3 * Copyright 2015 Glenn Randers-Pehrson
5 * This code is released under the libpng license.
6 * For conditions of distribution and use, see the disclaimer
11 * png-fix-itxt < bad.png > good.png
13 * Fixes a PNG file written with libpng-1.6.0 or 1.6.1 that has one or more
14 * uncompressed iTXt chunks. Assumes that the actual length is greater
15 * than or equal to the value in the length byte, and that the CRC is
16 * correct for the actual length. This program hunts for the CRC and
17 * adjusts the length byte accordingly. It is not an error to process a
18 * PNG file that has no iTXt chunks or one that has valid iTXt chunks;
19 * such files will simply be copied.
21 * Requires zlib (for crc32 and Z_NULL); build with
23 * gcc -O -o png-fix-itxt png-fix-itxt.c -lz
25 * If you need to handle iTXt chunks larger than 500000 kbytes you must
26 * rebuild png-fix-itxt with a larger values of MAX_LENGTH (or a smaller value
27 * if you know you will never encounter such huge iTXt chunks).
33 #define MAX_LENGTH 500000
35 /* Read one character (inchar), also return octet (c), break if EOF */
36 #define GETBREAK inchar=getchar(); \
38 if (inchar != c) break
43 unsigned char buf[MAX_LENGTH];
48 /* Skip 8-byte signature */
55 if (inchar == c) /* !EOF */
59 unsigned long length; /* must be 32 bits! */
60 GETBREAK; buf[0] = c; length = c; length <<= 8;
61 GETBREAK; buf[1] = c; length += c; length <<= 8;
62 GETBREAK; buf[2] = c; length += c; length <<= 8;
63 GETBREAK; buf[3] = c; length += c;
65 /* Read the chunkname */
72 /* The iTXt chunk type expressed as integers is (105, 84, 88, 116) */
73 if (buf[4] == 105 && buf[5] == 84 && buf[6] == 88 && buf[7] == 116)
75 if (length >= MAX_LENGTH-12)
76 break; /* To do: handle this more gracefully */
78 /* Initialize the CRC */
79 crc = crc32(0, Z_NULL, 0);
81 /* Copy the data bytes */
82 for (i=8; i < length + 12; i++)
87 if (inchar != c) /* EOF */
90 /* Calculate the CRC */
91 crc = crc32(crc, buf+4, (uInt)length+4);
96 if (((crc >> 24) & 0xffU) == buf[length+8] &&
97 ((crc >> 16) & 0xffU) == buf[length+9] &&
98 ((crc >> 8) & 0xffU) == buf[length+10] &&
99 ((crc ) & 0xffU) == buf[length+11])
104 if (length >= MAX_LENGTH-12)
111 crc = crc32(crc, buf+7+length, 1);
114 if (inchar != c) /* EOF */
117 /* Update length bytes */
118 buf[0] = (unsigned char)((length >> 24) & 0xffU);
119 buf[1] = (unsigned char)((length >> 16) & 0xffU);
120 buf[2] = (unsigned char)((length >> 8) & 0xffU);
121 buf[3] = (unsigned char)((length ) & 0xffU);
123 /* Write the fixed iTXt chunk (length, name, data, crc) */
124 for (i=0; i<length+12; i++)
130 if (inchar != c) /* EOF */
133 /* Copy bytes that were already read (length and chunk name) */
137 /* Copy data bytes and CRC */
138 for (i=8; i< length+12; i++)
144 if (inchar != c) /* EOF */
149 /* The IEND chunk type expressed as integers is (73, 69, 78, 68) */
150 if (buf[4] == 73 && buf[5] == 69 && buf[6] == 78 && buf[7] == 68)
154 if (inchar != c) /* EOF */
157 if (buf[4] == 73 && buf[5] == 69 && buf[6] == 78 && buf[7] == 68)