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revise unpremultiply, again

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We were not detecting division by zero carefully enough, nor clipping
the alpha range sufficiently in unpremultiply.

see https://github.com/libvips/libvips/issues/1941

also see https://github.com/libvips/libvips/pull/1675 for another
difficult test case
This commit is contained in:
John Cupitt 2021-02-27 15:16:25 +00:00
parent 3c60e9de60
commit d3ccadf212
3 changed files with 28 additions and 25 deletions
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1
ChangeLog
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@ -7,6 +7,7 @@
- fix includes of glib headers in C++ [lovell]
- fix build with more modern librsvg [lovell]
- fix a possible segv with very wide images [f1ac]
- revise premultiply, again [jjonesrs]
18/12/20 started 8.10.5
- fix potential /0 in animated webp load [lovell]

38
libvips/conversion/unpremultiply.c
View File

@ -7,6 +7,8 @@
* - max_alpha defaults to 65535 for RGB16/GREY16
* 24/11/17 lovell
* - match normalised alpha to output type
* 27/2/21 jjonesrs
* - revise range clipping and 1/x, again
*/
/*
@ -78,18 +80,12 @@ G_DEFINE_TYPE( VipsUnpremultiply, vips_unpremultiply, VIPS_TYPE_CONVERSION );
\
for( x = 0; x < width; x++ ) { \
IN alpha = p[alpha_band]; \
IN clip_alpha = VIPS_CLIP( 0.0, alpha, max_alpha ); \
OUT factor = max_alpha * vips_recip( clip_alpha ); \
\
if( alpha != 0 ) { \
OUT factor = max_alpha / alpha; \
\
for( i = 0; i < alpha_band; i++ ) \
q[i] = factor * p[i]; \
q[alpha_band] = alpha; \
} \
else \
for( i = 0; i < alpha_band + 1; i++ ) \
q[i] = 0; \
\
for( i = 0; i < alpha_band; i++ ) \
q[i] = factor * p[i]; \
q[alpha_band] = clip_alpha; \
for( i = alpha_band + 1; i < bands; i++ ) \
q[i] = p[i]; \
\
@ -106,21 +102,13 @@ G_DEFINE_TYPE( VipsUnpremultiply, vips_unpremultiply, VIPS_TYPE_CONVERSION );
\
for( x = 0; x < width; x++ ) { \
IN alpha = p[3]; \
IN clip_alpha = VIPS_CLIP( 0.0, alpha, max_alpha ); \
OUT factor = max_alpha * vips_recip( clip_alpha ); \
\
if( alpha != 0 ) { \
OUT factor = max_alpha / alpha; \
\
q[0] = factor * p[0]; \
q[1] = factor * p[1]; \
q[2] = factor * p[2]; \
q[3] = alpha; \
} \
else { \
q[0] = 0; \
q[1] = 0; \
q[2] = 0; \
q[3] = 0; \
} \
q[0] = factor * p[0]; \
q[1] = factor * p[1]; \
q[2] = factor * p[2]; \
q[3] = clip_alpha; \
\
p += 4; \
q += 4; \

14
libvips/include/vips/util.h
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@ -84,6 +84,20 @@ extern "C" {
#define VIPS_FMIN( A, B ) VIPS_MIN( A, B )
#endif
/* Calculate 1.0 / x, avoiding division by zero when near zero.
*/
static inline double
vips_recip( double x )
{
static const double epsilon = 1e-10;
int sign = x < 0.0 ? -1.0 : 1.0;
return( sign * x >= epsilon ?
1.0 / x :
sign / epsilon );
}
/* Testing status before the function call saves a lot of time.
*/
#define VIPS_ONCE( ONCE, FUNC, CLIENT ) \