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better 0 detection in unpremultiply

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We were avoiding /0 by testing for alpha==0, however, this will still
allow very small values of alpha to generate +/- Inf. Instead, check for
abs(alpha)<epsilon.

Fixes some artifacts after unpremul.
This commit is contained in:
John Cupitt 2022-08-08 16:16:10 +01:00
parent 2c4c039056
commit 29e80ca43f
2 changed files with 54 additions and 2 deletions
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1
ChangeLog
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@ -3,6 +3,7 @@
- add a summary table at the end of configure in meson
- fix libpng fallback when spng is disabled in meson
- add "unlimited" to jpegload
- better 0 detection in unpremultiply
21/11/21 started 8.13
- configure fails for requested but unmet dependencies [remicollet]

55
libvips/conversion/unpremultiply.c
View File

@ -9,6 +9,8 @@
* - match normalised alpha to output type
* 27/2/21 jjonesrs
* - revise range clipping and 1/x, again
* 8/8/22
* - look for alpha near 0, not just exactly 0
*/
/*
@ -123,6 +125,55 @@ G_DEFINE_TYPE( VipsUnpremultiply, vips_unpremultiply, VIPS_TYPE_CONVERSION );
} \
}
/* For float-style images, we need to check for alpha near zero, or we'll get
* +/- Inf in the output.
*/
#define FUNPRE_MANY( IN, OUT ) { \
IN * restrict p = (IN *) in; \
OUT * restrict q = (OUT *) out; \
\
for( x = 0; x < width; x++ ) { \
IN alpha = p[alpha_band]; \
OUT factor = VIPS_ABS( alpha ) < 0.01 ? 0 : max_alpha / alpha; \
\
for( i = 0; i < alpha_band; i++ ) \
q[i] = factor * p[i]; \
q[alpha_band] = VIPS_CLIP( 0, alpha, max_alpha ); \
for( i = alpha_band + 1; i < bands; i++ ) \
q[i] = p[i]; \
\
p += bands; \
q += bands; \
} \
}
#define FUNPRE_RGBA( IN, OUT ) { \
IN * restrict p = (IN *) in; \
OUT * restrict q = (OUT *) out; \
\
for( x = 0; x < width; x++ ) { \
IN alpha = p[3]; \
OUT factor = VIPS_ABS( alpha ) < 0.01 ? 0 : max_alpha / alpha; \
\
q[0] = factor * p[0]; \
q[1] = factor * p[1]; \
q[2] = factor * p[2]; \
q[3] = VIPS_CLIP( 0, alpha, max_alpha ); \
\
p += 4; \
q += 4; \
} \
}
#define FUNPRE( IN, OUT ) { \
if( bands == 4 ) { \
FUNPRE_RGBA( IN, OUT ); \
} \
else { \
FUNPRE_MANY( IN, OUT ); \
} \
}
static int
vips_unpremultiply_gen( VipsRegion *or, void *vseq, void *a, void *b,
gboolean *stop )
@ -171,11 +222,11 @@ vips_unpremultiply_gen( VipsRegion *or, void *vseq, void *a, void *b,
break;
case VIPS_FORMAT_FLOAT:
UNPRE( float, float );
FUNPRE( float, float );
break;
case VIPS_FORMAT_DOUBLE:
UNPRE( double, double );
FUNPRE( double, double );
break;
case VIPS_FORMAT_COMPLEX: