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cgifsave: allow to apply maxerror optimization to frames with alpha channel #2778

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Merged
merged 1 commit into from
May 7, 2022
Merged

cgifsave: allow to apply maxerror optimization to frames with alpha channel #2778

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46 changes: 40 additions & 6 deletions libvips/foreign/cgifsave.c
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Original file line number Diff line number Diff line change
Expand Up @@ -166,10 +166,14 @@ static gboolean
vips_foreign_save_cgif_pixels_are_equal( const VipsPel *cur, const VipsPel *bef,
double maxerror )
{
if( bef[3] != 0xFF )
/* Solid pixels only.
if( bef[3] != cur[3] )
/* Alpha channel must be identical.
*/
return FALSE;
if( bef[3] == 0 && cur[3] == 0)
/* RGB component can be ignored.
*/
return TRUE;

/* Test Euclidean distance between the two points.
*/
Expand All @@ -180,6 +184,25 @@ vips_foreign_save_cgif_pixels_are_equal( const VipsPel *cur, const VipsPel *bef,
return( sqrt( dR * dR + dG * dG + dB * dB ) <= maxerror );
}

/* Check if the alpha channel of the current frame matches the frame before.
* If the current frame has an alpha component which is not identical to the
* frame from before we are forced to use the transparency index for the
* alpha channel instead of for the transparency size optimization (maxerror).
*/
static gboolean
vips_foreign_save_cgif_check_alpha_constraint( const VipsPel *cur,
const VipsPel *bef, int n_pels )
{
while( n_pels-- ) {
if( cur[3] == 0 && bef[3] != 0) {
return TRUE;
}
cur += 4;
bef += 4;
}
return FALSE;
}

/* We have a complete frame --- write!
*/
static int
Expand Down Expand Up @@ -355,13 +378,24 @@ vips_foreign_save_cgif_write_frame( VipsForeignSaveCgif *cgif )
*/
if( cgif->frame_bytes_head ) {
VipsPel *cur, *bef;
gboolean has_alpha_constraint;

cur = frame_bytes;
bef = cgif->frame_bytes_head;
if( !cgif->has_transparency ) {
const uint8_t trans_index = cgif->lp->count;

has_alpha_constraint =
vips_foreign_save_cgif_check_alpha_constraint(cur, bef, n_pels);
/* Transparency trick is only possible
* when no alpha channel constraint is present.
*/
if( !has_alpha_constraint ) {
int i;
uint8_t trans_index;

trans_index = cgif->lp->count;
if( cgif->has_transparency ) {
trans_index = 0;
frame_config.attrFlags &= (~CGIF_FRAME_ATTR_HAS_ALPHA);
}

for( i = 0; i < n_pels; i++ ) {
if( vips_foreign_save_cgif_pixels_are_equal(
Expand All @@ -383,7 +417,7 @@ vips_foreign_save_cgif_write_frame( VipsForeignSaveCgif *cgif )
frame_config.transIndex = trans_index;
}
else {
/* Transparency trick not possible (alpha channel
/* Transparency trick not possible (constraining alpha channel
* present). Update head.
*/
memcpy( bef, cur, 4 * n_pels);
Expand Down