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improve pngsave compression for palette images 

set pngsave default filter to none for palette images, since they usually
have low local correlation
This commit is contained in:
John Cupitt 2021-02-14 11:29:22 +00:00
parent 8b791a072e
commit f27f681f1b
1 changed files with 9 additions and 1 deletions
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10
libvips/foreign/pngsave.c
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@ -136,6 +136,13 @@ vips_foreign_save_png_build( VipsObject *object )
if( !vips_object_argument_isset( object, "bitdepth" ) )
png->bitdepth = in->BandFmt == VIPS_FORMAT_UCHAR ? 8 : 16;
/* Filtering usually reduces the compression ratio for palette images,
* so default off.
*/
if( !vips_object_argument_isset( object, "filter" ) &&
png->palette )
png->filter = VIPS_FOREIGN_PNG_FILTER_NONE;
/* If this is a RGB or RGBA image and a low bit depth has been
* requested, enable palettization.
*/
@ -476,7 +483,8 @@ vips_foreign_save_png_buffer_init( VipsForeignSavePngBuffer *buffer )
* profile from the VIPS header will be attached.
*
* Use @filter to specify one or more filters (instead of adaptive filtering),
* see #VipsForeignPngFilter.
* see #VipsForeignPngFilter. @filter defaults to NONE for palette images,
* since they generally have very low local correlation.
*
* The image is automatically converted to RGB, RGBA, Monochrome or Mono +
* alpha before saving. Images with more than one byte per band element are