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handle transparency in vips_region_shrink()

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we weren't multiplying through alpha in vips_region_shrink(), causing
fringing on pyramids with transparent elements

thanks sickrandir!

see https://github.com/jcupitt/libvips/issues/599
This commit is contained in:
John Cupitt 2017-02-23 17:04:25 +00:00
parent f84825a634
commit 4bbd56fb0a
2 changed files with 91 additions and 2 deletions
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1
ChangeLog
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@ -31,6 +31,7 @@
- vips_gaussnoise() pixels are reproducible on recalc, thanks MvGulik
- max/min sort values by y and x coordinate
- tiff read uses libtiff scanline API if rows-per-strip is large
- vips_region_shrink() knows about alpha, helps dzsave and tiffsave
8/12/16 started 8.4.5
- allow libgsf-1.14.26 to help centos, thanks tdiprima

92
libvips/iofuncs/region.c
View File

@ -41,6 +41,8 @@
* - move invalid stuff to region
* 3/3/11
* - move on top of VipsObject, rename as VipsRegion
* 23/2/17
* - multiply transparent images through alpha in vips_region_shrink()
*/
/*
@ -1236,6 +1238,88 @@ vips_region_shrink_uncoded( VipsRegion *from, VipsRegion *to, VipsRect *target )
}
}
/* No point having an int path, this will always be horribly slow.
*/
#define SHRINK_ALPHA_TYPE( TYPE ) { \
TYPE *tp = (TYPE *) p; \
TYPE *tp1 = (TYPE *) (p + ls); \
TYPE *tq = (TYPE *) q; \
\
for( x = 0; x < target->width; x++ ) { \
/* Make the input alphas. \
*/ \
double a1 = tp[nb - 1]; \
double a2 = tp[nb + nb - 1]; \
double a3 = tp1[nb - 1]; \
double a4 = tp1[nb + nb - 1]; \
\
/* Output alpha. \
*/ \
double a = (a1 + a2 + a3 + a4) / 4.0; \
\
if( a == 0 ) { \
for( z = 0; z < nb; z++ ) \
tq[z] = 0; \
} \
else { \
for( z = 0; z < nb - 1; z++ ) \
tq[z] = (a1 * tp[z] + a2 * tp[z + nb] + \
a3 * tp1[z] + a4 * tp1[z + nb]) / \
(4.0 * a); \
tq[z] = a; \
} \
\
/* Move on two pels in input. \
*/ \
tp += nb << 1; \
tp1 += nb << 1; \
tq += nb; \
} \
}
/* Generate area @target in @to using pixels in @from. Non-complex. Use the
* last band as alpha.
*/
static void
vips_region_shrink_alpha( VipsRegion *from, VipsRegion *to, VipsRect *target )
{
int ls = VIPS_REGION_LSKIP( from );
int nb = from->im->Bands;
int x, y, z;
for( y = 0; y < target->height; y++ ) {
VipsPel *p = VIPS_REGION_ADDR( from,
target->left * 2, (target->top + y) * 2 );
VipsPel *q = VIPS_REGION_ADDR( to,
target->left, target->top + y );
/* Process this line of pels.
*/
switch( from->im->BandFmt ) {
case VIPS_FORMAT_UCHAR:
SHRINK_ALPHA_TYPE( unsigned char ); break;
case VIPS_FORMAT_CHAR:
SHRINK_ALPHA_TYPE( signed char ); break;
case VIPS_FORMAT_USHORT:
SHRINK_ALPHA_TYPE( unsigned short ); break;
case VIPS_FORMAT_SHORT:
SHRINK_ALPHA_TYPE( signed short ); break;
case VIPS_FORMAT_UINT:
SHRINK_ALPHA_TYPE( unsigned int ); break;
case VIPS_FORMAT_INT:
SHRINK_ALPHA_TYPE( signed int ); break;
case VIPS_FORMAT_FLOAT:
SHRINK_ALPHA_TYPE( float ); break;
case VIPS_FORMAT_DOUBLE:
SHRINK_ALPHA_TYPE( double ); break;
default:
g_assert_not_reached();
}
}
}
/**
* vips_region_shrink:
* @from: source region
@ -1243,7 +1327,8 @@ vips_region_shrink_uncoded( VipsRegion *from, VipsRegion *to, VipsRect *target )
* @target: #VipsRect of pixels you need to copy
*
* Write the pixels @target in @to from the x2 larger area in @from.
* Non-complex uncoded images and LABQ only.
* Non-complex uncoded images and LABQ only. Images with alpha (see
* vips_image_hasalpha()) shrink with pixels scaled by alpha to avoid fringing.
*
* See also: vips_region_copy().
*/
@ -1259,7 +1344,10 @@ vips_region_shrink( VipsRegion *from, VipsRegion *to, VipsRect *target )
if( vips_check_noncomplex( "vips_region_shrink", image ) )
return( -1 );
vips_region_shrink_uncoded( from, to, target );
if( vips_image_hasalpha( image ) )
vips_region_shrink_alpha( from, to, target );
else
vips_region_shrink_uncoded( from, to, target );
}
else
vips_region_shrink_labpack( from, to, target );