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revise cgifsave checksum

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We were calling it a sum and looking for a change threshold, but really
it's only useful as a checksum and does not have a magnitude. Revise
comments and variable names to reflect this.
This commit is contained in:
John Cupitt 2022-05-23 13:17:20 +01:00
parent 89bbb7981d
commit a9bfd508f5
1 changed files with 21 additions and 23 deletions
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44
libvips/foreign/cgifsave.c
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@ -76,10 +76,6 @@ typedef struct _VipsForeignSaveCgif {
int *global_colour_table;
int global_colour_table_length;
/* We save ->ready a frame at a time, regenerating the
* palette if we see a significant frame to frame change.
*/
/* The current frame coming from libvips, the y position we write to,
* and some spare pixels we copy down when we move to the next frame.
*/
@ -96,7 +92,7 @@ typedef struct _VipsForeignSaveCgif {
/* The current colourmap, updated on a significant frame change.
*/
VipsPel *palette_rgb;
gint64 frame_sum;
gint64 frame_checksum;
/* The index frame we get libimagequant to generate.
*/
@ -222,10 +218,8 @@ vips_foreign_save_cgif_write_frame( VipsForeignSaveCgif *cgif )
VIPS_REGION_ADDR( cgif->frame, 0, frame_rect->top );
VipsPel * restrict p;
VipsPel *rgb;
gint64 sum;
double change;
int i;
VipsPel *rgb;
CGIF_FrameConfig frame_config;
#ifdef DEBUG_VERBOSE
@ -250,27 +244,31 @@ vips_foreign_save_cgif_write_frame( VipsForeignSaveCgif *cgif )
/* Do we need to compute a new palette? Do it if the frame sum
* changes.
*
* frame_sum 0 means no current colourmap.
* frame_checksum 0 means no current colourmap.
*/
if( !cgif->global_colour_table ) {
sum = 0;
gint64 checksum;
/* We need a checksum which detects colour changes, but
* doesn't care about pixel ordering.
*
* Scale RGBA differently so that changes like [0, 255, 0]
* to [255, 0, 0] are detected.
*/
checksum = 0;
p = frame_bytes;
for( i = 0; i < n_pels; i++ ) {
/* Scale RGBA differently so that changes like
* [0, 255, 0] to [255, 0, 0] are detected.
*/
sum += p[0] * 1000;
sum += p[1] * 100;
sum += p[2] * 10;
sum += p[3];
checksum += p[0] * 1000;
checksum += p[1] * 100;
checksum += p[2] * 10;
checksum += p[3];
p += 4;
}
change = VIPS_ABS( ((double) sum - cgif->frame_sum) ) / n_pels;
if( cgif->frame_sum == 0 ||
change > 0 ) {
cgif->frame_sum = sum;
if( cgif->frame_checksum == 0 ||
checksum != cgif->frame_checksum ) {
cgif->frame_checksum = checksum;
/* If this is not our first cmap, make a note that we
* need to attach it as a local cmap when we write.
@ -292,8 +290,8 @@ vips_foreign_save_cgif_write_frame( VipsForeignSaveCgif *cgif )
cgif->n_cmaps_generated += 1;
cgif->lp = vips__quantise_get_palette(
cgif->quantisation_result );
printf( "frame %d, change %g, new %d item colourmap\n",
page_index, change, cgif->lp->count );
printf( "frame %d, new %d item colourmap\n",
page_index, cgif->lp->count );
#endif/*DEBUG_PERCENT*/
}
}