diff options
Diffstat (limited to 'jutils.c')
| -rw-r--r-- | jutils.c | 52 |
1 files changed, 48 insertions, 4 deletions
@@ -1,12 +1,12 @@ /* * jutils.c * - * Copyright (C) 1991-1994, Thomas G. Lane. + * Copyright (C) 1991-1995, 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. * - * This file contains miscellaneous utility routines needed for both - * compression and decompression. + * This file contains tables and miscellaneous utility routines needed + * for both compression and decompression. * Note we prefix all global names with "j" to minimize conflicts with * a surrounding application. */ @@ -17,6 +17,50 @@ /* + * jpeg_zigzag_order[i] is the zigzag-order position of the i'th element + * of a DCT block read in natural order (left to right, top to bottom). + */ + +const int jpeg_zigzag_order[DCTSIZE2] = { + 0, 1, 5, 6, 14, 15, 27, 28, + 2, 4, 7, 13, 16, 26, 29, 42, + 3, 8, 12, 17, 25, 30, 41, 43, + 9, 11, 18, 24, 31, 40, 44, 53, + 10, 19, 23, 32, 39, 45, 52, 54, + 20, 22, 33, 38, 46, 51, 55, 60, + 21, 34, 37, 47, 50, 56, 59, 61, + 35, 36, 48, 49, 57, 58, 62, 63 +}; + +/* + * jpeg_natural_order[i] is the natural-order position of the i'th element + * of zigzag order. + * + * When reading corrupted data, the Huffman decoders could attempt + * to reference an entry beyond the end of this array (if the decoded + * zero run length reaches past the end of the block). To prevent + * wild stores without adding an inner-loop test, we put some extra + * "63"s after the real entries. This will cause the extra coefficient + * to be stored in location 63 of the block, not somewhere random. + * The worst case would be a run-length of 15, which means we need 16 + * fake entries. + */ + +const int jpeg_natural_order[DCTSIZE2+16] = { + 0, 1, 8, 16, 9, 2, 3, 10, + 17, 24, 32, 25, 18, 11, 4, 5, + 12, 19, 26, 33, 40, 48, 41, 34, + 27, 20, 13, 6, 7, 14, 21, 28, + 35, 42, 49, 56, 57, 50, 43, 36, + 29, 22, 15, 23, 30, 37, 44, 51, + 58, 59, 52, 45, 38, 31, 39, 46, + 53, 60, 61, 54, 47, 55, 62, 63, + 63, 63, 63, 63, 63, 63, 63, 63, /* extra entries for safety in decoder */ + 63, 63, 63, 63, 63, 63, 63, 63 +}; + + +/* * Arithmetic utilities */ @@ -116,7 +160,7 @@ jcopy_block_row (JBLOCKROW input_row, JBLOCKROW output_row, GLOBAL void jzero_far (void FAR * target, size_t bytestozero) /* Zero out a chunk of FAR memory. */ -/* This might be sample-array data, block-array data, or alloc_medium data. */ +/* This might be sample-array data, block-array data, or alloc_large data. */ { #ifdef FMEMZERO FMEMZERO(target, bytestozero); |