Increase lookback length for PNG to improve compression

aaaaaa123456789 · Jan 24, 2022 · e3f47d1 · e3f47d1
1 parent 294423c
commit e3f47d1
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Showing 2 changed files with 2 additions and 2 deletions.
diff --git a/docs/changelog.md b/docs/changelog.md
@@ -17,7 +17,7 @@ Note: releases are listed from latest to oldest.
 - Added some warning flags for debug builds, and cleared some warnings that would be raised by them
 - Added and improved some safety checks that detect maliciously-crafted and other pathological files
 - Added detection for empty GIF files (erroring out with `PLUM_ERR_NO_DATA` instead of `PLUM_ERR_INVALID_FILE_FORMAT`)
-- Improved PNG compression by fixing a lookback bug in the compressor
+- Improved PNG compression by fixing a lookback bug in the compressor and adjusting the lookback length
 - Added and documented a restriction requiring `size_t` to be at least 32 bits wide
 - Some minor documentation updates and code cleanup
 

diff --git a/src/pngcompress.c b/src/pngcompress.c
@@ -1,6 +1,6 @@
 #include "proto.h"
 
-#define PNG_MAX_LOOKBACK_COUNT 16
+#define PNG_MAX_LOOKBACK_COUNT 64
 
 unsigned char * compress_PNG_data (struct context * context, const unsigned char * restrict data, size_t size, size_t extra, size_t * restrict output_size) {
   // extra is the number of zero bytes inserted before the compressed data; they are not included in the size