png_ptr, 8); |
| |
| You now need to fill in the png_info structure with all the data you |
| wish to write before the actual image. Note that the only thing you |
| are allowed to write after the image is the text chunks and the time |
| chunk (as of PNG Specification 1.0, anyway). See png_write_end() and |
| the latest PNG specification for more information on that. If you |
| wish to write them before the image, fill them in now, and flag that |
| data as being valid. If you want to wait until after the data, don't |
| fill them until png_write_end(). For all the fields in png_info and |
| their data types, see png.h. For explanations of what the fields |
| contain, see the PNG specification. |
| |
| Some of the more important parts of the png_info are: |
| |
| width - holds the width of the file |
| height - holds the height of the file |
| bit_depth - holds the bit depth of one of the image channels |
| color_type - describes the channels and what they mean |
| see the PNG_COLOR_TYPE_ defines for more information |
| interlace_type - allowed values are 0 for none, 1 for interlaced |
| valid - this describes which optional chunks to write to the |
| file. Note that if you are writing a |
| PNG_COLOR_TYPE_PALETTE file, the PLTE chunk is not |
| optional, but must still be marked for writing. To |
| mark chunks for writing, logical OR '|' valid with |
| the appropriate PNG_INFO_<chunk name> define. |
| palette - the palette for the file (PNG_INFO_PLTE) |
| num_palette - number of entries in the palette |
| gamma - the gamma the file is written at (PNG_INFO_gAMA) |
| sig_bit - the number of significant bits (PNG_INFO_sBIT) |
| for the gray, red, green, and blue channels, whichever |
| are appropriate for the given color type. |
| trans_values - transparent pixel for non-paletted images (PNG_INFO_tRNS) |
| trans - array of transparent entries for paletted images |
| num_trans - number of transparent entries |
| hist - histogram of palette (PNG_INFO_hIST) |
| mod_time - time image was last modified (PNG_VALID_tIME) |
| background - background color (PNG_VALID_bKGD) |
| text - text comments in the file. |
| num_text - number of comments |
| |
| A quick word about text and num_text. text is an array of png_text |
| structures. num_text is the number of valid structures in the array. |
| If you want, you can use max_text to hold the size of the array, but |
| libpng ignores it for writing (it does use it for reading). Each |
| png_text structure holds a keyword-text value, and a compression type. |
| The compression types have the same valid numbers as the compression |
| types of the image data. Currently, the only valid number is zero. |
| However, you can store text either compressed or uncompressed, unlike |
| images which always have to be compressed. So if you don't want the |
| text compressed, set the compression type to -1. Until text gets |
| around 1000 bytes, it is not worth compressing it. |
| |
| The keywords that are given in the PNG Specification are: |
| |
| Title Short (one line) title or caption for image |
| Author Name of image's creator |
| Description Description of image (possibly long) |
| Copyright Copyright notice |
| Creation Time Time of original image creation |
| Software Software used to create the image |
| Disclaimer Legal disclaimer |
| Warning Warning of nature of content |
| Source Device used to create the image |
| Comment Miscellaneous comment; conversion from other |
| image format |
| |
| The keyword-text pairs work like this. Keywords should be short |
| simple descriptions of what the comment is about. Some typical |
| keywords are found in the PNG specification, as is some recomendations |
| on keywords. You can repeat keywords in a file. You can even write |
| some text before the image and some after. For example, you may want |
| to put a description of the image before the image, but leave the |
| disclaimer until after, so viewers working over modem connections |
| don't have to wait for the disclaimer to go over the modem before |
| they start seeing the image. Finally, keywords should be full |
| words, not abbreviations. Keywords and text are in the ISO 8859-1 |
| (Latin-1) character set (a superset of regular ASCII) and can not |
| contain NUL characters, and should not contain control or other |
| unprintable characters. To make the comments widely readable, stick |
| with basic ASCII, and avoid machine specific character set extensions |
| like the IBM-PC character set. The keyword must be present, but |
| you can leave off the text string on non-compressed pairs. |
| Compressed pairs must have a text string, as only the text string |
| is compressed anyway, so the compression would be meaningless. |
| |
| PNG supports modification time via the png_time structure. Two |
| conversion routines are proved, png_convert_from_time_t() for |
| time_t and png_convert_from_struct_tm() for struct tm. The |
| time_t routine uses gmtime(). You don't have to use either of |
| these, but if you wish to fill in the png_time structure directly, |
| you should provide the time in universal time (GMT) if possible |
| instead of your local time. Note that the year number is the full |
| year (ie 1996, rather than 96), and that months start with 1. |
| |
| You are now ready to write all the file information up to the actual |
| image data. You do this with a call to png_write_info(). |
| |
| png_write_info(png_ptr, info_ptr); |
| |
| After you've written the file information, you can set up the library |
| to handle any special transformations of the image data. The various |
| ways to transform the data will be described in the order that they |
| should occur. This is important, as some of these change the color |
| type and/or bit depth of the data, and some others only work on |
| certain color types and bit depths. Even though each transformation |
| checks to see if it has data that it can do somthing with, you should |
| make sure to only enable a transformation if it will be valid for the |
| data. For example, don't swap red and blue on grayscale data. |
| |
| PNG files store RGB pixels packed into 3 bytes. This code tells |
| the library to expect input data with 4 bytes per pixel |
| |
| png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE); |
| |
| where the 0 is the value that will be put in the 4th byte, and the |
| location is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending |
| upon whether the filler byte is stored XRGB or RGBX. |
| |
| PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as |
| they can, resulting in, for example, 8 pixels per byte for 1 bit files. |
| If the data is supplied at 1 pixel per byte, use this code, which will |
| correctly pack the pixels into a single byte: |
| |
| png_set_packing(png_ptr); |
| |
| PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your |
| data is of another bit depth, you can write an sBIT chunk into the |
| file so that decoders can get the original data if desired. |
| |
| /* Do this before png_write_info() */ |
| info_ptr->valid |= PNG_INFO_sBIT; |
| |
| /* Set the true bit depth of the image data */ |
| if (info_ptr->color_type & PNG_COLOR_MASK_COLOR) |
| { |
| info_ptr->sig_bit.red = true_bit_depth; |
| info_ptr->sig_bit.green = true_bit_depth; |
| info_ptr->sig_bit.blue = true_bit_depth; |
| } |
| else |
| { |
| info_ptr->sig_bit.gray = true_bit_depth; |
| } |
| |
| if (info_ptr->color_type & PNG_COLOR_MASK_ALPHA) |
| { |
| info_ptr->sig_bit.alpha = true_bit_depth; |
| } |
| |
| If the data is stored in the row buffer in a bit depth other than |
| one supported by PNG (ie 3 bit data in the range 0-7 for a 4-bit PNG), |
| this will scale the values to appear to be the correct bit depth as |
| is required by PNG. |
| |
| png_set_shift(png_ptr, &(info_ptr->sig_bit)); |
| |
| PNG files store 16 bit pixels in network byte order (big-endian, |
| ie. most significant bits first). This code would be used if they are |
| supplied the other way (little-endian, ie. least significant bits |
| first, eg. the way PCs store them): |
| |
| png_set_swap(png_ptr); |
| |
| PNG files store 3 color pixels in red, green, blue order. This code |
| would be used if they are supplied as blue, green, red: |
| |
| png_set_bgr(png_ptr); |
| |
| PNG files describe monochrome as black being zero and white being |
| one. This code would be used if the pixels are supplied with this reversed |
| (black being one and white being zero): |
| |
| png_set_invert(png_ptr); |
| |
| It is possible to have libpng flush any pending output, either manually, |
| or automatically after a certain number of lines have been written. To |
| flush the output stream a single time call: |
| |
| png_write_flush(png_ptr); |
| |
| and to have libpng flush the output stream periodically after a certain |
| number of scanlines have been written, call: |
| |
| png_set_flush(png_ptr, nrows); |
| |
| Note that the distance between rows is from the last time png_write_flush() |
| was called, or the first row of the image if it has never been called. |
| So if you write 50 lines, and then png_set_flush 25, it will flush the |
| output on the next scanline, and every 25 lines thereafter, unless |
| png_write_flush()ls is called before 25 more lines have been written. |
| If nrows is too small (less than about 10 lines for a 640 pixel wide |
| RGB image) the image compression may decrease noticably (although this |
| may be acceptable for real-time applications). Infrequent flushing will |
| only degrade the compression performance by a few percent over images |
| that do not use flushing. |
| |
| That's it for the transformations. Now you can write the image data. |
| The simplest way to do this is in one function call. If have the |
| whole image in memory, you can just call png_write_image() and libpng |
| will write the image. You will need to pass in an array of pointers to |
| each row. This function automatically handles interlacing, so you don't |
| need to call png_set_interlace_handling() or call this function multiple |
| times, or any of that other stuff necessary with png_write_rows(). |
| |
| png_write_image(png_ptr, row_pointers); |
| |
| where row_pointers is: |
| |
| png_bytef *row_pointers[height]; |
| |
| You can point to void or char or whatever you use for pixels. |
| |
| If you can't want to write the whole image at once, you can |
| use png_write_rows() instead. If the file is not interlaced, |
| this is simple: |
| |
| png_write_rows(png_ptr, row_pointers, number_of_rows); |
| |
| row_pointers is the same as in the png_write_image() call. |
| |
| If you are just writing one row at a time, you can do this with |
| row_pointers: |
| |
| png_bytep row_pointer = row; |
| |
| png_write_row(png_ptr, &row_pointer); |
| |
| When the file is interlaced, things can get a good deal more |
| complicated. The only currently (as of 6/96 -- PNG Specification |
| version 1.0) defined interlacing scheme for PNG files is a |
| compilcated interlace scheme, known as Adam7, that breaks down an |
| image into seven smaller images of varying size. libpng will build |
| these images for you, or you can do them yourself. If you want to |
| build them yourself, see the PNG specification for details of which |
| pixels to write when. |
| |
| If you don't want libpng to handle the interlacing details, just |
| use png_set_interlace_handling() and call png_write_rows() the |
| correct number of times to write all seven sub-images. |
| |
| If you want libpng to build the sub-images, call this before you start |
| writing any rows: |
| |
| number_passes = png_set_interlace_handling(png_ptr); |
| |
| This will return the number of passes needed. Currently, this |
| is seven, but may change if another interlace type is added. |
| |
| Then write the complete image number_passes times. |
| |
| png_write_rows(png_ptr, row_pointers, number_of_rows); |
| |
| As some of these rows are not used, and thus return immediately, |
| you may want to read about interlacing in the PNG specification, |
| and only update the rows that are actually used. |
| |
| After you are finished writing the image, you should finish writing |
| the file. If you are interested in writing comments or time, you should |
| pass the an appropriately filled png_info pointer. If you |
| are not interested, you can pass NULL. If you have written text at |
| the beginning and are not writing more at the end, you should set |
| info_ptr->num_text = 0, or the text will be written again here. |
| |
| png_write_end(png_ptr, info_ptr); |
| |
| When you are done, you can free all memory used by libpng like this: |
| |
| png_destroy_write_struct(&png_ptr, &info_ptr); |
| |
| You must free any data you allocated for info_ptr, such as comments, |
| palette, or histogram, before the call to png_destroy_write_struct(); |
| |
| For a more compact example of writing a PNG image, see the file example.c. |
| |
| |
| V. Modifying/Customizing libpng: |
| |
| There are two issues here. The first is changing how libpng does |
| standard things like memory allocation, input/output, and error handling. |
| The second deals with more complicated things like adding new chunks, |
| adding new transformations, and generally changing how libpng works. |
| |
| All of the memory allocation, input/output, and error handling in |
| libpng goes through callbacks which are user setable. The default |
| routines are in pngmem.c, pngrio.c, pngwio.c, and pngerror.c respectively. |
| To change these functions, call the approprate png_set_???_fn() function. |
| |
| Memory allocation is done through the functions png_large_malloc(), |
| png_malloc(), png_realloc(), png_large_free(), and png_free(). These |
| currently just call the standard C functions. The large functions must |
| handle exactly 64K, but they don't have to handle more than that. If |
| your pointers can't access more then 64K at a time, you will want to |
| set MAXSEG_64K in zlib.h. Since it is unlikely that the method of |
| handling memory allocation on a platform will change between applications, |
| these functions must be modified in the library at compile time. |
| |
| Input/Output in libpng is done throught png_read() and png_write(), which |
| currently just call fread() and fwrite(). The FILE * is stored in |
| png_struct, and is initialized via png_init_io(). If you wish to change |
| the method of I/O, the library supplies callbacks that you can set through |
| the function png_set_read_fn() and png_set_write_fn() at run time. These |
| functions also provide a void pointer that can be retrieved via the function |
| png_get_io_ptr(). For example: |
| |
| png_set_read_fn(png_structp png_ptr, voidp io_ptr, |
| png_rw_ptr read_data_fn) |
| |
| png_set_write_fn(png_structp png_ptr, voidp io_ptr, |
| png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn); |
| |
| voidp io_ptr = png_get_io_ptr(png_ptr); |
| |
| The replacement I/O functions should have prototypes as follows: |
| |
| void user_read_data(png_structp png_ptr, png_bytep data, |
| png_uint_32 length); |
| void user_write_data(png_structp png_ptr, png_bytep data, |
| png_uint_32 length); |
| void user_flush_data(png_structp png_ptr); |
| |
| Supplying NULL for the read, write, or flush functions sets them back |
| to using the default C stream functions. It is an error to read from |
| a write stream, and vice versa. |
| |
| Error handling in libpng is done through png_error() and png_warning(). |
| Errors handled through png_error() are fatal, meaning that png_error() |
| should never return to it's caller. Currently, this is handled via |
| setjmp() and longjmp(), but you could change this to do things like |
| exit() if you should wish. On non-fatal errors, png_warning() is called |
| to print a warning message, and then control returns to the calling code. |
| By default png_error() and png_warning() print a message on stderr via |
| fprintf() unless the library is compiled with PNG_NO_STDIO defined. If |
| you wish to change the behavior of the error functions, you will need to |
| set up your own message callbacks. These functions are normally supplied |
| at the time that the png_struct is created. It is also possible to change |
| these functions after png_create_???_struct() has been called by calling: |
| |
| png_set_error_fn(png_structp png_ptr, png_voidp error_ptr, |
| png_error_ptr error_fn, png_error_ptr warning_fn); |
| |
| png_voidp error_ptr = png_get_error_ptr(png_ptr); |
| |
| If NULL is supplied for either error_fn or warning_fn, then the libpng |
| default function will be used, calling fprintf() and/or longjmp() if a |
| problem is encountered. The replacement error functions should have |
| parameters as follows: |
| |
| void user_error_fn(png_struct png_ptr, png_const_charp error_msg); |
| void user_warning_fn(png_struct png_ptr, png_const_charp warning_msg); |
| |
| The motivation behind using setjmp() and longjmp() is the C++ throw and |
| catch exception handling methods. This makes the code much easier to write, |
| as there is no need to check every return code of every function call. |
| However, there are some uncertainties about the status of local variables |
| after a longjmp, so the user may want to be careful about doing anything after |
| setjmp returns non zero besides returning itself. Consult your compiler |
| documentation for more details. |
| |
| If you need to read or write custom chunks, you will need to get deeper |
| into the libpng code, as a mechanism has not yet been supplied for user |
| callbacks with custom chunks. First, read the PNG specification, and have |
| a first level of understanding of how it works. Pay particular attention |
| to the sections that describe chunk names, and look at how other chunks |
| were designed, so you can do things similarly. Second, check out the |
| sections of libpng that read and write chunks. Try to find a chunk that |
| is similar to yours and copy off of it. More details can be found in the |
| comments inside the code. A way of handling unknown chunks in a generic |
| method, potentially via callback functions, would be best. |
| |
| If you wish to write your own transformation for the data, look through |
| the part of the code that does the transformations, and check out some of |
| the simpler ones to get an idea of how they work. Try to find a similar |
| transformation to the one you want to add and copy off of it. More details |
| can be found in the comments inside the code itself. |
| |
| Configuring for 16 bit platforms: |
| |
| You may need to change the png_large_malloc() and png_large_free() |
| routines in pngmem.c, as these are requred to allocate 64K, although |
| there is already support for many of the common DOS compilers. Also, |
| you will want to look into zconf.h to tell zlib (and thus libpng) that |
| it cannot allocate more then 64K at a time. Even if you can, the memory |
| won't be accessable. So limit zlib and libpng to 64K by defining MAXSEG_64K. |
| |
| Configuring for DOS: |
| |
| For DOS users which only have access to the lower 640K, you will |
| have to limit zlib's memory usage via a png_set_compression_mem_level() |
| call. See zlib.h or zconf.h in the zlib library for more information. |
| |
| Configuring for Medium Model: |
| |
| Libpng's support for medium model has been tested on most of the popular |
| complers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets |
| defined, and FAR gets defined to far in pngconf.h, and you should be |
| all set. Everything in the library (except for zlib's structure) is |
| expecting far data. You must use the typedefs with the p or pp on |
| the end for pointers (or at least look at them and be careful). Make |
| note that the row's of data are defined as png_bytepp which is a |
| unsigned char far * far *. |
| |
| Configuring for gui/windowing platforms: |
| |
| You will need to write new error and warning functions that use the GUI |
| interface, as described previously, and set them to be the error and |
| warning functions at the time that png_create_???_struct() is called, |
| in order to have them available during the structure initialization. |
| They can be changed later via png_set_error_fn(). On some compliers, |
| you may also have to change the memory allocators (png_malloc, etc.). |
| |
| Configuring for compiler xxx: |
| |
| All includes for libpng are in pngconf.h. If you need to add/change/delete |
| an include, this is the place to do it. The includes that are not |
| needed outside libpng are protected by the PNG_INTERNAL definition, |
| which is only defined for those routines inside libpng itself. The |
| files in libpng proper only include png.h, which includes pngconf.h. |
| |
| Configuring zlib: |
| |
| There are special functions to configure the compression. Perhaps the |
| most useful one changes the compression level, which currently uses |
| input compression values in the range 0 - 9. The library normally |
| uses the default compression level (Z_DEFAULT_COMPRESSION = 6), but if |
| speed is not critical it is possible to configure it for maximum |
| compression (Z_BEST_COMPRESSION = 9) to generate smaller PNG files. |
| For online applications it may be desirable to have maximum speed |
| (Z_BEST_SPEED = 1). With versions of zlib after v0.99, you can also |
| specify no compression (Z_NO_COMPRESSION = 0), but this would create |
| files larger than just storing the raw bitmap. You can specify the |
| compression level by calling: |
| |
| png_set_compression_mem_level(png_ptr, level); |
| |
| Another useful one is to reduce the memory level used by the library. |
| The memory level defaults to 8, but it can be lowered if you are |
| short on memory (running DOS, for example, where you only have 640K). |
| |
| png_set_compression_mem_level(png_ptr, level); |
| |
| If you want to control whether libpng uses filtering or not, you |
| can call this function. Filtering is enabled by default for RGB |
| and grayscale images (with and without alpha), and for 8-bit |
| paletted images, but not for paletted images with bit depths less |
| than 8 bits/pixel. The 'method' parameter sets the main filtering |
| method, which is currently only '0' in the PNG 1.0 specification. |
| The 'filters' parameter sets which filter(s), if any, should be |
| used for each scanline. Possible values are PNG_ALL_FILTERS and |
| PNG_NO_FILTERS to turn filtering on and off, respectively. |
| Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB, |
| PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise |
| ORed together '|' to specify one or more filters to use. These |
| filters are described in more detail in the PNG specification. If |
| you intend to change the filter type during the course of writing |
| the image, you should start with flags set for all of the filters |
| you intend to use so that libpng can initialize its internal |
| structures appropriately for all of the filter types. |
| |
| png_set_filter(png_ptr, method, filters); |
| |
| The other functions are for configuring zlib. They are not recommended |
| for normal use and may result in writing an invalid PNG file. See |
| zlib.h for more information on what these mean. |
| |
| png_set_compression_strategy(png_ptr, strategy); |
| png_set_compression_window_bits(png_ptr, window_bits); |
| png_set_compression_method(png_ptr, method); |
| |
| Except for png_set_filter(), all of these are just controlling zlib, |
| so see the zlib documentation (zlib.h and zconf.h) for more information. |
| |
| Removing unwanted object code: |
| |
| There are a bunch of #define's in pngconf.h that control what parts of |
| libpng are compiled. All the defines end in _SUPPORT. If you are |
| never going to use an ability, you can change the #define to #undef |
| before recompiling libpng and save yourself code and data space. All |
| the reading and writing specific code are in seperate files, so the |
| linker should only grab the files it needs. However, if you want to |
| make sure, or if you are building a stand alone library, all the |
| reading files start with pngr and all the writing files start with |
| pngw. The files that don't match either (like png.c, pngtrans.c, etc.) |
| are used for both reading and writing, and always need to be included. |
| The progressive reader is in pngpread.c |
| |
| If you are creating or distributing a dynamically linked library (a .so |
| or DLL file), you should not remove or disable any parts of the |
| library, as this will cause applications linked with different versions |
| of the library to fail if they call functions not available in your |
| library. The size of the library itself should not be an issue, because |
| only those sections which are actually used will be loaded into memory. |
| |
| Changes to Libpng from version 0.88 to version 0.89 |
| |
| It should be noted that version 0.89 of libpng is not distributed by |
| the original author, Guy Schalnat, but rather Andreas Dilger, although |
| all of the copyright messages have been left in Guy's name. |
| |
| The old libpng functions png_read_init(), png_write_init() and |
| png_info_init() still exist in the 0.89 version of the library, as |
| do png_read_destroy() and png_write_destroy(). The preferred method |
| of creating and initializing the libpng structures is via the |
| png_create_read_struct(), png_create_write_struct(), and |
| png_create_info_struct() because they isolate the size of the structures |
| from the application, allow version error checking, and also allow |
| the use of custom error handling routines during the initialization, |
| which the old functions do not. The functions png_read_destroy() and |
| png_write_destroy() do not actually free the memory that libpng allocated |
| for these structs, but just reset the data structures, so they can be |
| used instead of png_destroy_read_struct() and png_destroy_write_struct() |
| if you feel there is too much system overhead allocating and freeing the |
| png_struct for each image read. |
| |
| Setting the error callbacks via png_set_message_fn() before |
| png_read_init() as was suggested in libpng-0.88 is no longer supported |
| because this caused applications which do not use custom error functions |
| to fail if the png_ptr was not initialized to zero. It is still possible |
| to set the error callbacks AFTER png_read_init(), or to change them with |
| png_set_error_fn(), which is essentially the same function, but with a |
| new name to force compilation errors with the new library. |
| |