GIF的C语言解码程序

/* DECODE.C - An LZW decoder for GIF * Copyright (C) 1987, by Steven A. Bennett * * Permission is given by the author to freely redistribute and include * this code in any program as long as this credit is given where due. * * In accordance with the above, I want to credit Steve Wilhite who wrote * the code which this is heavily inspired by... * * GIF and 'Graphics Interchange Format' are trademarks (tm) of * Compuserve, Incorporated, an H&R Block Company. * * Release Notes: This file contains a decoder routine for GIF images * which is similar, structurally, to the original routine by Steve Wilhite. * It is, however, somewhat noticably faster in most cases. * */ #include "std.h" #include "errs.h" IMPORT TEXT *malloc(); /* Standard C library allocation */ /* IMPORT INT get_byte() * * - This external (machine specific) function is expected to return * either the next byte from the GIF file, or a negative number, as * defined in ERRS.H. */ IMPORT INT get_byte(); /* IMPORT INT out_line(pixels, linelen) * UBYTE pixels[]; * INT linelen; * * - This function takes a full line of pixels (one byte per pixel) and * displays them (or does whatever your program wants with them...). It * should return zero, or negative if an error or some other event occurs * which would require aborting the decode process... Note that the length * passed will almost always be equal to the line length passed to the * decoder function, with the sole exception occurring when an ending code * occurs in an odd place in the GIF file... In any case, linelen will be * equal to the number of pixels passed... */ IMPORT INT out_line(); /* IMPORT INT bad_code_count; * * This value is the only other global required by the using program, and * is incremented each time an out of range code is read by the decoder. * When this value is non-zero after a decode, your GIF file is probably * corrupt in some way... */ IMPORT INT bad_code_count; #define NULL 0L #define MAX_CODES 4095 /* Static variables */ LOCAL WORD curr_size; /* The current code size */ LOCAL WORD clear; /* Value for a clear code */ LOCAL WORD ending; /* Value for a ending code */ LOCAL WORD newcodes; /* First available code */ LOCAL WORD top_slot; /* Highest code for current size */ LOCAL WORD slot; /* Last read code */ /* The following static variables are used * for seperating out codes */ LOCAL WORD navail_bytes = 0; /* # bytes left in block */ LOCAL WORD nbits_left = 0; /* # bits left in current byte */ LOCAL UTINY b1; /* Current byte */ LOCAL UTINY byte_buff[257]; /* Current block */ LOCAL UTINY *pbytes; /* Pointer to next byte in block */ LOCAL LONG code_mask[13] = { 0, 0x0001, 0x0003, 0x0007, 0x000F, 0x001F, 0x003F, 0x007F, 0x00FF, 0x01FF, 0x03FF, 0x07FF, 0x0FFF }; /* This function initializes the decoder for reading a new image. */ LOCAL WORD init_exp(size) WORD size; { curr_size = size + 1; top_slot = 1 << curr_size; clear = 1 << size; ending = clear + 1; slot = newcodes = ending + 1; navail_bytes = nbits_left = 0; return(0); } /* get_next_code() * - gets the next code from the GIF file. Returns the code, or else * a negative number in case of file errors... */ LOCAL WORD get_next_code() { WORD i, x; ULONG ret; if (nbits_left == 0) { if (navail_bytes <= 0) { /* Out of bytes in current block, so read next block */ pbytes = byte_buff; if ((navail_bytes = get_byte()) < 0) return(navail_bytes); else if (navail_bytes) { for (i = 0; i < navail_bytes; ++i) { if ((x = get_byte()) < 0) return(x); byte_buff[i] = x; } } } b1 = *pbytes++; nbits_left = 8; --navail_bytes; } ret = b1 >> (8 - nbits_left); while (curr_size > nbits_left) { if (navail_bytes <= 0) { /* Out of bytes in current block, so read next block */ pbytes = byte_buff; if ((navail_bytes = get_byte()) < 0) return(navail_bytes); else if (navail_bytes) { for (i = 0; i < navail_bytes; ++i) { if ((x = get_byte()) < 0) return(x); byte_buff[i] = x; } } } b1 = *pbytes++; ret |= b1 << nbits_left; nbits_left += 8; --navail_bytes; } nbits_left -= curr_size; ret &= code_mask[curr_size]; return((WORD)(ret)); } /* The reason we have these seperated like this instead of using * a structure like the original Wilhite code did, is because this * stuff generally produces significantly faster code when compiled... * This code is full of similar speedups... (For a good book on writing * C for speed or for space optomisation, see Efficient C by Tom Plum, * published by Plum-Hall Associates...) */ LOCAL UTINY stack[MAX_CODES + 1]; /* Stack for storing pixels */ LOCAL UTINY suffix[MAX_CODES + 1]; /* Suffix table */ LOCAL UWORD prefix[MAX_CODES + 1]; /* Prefix linked list */ /* WORD decoder(linewidth) * WORD linewidth; * Pixels per line of image * * * - This function decodes an LZW image, according to the method used * in the GIF spec. Every *linewidth* "characters" (ie. pixels) decoded * will generate a call to out_line(), which is a user specific function * to display a line of pixels. The function gets it's codes from * get_next_code() which is responsible for reading blocks of data and * seperating them into the proper size codes. Finally, get_byte() is * the global routine to read the next byte from the GIF file. * * It is generally a good idea to have linewidth correspond to the actual * width of a line (as specified in the Image header) to make your own * code a bit simpler, but it isn't absolutely necessary. * * Returns: 0 if successful, else negative. (See ERRS.H) * */ WORD decoder(linewidth) WORD linewidth; { FAST UTINY *sp, *bufptr; UTINY *buf; FAST WORD code, fc, oc, bufcnt; WORD c, size, ret; /* Initialize for decoding a new image... */ if ((size = get_byte()) < 0) return(size); if (size < 2 || 9 < size) return(BAD_CODE_SIZE); init_exp(size); /* Initialize in case they forgot to put in a clear code. * (This shouldn't happen, but we'll try and decode it anyway...) */ oc = fc = 0; /* Allocate space for the decode buffer */ if ((buf = (UTINY *)malloc(linewidth + 1)) == NULL) return(OUT_OF_MEMORY); /* Set up the stack pointer and decode buffer pointer */ sp = stack; bufptr = buf; bufcnt = linewidth; /* This is the main loop. For each code we get we pass through the * linked list of prefix codes, pushing the corresponding "character" for * each code onto the stack. When the list reaches a single "character" * we push that on the stack too, and then start unstacking each * character for output in the correct order. Special handling is * included for the clear code, and the whole thing ends when we get * an ending code. */ while ((c = get_next_code()) != ending) { /* If we had a file error, return without completing the decode */ if (c < 0) { free(buf); return(0); } /* If the code is a clear code, reinitialize all necessary items. */ if (c == clear) { curr_size = size + 1; slot = newcodes; top_slot = 1