Chris Craik | b50c217 | 2013-07-29 15:28:30 -0700 | [diff] [blame^] | 1 | /*- pngpixel |
| 2 | * |
| 3 | * COPYRIGHT: Written by John Cunningham Bowler, 2011. |
| 4 | * To the extent possible under law, the author has waived all copyright and |
| 5 | * related or neighboring rights to this work. This work is published from: |
| 6 | * United States. |
| 7 | * |
| 8 | * Read a single pixel value from a PNG file. |
| 9 | * |
| 10 | * This code illustrates basic 'by-row' reading of a PNG file using libpng. |
| 11 | * Rows are read until a particular pixel is found, the value of this pixel is |
| 12 | * then printed on stdout. |
| 13 | * |
| 14 | * The code illustrates how to do this on interlaced as well as non-interlaced |
| 15 | * images. Normally you would call png_set_interlace_handling() to have libpng |
| 16 | * deal with the interlace for you, but that obliges you to buffer half of the |
| 17 | * image to assemble the interlaced rows. In this code |
| 18 | * png_set_interlace_handling() is not called and, instead, the code handles the |
| 19 | * interlace passes directly looking for the required pixel. |
| 20 | */ |
| 21 | #include <stdlib.h> |
| 22 | #include <stdio.h> |
| 23 | #include <setjmp.h> /* required for error handling */ |
| 24 | |
| 25 | /* Normally use <png.h> here to get the installed libpng, but this is done to |
| 26 | * ensure the code picks up the local libpng implementation: |
| 27 | */ |
| 28 | #include "../../png.h" |
| 29 | |
| 30 | /* Return component 'c' of pixel 'x' from the given row. */ |
| 31 | static unsigned int |
| 32 | component(png_const_bytep row, png_uint_32 x, unsigned int c, |
| 33 | unsigned int bit_depth, unsigned int channels) |
| 34 | { |
| 35 | /* PNG images can be up to 2^31 pixels wide, but this means they can be up to |
| 36 | * 2^37 bits wide (for a 64-bit pixel - the largest possible) and hence 2^34 |
| 37 | * bytes wide. Since the row fitted into memory, however, the following must |
| 38 | * work: |
| 39 | */ |
| 40 | png_uint_32 bit_offset_hi = bit_depth * ((x >> 6) * channels); |
| 41 | png_uint_32 bit_offset_lo = bit_depth * ((x & 0x3f) * channels + c); |
| 42 | |
| 43 | row = (png_const_bytep)(((PNG_CONST png_byte (*)[8])row) + bit_offset_hi); |
| 44 | row += bit_offset_lo >> 3; |
| 45 | bit_offset_lo &= 0x07; |
| 46 | |
| 47 | /* PNG pixels are packed into bytes to put the first pixel in the highest |
| 48 | * bits of the byte and into two bytes for 16-bit values with the high 8 bits |
| 49 | * first, so: |
| 50 | */ |
| 51 | switch (bit_depth) |
| 52 | { |
| 53 | case 1: return (row[0] >> (7-bit_offset_lo)) & 0x01; |
| 54 | case 2: return (row[0] >> (6-bit_offset_lo)) & 0x03; |
| 55 | case 4: return (row[0] >> (4-bit_offset_lo)) & 0x0f; |
| 56 | case 8: return row[0]; |
| 57 | case 16: return (row[0] << 8) + row[1]; |
| 58 | default: |
| 59 | /* This should never happen, it indicates a bug in this program or in |
| 60 | * libpng itself: |
| 61 | */ |
| 62 | fprintf(stderr, "pngpixel: invalid bit depth %u\n", bit_depth); |
| 63 | exit(1); |
| 64 | } |
| 65 | } |
| 66 | |
| 67 | /* Print a pixel from a row returned by libpng; determine the row format, find |
| 68 | * the pixel, and print the relevant information to stdout. |
| 69 | */ |
| 70 | static void |
| 71 | print_pixel(png_structp png_ptr, png_infop info_ptr, png_const_bytep row, |
| 72 | png_uint_32 x) |
| 73 | { |
| 74 | PNG_CONST unsigned int bit_depth = png_get_bit_depth(png_ptr, info_ptr); |
| 75 | |
| 76 | switch (png_get_color_type(png_ptr, info_ptr)) |
| 77 | { |
| 78 | case PNG_COLOR_TYPE_GRAY: |
| 79 | printf("GRAY %u\n", component(row, x, 0, bit_depth, 1)); |
| 80 | return; |
| 81 | |
| 82 | /* The palette case is slightly more difficult - the palette and, if |
| 83 | * present, the tRNS ('transparency', though the values are really |
| 84 | * opacity) data must be read to give the full picture: |
| 85 | */ |
| 86 | case PNG_COLOR_TYPE_PALETTE: |
| 87 | { |
| 88 | PNG_CONST unsigned int index = component(row, x, 0, bit_depth, 1); |
| 89 | png_colorp palette = NULL; |
| 90 | int num_palette = 0; |
| 91 | |
| 92 | if ((png_get_PLTE(png_ptr, info_ptr, &palette, &num_palette) & |
| 93 | PNG_INFO_PLTE) && num_palette > 0 && palette != NULL) |
| 94 | { |
| 95 | png_bytep trans_alpha = NULL; |
| 96 | int num_trans = 0; |
| 97 | if ((png_get_tRNS(png_ptr, info_ptr, &trans_alpha, &num_trans, |
| 98 | NULL) & PNG_INFO_tRNS) && num_trans > 0 && |
| 99 | trans_alpha != NULL) |
| 100 | printf("INDEXED %u = %d %d %d %d\n", index, |
| 101 | palette[index].red, palette[index].green, |
| 102 | palette[index].blue, |
| 103 | index < num_trans ? trans_alpha[index] : 255); |
| 104 | |
| 105 | else /* no transparency */ |
| 106 | printf("INDEXED %u = %d %d %d\n", index, |
| 107 | palette[index].red, palette[index].green, |
| 108 | palette[index].blue); |
| 109 | } |
| 110 | |
| 111 | else |
| 112 | printf("INDEXED %u = invalid index\n", index); |
| 113 | } |
| 114 | return; |
| 115 | |
| 116 | case PNG_COLOR_TYPE_RGB: |
| 117 | printf("RGB %u %u %u\n", component(row, x, 0, bit_depth, 3), |
| 118 | component(row, x, 1, bit_depth, 3), |
| 119 | component(row, x, 2, bit_depth, 3)); |
| 120 | return; |
| 121 | |
| 122 | case PNG_COLOR_TYPE_GRAY_ALPHA: |
| 123 | printf("GRAY+ALPHA %u %u\n", component(row, x, 0, bit_depth, 2), |
| 124 | component(row, x, 1, bit_depth, 2)); |
| 125 | return; |
| 126 | |
| 127 | case PNG_COLOR_TYPE_RGB_ALPHA: |
| 128 | printf("RGBA %u %u %u %u\n", component(row, x, 0, bit_depth, 4), |
| 129 | component(row, x, 1, bit_depth, 4), |
| 130 | component(row, x, 2, bit_depth, 4), |
| 131 | component(row, x, 3, bit_depth, 4)); |
| 132 | return; |
| 133 | |
| 134 | default: |
| 135 | png_error(png_ptr, "invalid color type"); |
| 136 | } |
| 137 | } |
| 138 | |
| 139 | int main(int argc, const char **argv) |
| 140 | { |
| 141 | /* This program uses the default, <setjmp.h> based, libpng error handling |
| 142 | * mechanism, therefore any local variable that exists before the call to |
| 143 | * setjmp and is changed after the call to setjmp returns successfully must |
| 144 | * be declared with 'volatile' to ensure that their values don't get |
| 145 | * destroyed by longjmp: |
| 146 | */ |
| 147 | volatile int result = 1/*fail*/; |
| 148 | |
| 149 | if (argc == 4) |
| 150 | { |
| 151 | long x = atol(argv[1]); |
| 152 | long y = atol(argv[2]); |
| 153 | FILE *f = fopen(argv[3], "rb"); |
| 154 | volatile png_bytep row = NULL; |
| 155 | |
| 156 | if (f != NULL) |
| 157 | { |
| 158 | /* libpng requires a callback function for handling errors; this |
| 159 | * callback must not return. The default callback function uses a |
| 160 | * stored <setjmp.h> style jmp_buf which is held in a png_struct and |
| 161 | * writes error messages to stderr. Creating the png_struct is a |
| 162 | * little tricky; just copy the following code. |
| 163 | */ |
| 164 | png_structp png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, |
| 165 | NULL, NULL, NULL); |
| 166 | |
| 167 | if (png_ptr != NULL) |
| 168 | { |
| 169 | png_infop info_ptr = png_create_info_struct(png_ptr); |
| 170 | |
| 171 | if (info_ptr != NULL) |
| 172 | { |
| 173 | /* Declare stack variables to hold pointers to locally allocated |
| 174 | * data. |
| 175 | */ |
| 176 | |
| 177 | /* Initialize the error control buffer: */ |
| 178 | if (setjmp(png_jmpbuf(png_ptr)) == 0) |
| 179 | { |
| 180 | png_uint_32 width, height; |
| 181 | int bit_depth, color_type, interlace_method, |
| 182 | compression_method, filter_method; |
| 183 | png_bytep row_tmp; |
| 184 | |
| 185 | /* Now associate the recently opened (FILE*) with the default |
| 186 | * libpng initialization functions. Sometimes libpng is |
| 187 | * compiled without stdio support (it can be difficult to do |
| 188 | * in some environments); in that case you will have to write |
| 189 | * your own read callback to read data from the (FILE*). |
| 190 | */ |
| 191 | png_init_io(png_ptr, f); |
| 192 | |
| 193 | /* And read the first part of the PNG file - the header and |
| 194 | * all the information up to the first pixel. |
| 195 | */ |
| 196 | png_read_info(png_ptr, info_ptr); |
| 197 | |
| 198 | /* This fills in enough information to tell us the width of |
| 199 | * each row in bytes, allocate the appropriate amount of |
| 200 | * space. In this case png_malloc is used - it will not |
| 201 | * return if memory isn't available. |
| 202 | */ |
| 203 | row = png_malloc(png_ptr, png_get_rowbytes(png_ptr, |
| 204 | info_ptr)); |
| 205 | |
| 206 | /* To avoid the overhead of using a volatile auto copy row_tmp |
| 207 | * to a local here - just use row for the png_free below. |
| 208 | */ |
| 209 | row_tmp = row; |
| 210 | |
| 211 | /* All the information we need is in the header is returned by |
| 212 | * png_get_IHDR, if this fails we can now use 'png_error' to |
| 213 | * signal the error and return control to the setjmp above. |
| 214 | */ |
| 215 | if (png_get_IHDR(png_ptr, info_ptr, &width, &height, |
| 216 | &bit_depth, &color_type, &interlace_method, |
| 217 | &compression_method, &filter_method)) |
| 218 | { |
| 219 | int passes, pass; |
| 220 | |
| 221 | /* png_set_interlace_handling returns the number of |
| 222 | * passes required as well as turning on libpng's |
| 223 | * handling, but since we do it ourselves this is |
| 224 | * necessary: |
| 225 | */ |
| 226 | switch (interlace_method) |
| 227 | { |
| 228 | case PNG_INTERLACE_NONE: |
| 229 | passes = 1; |
| 230 | break; |
| 231 | |
| 232 | case PNG_INTERLACE_ADAM7: |
| 233 | passes = PNG_INTERLACE_ADAM7_PASSES; |
| 234 | break; |
| 235 | |
| 236 | default: |
| 237 | png_error(png_ptr, "pngpixel: unknown interlace"); |
| 238 | } |
| 239 | |
| 240 | /* Now read the pixels, pass-by-pass, row-by-row: */ |
| 241 | png_start_read_image(png_ptr); |
| 242 | |
| 243 | for (pass=0; pass<passes; ++pass) |
| 244 | { |
| 245 | png_uint_32 ystart, xstart, ystep, xstep; |
| 246 | png_uint_32 py; |
| 247 | |
| 248 | if (interlace_method == PNG_INTERLACE_ADAM7) |
| 249 | { |
| 250 | /* Sometimes the whole pass is empty because the |
| 251 | * image is too narrow or too short. libpng |
| 252 | * expects to be called for each row that is |
| 253 | * present in the pass, so it may be necessary to |
| 254 | * skip the loop below (over py) if the image is |
| 255 | * too narrow. |
| 256 | */ |
| 257 | if (PNG_PASS_COLS(width, pass) == 0) |
| 258 | continue; |
| 259 | |
| 260 | /* We need the starting pixel and the offset |
| 261 | * between each pixel in this pass; use the macros |
| 262 | * in png.h: |
| 263 | */ |
| 264 | xstart = PNG_PASS_START_COL(pass); |
| 265 | ystart = PNG_PASS_START_ROW(pass); |
| 266 | xstep = PNG_PASS_COL_OFFSET(pass); |
| 267 | ystep = PNG_PASS_ROW_OFFSET(pass); |
| 268 | } |
| 269 | |
| 270 | else |
| 271 | { |
| 272 | ystart = xstart = 0; |
| 273 | ystep = xstep = 1; |
| 274 | } |
| 275 | |
| 276 | /* To find the pixel, loop over 'py' for each pass |
| 277 | * reading a row and then checking to see if it |
| 278 | * contains the pixel. |
| 279 | */ |
| 280 | for (py = ystart; py < height; py += ystep) |
| 281 | { |
| 282 | png_uint_32 px, ppx; |
| 283 | |
| 284 | /* png_read_row takes two pointers. When libpng |
| 285 | * handles the interlace the first is filled in |
| 286 | * pixel-by-pixel, and the second receives the same |
| 287 | * pixels but they are replicated across the |
| 288 | * unwritten pixels so far for each pass. When we |
| 289 | * do the interlace, however, they just contain |
| 290 | * the pixels from the interlace pass - giving |
| 291 | * both is wasteful and pointless, so we pass a |
| 292 | * NULL pointer. |
| 293 | */ |
| 294 | png_read_row(png_ptr, row_tmp, NULL); |
| 295 | |
| 296 | /* Now find the pixel if it is in this row; there |
| 297 | * are, of course, much better ways of doing this |
| 298 | * than using a for loop: |
| 299 | */ |
| 300 | if (y == py) for (px = xstart, ppx = 0; |
| 301 | px < width; px += xstep, ++ppx) if (x == px) |
| 302 | { |
| 303 | /* 'ppx' is the index of the pixel in the row |
| 304 | * buffer. |
| 305 | */ |
| 306 | print_pixel(png_ptr, info_ptr, row_tmp, ppx); |
| 307 | |
| 308 | /* Now terminate the loops early - we have |
| 309 | * found and handled the required data. |
| 310 | */ |
| 311 | goto pass_loop_end; |
| 312 | } /* x loop */ |
| 313 | } /* y loop */ |
| 314 | } /* pass loop */ |
| 315 | |
| 316 | /* Finally free the temporary buffer: */ |
| 317 | pass_loop_end: |
| 318 | row = NULL; |
| 319 | png_free(png_ptr, row_tmp); |
| 320 | } |
| 321 | |
| 322 | else |
| 323 | png_error(png_ptr, "pngpixel: png_get_IHDR failed"); |
| 324 | |
| 325 | } |
| 326 | |
| 327 | else |
| 328 | { |
| 329 | /* Else libpng has raised an error. An error message has |
| 330 | * already been output, so it is only necessary to clean up |
| 331 | * locally allocated data: |
| 332 | */ |
| 333 | if (row != NULL) |
| 334 | { |
| 335 | /* The default implementation of png_free never errors out |
| 336 | * (it just crashes if something goes wrong), but the safe |
| 337 | * way of using it is still to clear 'row' before calling |
| 338 | * png_free: |
| 339 | */ |
| 340 | png_bytep row_tmp = row; |
| 341 | row = NULL; |
| 342 | png_free(png_ptr, row_tmp); |
| 343 | } |
| 344 | } |
| 345 | |
| 346 | png_destroy_info_struct(png_ptr, &info_ptr); |
| 347 | } |
| 348 | |
| 349 | else |
| 350 | fprintf(stderr, "pngpixel: out of memory allocating png_info\n"); |
| 351 | |
| 352 | png_destroy_read_struct(&png_ptr, NULL, NULL); |
| 353 | } |
| 354 | |
| 355 | else |
| 356 | fprintf(stderr, "pngpixel: out of memory allocating png_struct\n"); |
| 357 | } |
| 358 | |
| 359 | else |
| 360 | fprintf(stderr, "pngpixel: %s: could not open file\n", argv[3]); |
| 361 | } |
| 362 | |
| 363 | else |
| 364 | /* Wrong number of arguments */ |
| 365 | fprintf(stderr, "pngpixel: usage: pngpixel x y png-file\n"); |
| 366 | |
| 367 | return result; |
| 368 | } |