2 /* png-fix-itxt version 1.0.0
4 * Copyright 2015 Glenn Randers-Pehrson
5 * Last changed in libpng 1.6.18 [July 23, 2015]
7 * This code is released under the libpng license.
8 * For conditions of distribution and use, see the disclaimer
13 * png-fix-itxt.exe < bad.png > good.png
15 * Fixes a PNG file written with libpng-1.6.0 or 1.6.1 that has one or more
16 * uncompressed iTXt chunks. Assumes that the actual length is greater
17 * than or equal to the value in the length byte, and that the CRC is
18 * correct for the actual length. This program hunts for the CRC and
19 * adjusts the length byte accordingly. It is not an error to process a
20 * PNG file that has no iTXt chunks or one that has valid iTXt chunks;
21 * such files will simply be copied.
23 * Requires zlib (for crc32 and Z_NULL); build with
25 * gcc -O -o png-fix-itxt png-fix-itxt.c -lz
27 * If you need to handle iTXt chunks larger than 500000 kbytes you must
28 * rebuild png-fix-itxt with a larger values of MAX_LENGTH (or a smaller value
29 * if you know you will never encounter such huge iTXt chunks).
35 #define MAX_LENGTH 500000
37 /* Read one character (inchar), also return octet (c), break if EOF */
38 #define GETBREAK inchar=getchar(); \
40 if (inchar != c) break
45 unsigned char buf[MAX_LENGTH];
50 /* Skip 8-byte signature */
57 if (inchar == c) /* !EOF */
61 unsigned long length; /* must be 32 bits! */
62 GETBREAK; buf[0] = c; length = c; length <<= 8;
63 GETBREAK; buf[1] = c; length += c; length <<= 8;
64 GETBREAK; buf[2] = c; length += c; length <<= 8;
65 GETBREAK; buf[3] = c; length += c;
67 /* Read the chunkname */
74 /* The iTXt chunk type expressed as integers is (105, 84, 88, 116) */
75 if (buf[4] == 105 && buf[5] == 84 && buf[6] == 88 && buf[7] == 116)
77 if (length >= MAX_LENGTH-12)
78 break; /* To do: handle this more gracefully */
80 /* Initialize the CRC */
81 crc = crc32(0, Z_NULL, 0);
83 /* Copy the data bytes */
84 for (i=8; i < length + 12; i++)
89 if (inchar != c) /* EOF */
92 /* Calculate the CRC */
93 crc = crc32(crc, buf+4, (uInt)length+4);
98 if (((crc >> 24) & 0xffU) == buf[length+8] &&
99 ((crc >> 16) & 0xffU) == buf[length+9] &&
100 ((crc >> 8) & 0xffU) == buf[length+10] &&
101 ((crc ) & 0xffU) == buf[length+11])
106 if (length >= MAX_LENGTH-12)
113 crc = crc32(crc, buf+7+length, 1);
116 if (inchar != c) /* EOF */
119 /* Update length bytes */
120 buf[0] = (unsigned char)((length >> 24) & 0xffU);
121 buf[1] = (unsigned char)((length >> 16) & 0xffU);
122 buf[2] = (unsigned char)((length >> 8) & 0xffU);
123 buf[3] = (unsigned char)((length ) & 0xffU);
125 /* Write the fixed iTXt chunk (length, name, data, crc) */
126 for (i=0; i<length+12; i++)
132 if (inchar != c) /* EOF */
135 /* Copy bytes that were already read (length and chunk name) */
139 /* Copy data bytes and CRC */
140 for (i=8; i< length+12; i++)
146 if (inchar != c) /* EOF */
151 /* The IEND chunk type expressed as integers is (73, 69, 78, 68) */
152 if (buf[4] == 73 && buf[5] == 69 && buf[6] == 78 && buf[7] == 68)
156 if (inchar != c) /* EOF */
159 if (buf[4] == 73 && buf[5] == 69 && buf[6] == 78 && buf[7] == 68)