The Independent JPEG Group's JPEG software v6b
diff --git a/wrgif.c b/wrgif.c
index 85cfaa8..5fe8328 100644
--- a/wrgif.c
+++ b/wrgif.c
@@ -1,17 +1,18 @@
/*
* wrgif.c
*
- * Copyright (C) 1991-1996, Thomas G. Lane.
+ * Copyright (C) 1991-1997, Thomas G. Lane.
* This file is part of the Independent JPEG Group's software.
* For conditions of distribution and use, see the accompanying README file.
*
- **************************************************************************
- * WARNING: You will need an LZW patent license from Unisys in order to *
- * use this file legally in any commercial or shareware application. *
- **************************************************************************
- *
* This file contains routines to write output images in GIF format.
*
+ **************************************************************************
+ * NOTE: to avoid entanglements with Unisys' patent on LZW compression, *
+ * this code has been modified to output "uncompressed GIF" files. *
+ * There is no trace of the LZW algorithm in this file. *
+ **************************************************************************
+ *
* These routines may need modification for non-Unix environments or
* specialized applications. As they stand, they assume output to
* an ordinary stdio stream.
@@ -41,40 +42,6 @@
#ifdef GIF_SUPPORTED
-#define MAX_LZW_BITS 12 /* maximum LZW code size (4096 symbols) */
-
-typedef INT16 code_int; /* must hold -1 .. 2**MAX_LZW_BITS */
-
-#define LZW_TABLE_SIZE ((code_int) 1 << MAX_LZW_BITS)
-
-#define HSIZE 5003 /* hash table size for 80% occupancy */
-
-typedef int hash_int; /* must hold -2*HSIZE..2*HSIZE */
-
-#define MAXCODE(n_bits) (((code_int) 1 << (n_bits)) - 1)
-
-
-/*
- * The LZW hash table consists of two parallel arrays:
- * hash_code[i] code of symbol in slot i, or 0 if empty slot
- * hash_value[i] symbol's value; undefined if empty slot
- * where slot values (i) range from 0 to HSIZE-1. The symbol value is
- * its prefix symbol's code concatenated with its suffix character.
- *
- * Algorithm: use open addressing double hashing (no chaining) on the
- * prefix code / suffix character combination. We do a variant of Knuth's
- * algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime
- * secondary probe.
- *
- * The hash_value[] table is allocated from FAR heap space since it would
- * use up rather a lot of the near data space in a PC.
- */
-
-typedef INT32 hash_entry; /* must hold (code_int<<8) | byte */
-
-#define HASH_ENTRY(prefix,suffix) ((((hash_entry) (prefix)) << 8) | (suffix))
-
-
/* Private version of data destination object */
typedef struct {
@@ -84,23 +51,14 @@
/* State for packing variable-width codes into a bitstream */
int n_bits; /* current number of bits/code */
- code_int maxcode; /* maximum code, given n_bits */
- int init_bits; /* initial n_bits ... restored after clear */
+ int maxcode; /* maximum code, given n_bits */
INT32 cur_accum; /* holds bits not yet output */
int cur_bits; /* # of bits in cur_accum */
- /* LZW string construction */
- code_int waiting_code; /* symbol not yet output; may be extendable */
- boolean first_byte; /* if TRUE, waiting_code is not valid */
-
- /* State for LZW code assignment */
- code_int ClearCode; /* clear code (doesn't change) */
- code_int EOFCode; /* EOF code (ditto) */
- code_int free_code; /* first not-yet-used symbol code */
-
- /* LZW hash table */
- code_int *hash_code; /* => hash table of symbol codes */
- hash_entry FAR *hash_value; /* => hash table of symbol values */
+ /* State for GIF code assignment */
+ int ClearCode; /* clear code (doesn't change) */
+ int EOFCode; /* EOF code (ditto) */
+ int code_counter; /* counts output symbols */
/* GIF data packet construction buffer */
int bytesinpkt; /* # of bytes in current packet */
@@ -110,9 +68,12 @@
typedef gif_dest_struct * gif_dest_ptr;
+/* Largest value that will fit in N bits */
+#define MAXCODE(n_bits) ((1 << (n_bits)) - 1)
+
/*
- * Routines to package compressed data bytes into GIF data blocks.
+ * Routines to package finished data bytes into GIF data blocks.
* A data block consists of a count byte (1..255) and that many data bytes.
*/
@@ -141,7 +102,7 @@
/* Routine to convert variable-width codes into a byte stream */
LOCAL(void)
-output (gif_dest_ptr dinfo, code_int code)
+output (gif_dest_ptr dinfo, int code)
/* Emit a code of n_bits bits */
/* Uses cur_accum and cur_bits to reblock into 8-bit bytes */
{
@@ -153,123 +114,67 @@
dinfo->cur_accum >>= 8;
dinfo->cur_bits -= 8;
}
-
- /*
- * If the next entry is going to be too big for the code size,
- * then increase it, if possible. We do this here to ensure
- * that it's done in sync with the decoder's codesize increases.
- */
- if (dinfo->free_code > dinfo->maxcode) {
- dinfo->n_bits++;
- if (dinfo->n_bits == MAX_LZW_BITS)
- dinfo->maxcode = LZW_TABLE_SIZE; /* free_code will never exceed this */
- else
- dinfo->maxcode = MAXCODE(dinfo->n_bits);
- }
}
-/* The LZW algorithm proper */
-
-
-LOCAL(void)
-clear_hash (gif_dest_ptr dinfo)
-/* Fill the hash table with empty entries */
-{
- /* It's sufficient to zero hash_code[] */
- MEMZERO(dinfo->hash_code, HSIZE * SIZEOF(code_int));
-}
-
-
-LOCAL(void)
-clear_block (gif_dest_ptr dinfo)
-/* Reset compressor and issue a Clear code */
-{
- clear_hash(dinfo); /* delete all the symbols */
- dinfo->free_code = dinfo->ClearCode + 2;
- output(dinfo, dinfo->ClearCode); /* inform decoder */
- dinfo->n_bits = dinfo->init_bits; /* reset code size */
- dinfo->maxcode = MAXCODE(dinfo->n_bits);
-}
-
+/* The pseudo-compression algorithm.
+ *
+ * In this module we simply output each pixel value as a separate symbol;
+ * thus, no compression occurs. In fact, there is expansion of one bit per
+ * pixel, because we use a symbol width one bit wider than the pixel width.
+ *
+ * GIF ordinarily uses variable-width symbols, and the decoder will expect
+ * to ratchet up the symbol width after a fixed number of symbols.
+ * To simplify the logic and keep the expansion penalty down, we emit a
+ * GIF Clear code to reset the decoder just before the width would ratchet up.
+ * Thus, all the symbols in the output file will have the same bit width.
+ * Note that emitting the Clear codes at the right times is a mere matter of
+ * counting output symbols and is in no way dependent on the LZW patent.
+ *
+ * With a small basic pixel width (low color count), Clear codes will be
+ * needed very frequently, causing the file to expand even more. So this
+ * simplistic approach wouldn't work too well on bilevel images, for example.
+ * But for output of JPEG conversions the pixel width will usually be 8 bits
+ * (129 to 256 colors), so the overhead added by Clear symbols is only about
+ * one symbol in every 256.
+ */
LOCAL(void)
compress_init (gif_dest_ptr dinfo, int i_bits)
-/* Initialize LZW compressor */
+/* Initialize pseudo-compressor */
{
/* init all the state variables */
- dinfo->n_bits = dinfo->init_bits = i_bits;
+ dinfo->n_bits = i_bits;
dinfo->maxcode = MAXCODE(dinfo->n_bits);
- dinfo->ClearCode = ((code_int) 1 << (i_bits - 1));
+ dinfo->ClearCode = (1 << (i_bits - 1));
dinfo->EOFCode = dinfo->ClearCode + 1;
- dinfo->free_code = dinfo->ClearCode + 2;
- dinfo->first_byte = TRUE; /* no waiting symbol yet */
+ dinfo->code_counter = dinfo->ClearCode + 2;
/* init output buffering vars */
dinfo->bytesinpkt = 0;
dinfo->cur_accum = 0;
dinfo->cur_bits = 0;
- /* clear hash table */
- clear_hash(dinfo);
/* GIF specifies an initial Clear code */
output(dinfo, dinfo->ClearCode);
}
LOCAL(void)
-compress_byte (gif_dest_ptr dinfo, int c)
-/* Accept and compress one 8-bit byte */
+compress_pixel (gif_dest_ptr dinfo, int c)
+/* Accept and "compress" one pixel value.
+ * The given value must be less than n_bits wide.
+ */
{
- register hash_int i;
- register hash_int disp;
- register hash_entry probe_value;
-
- if (dinfo->first_byte) { /* need to initialize waiting_code */
- dinfo->waiting_code = c;
- dinfo->first_byte = FALSE;
- return;
- }
-
- /* Probe hash table to see if a symbol exists for
- * waiting_code followed by c.
- * If so, replace waiting_code by that symbol and return.
+ /* Output the given pixel value as a symbol. */
+ output(dinfo, c);
+ /* Issue Clear codes often enough to keep the reader from ratcheting up
+ * its symbol size.
*/
- i = ((hash_int) c << (MAX_LZW_BITS-8)) + dinfo->waiting_code;
- /* i is less than twice 2**MAX_LZW_BITS, therefore less than twice HSIZE */
- if (i >= HSIZE)
- i -= HSIZE;
-
- probe_value = HASH_ENTRY(dinfo->waiting_code, c);
-
- if (dinfo->hash_code[i] != 0) { /* is first probed slot empty? */
- if (dinfo->hash_value[i] == probe_value) {
- dinfo->waiting_code = dinfo->hash_code[i];
- return;
- }
- if (i == 0) /* secondary hash (after G. Knott) */
- disp = 1;
- else
- disp = HSIZE - i;
- for (;;) {
- i -= disp;
- if (i < 0)
- i += HSIZE;
- if (dinfo->hash_code[i] == 0)
- break; /* hit empty slot */
- if (dinfo->hash_value[i] == probe_value) {
- dinfo->waiting_code = dinfo->hash_code[i];
- return;
- }
- }
+ if (dinfo->code_counter < dinfo->maxcode) {
+ dinfo->code_counter++;
+ } else {
+ output(dinfo, dinfo->ClearCode);
+ dinfo->code_counter = dinfo->ClearCode + 2; /* reset the counter */
}
-
- /* here when hashtable[i] is an empty slot; desired symbol not in table */
- output(dinfo, dinfo->waiting_code);
- if (dinfo->free_code < LZW_TABLE_SIZE) {
- dinfo->hash_code[i] = dinfo->free_code++; /* add symbol to hashtable */
- dinfo->hash_value[i] = probe_value;
- } else
- clear_block(dinfo);
- dinfo->waiting_code = c;
}
@@ -277,9 +182,6 @@
compress_term (gif_dest_ptr dinfo)
/* Clean up at end */
{
- /* Flush out the buffered code */
- if (! dinfo->first_byte)
- output(dinfo, dinfo->waiting_code);
/* Send an EOF code */
output(dinfo, dinfo->EOFCode);
/* Flush the bit-packing buffer */
@@ -387,7 +289,7 @@
/* Write Initial Code Size byte */
putc(InitCodeSize, dinfo->pub.output_file);
- /* Initialize for LZW compression of image data */
+ /* Initialize for "compression" of image data */
compress_init(dinfo, InitCodeSize+1);
}
@@ -423,7 +325,7 @@
ptr = dest->pub.buffer[0];
for (col = cinfo->output_width; col > 0; col--) {
- compress_byte(dest, GETJSAMPLE(*ptr++));
+ compress_pixel(dest, GETJSAMPLE(*ptr++));
}
}
@@ -437,7 +339,7 @@
{
gif_dest_ptr dest = (gif_dest_ptr) dinfo;
- /* Flush LZW mechanism */
+ /* Flush "compression" mechanism */
compress_term(dest);
/* Write a zero-length data block to end the series */
putc(0, dest->pub.output_file);
@@ -491,14 +393,6 @@
((j_common_ptr) cinfo, JPOOL_IMAGE, cinfo->output_width, (JDIMENSION) 1);
dest->pub.buffer_height = 1;
- /* Allocate space for hash table */
- dest->hash_code = (code_int *)
- (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
- HSIZE * SIZEOF(code_int));
- dest->hash_value = (hash_entry FAR *)
- (*cinfo->mem->alloc_large) ((j_common_ptr) cinfo, JPOOL_IMAGE,
- HSIZE * SIZEOF(hash_entry));
-
return (djpeg_dest_ptr) dest;
}