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revise kernel mask calculations

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We were seeing small displacements at some shrink factors because of
rounding in the mask size calcualation. This PR takes the rounding into
account when positioning the mask.

See https://github.com/libvips/libvips/pull/1592#issuecomment-626363031
This commit is contained in:
John Cupitt 2020-05-10 18:45:27 +01:00
parent b9eddecca3
commit d64385ae99
2 changed files with 13 additions and 10 deletions
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8
libvips/resample/reduceh.cpp
View File

@ -101,19 +101,19 @@ vips_reduce_get_points( VipsKernel kernel, double shrink )
return( 1 ); return( 1 );
case VIPS_KERNEL_LINEAR: case VIPS_KERNEL_LINEAR:
return( rint( 2 * shrink ) + 1 ); return( 2 * rint( shrink ) + 1 );
case VIPS_KERNEL_CUBIC: case VIPS_KERNEL_CUBIC:
case VIPS_KERNEL_MITCHELL: case VIPS_KERNEL_MITCHELL:
return( rint( 4 * shrink ) + 1 ); return( 2 * rint( 2 * shrink ) + 1 );
case VIPS_KERNEL_LANCZOS2: case VIPS_KERNEL_LANCZOS2:
/* Needs to be in sync with calculate_coefficients_lanczos(). /* Needs to be in sync with calculate_coefficients_lanczos().
*/ */
return( rint( 2 * 2 * shrink ) + 1 ); return( 2 * rint( 2 * shrink ) + 1 );
case VIPS_KERNEL_LANCZOS3: case VIPS_KERNEL_LANCZOS3:
return( rint( 2 * 3 * shrink ) + 1 ); return( 2 * rint( 3 * shrink ) + 1 );
default: default:
g_assert_not_reached(); g_assert_not_reached();

15
libvips/resample/templates.h
View File

@ -321,14 +321,15 @@ calculate_coefficients_triangle( double *c,
{ {
/* Needs to be in sync with vips_reduce_get_points(). /* Needs to be in sync with vips_reduce_get_points().
*/ */
const int n_points = rint( 2 * shrink ) + 1; const int n_points = 2 * rint( shrink ) + 1;
const double half = x + n_points / 2.0 - 1;
int i; int i;
double sum; double sum;
sum = 0; sum = 0;
for( i = 0; i < n_points; i++ ) { for( i = 0; i < n_points; i++ ) {
double xp = (i - (shrink - 0.5) - x) / shrink; const double xp = (i - half) / shrink;
double l; double l;
@ -358,14 +359,15 @@ calculate_coefficients_cubic( double *c,
{ {
/* Needs to be in sync with vips_reduce_get_points(). /* Needs to be in sync with vips_reduce_get_points().
*/ */
const int n_points = rint( 4 * shrink ) + 1; const int n_points = 2 * rint( 2 * shrink ) + 1;
const double half = x + n_points / 2.0 - 1;
int i; int i;
double sum; double sum;
sum = 0; sum = 0;
for( i = 0; i < n_points; i++ ) { for( i = 0; i < n_points; i++ ) {
const double xp = (i - (2 * shrink - 1) - x) / shrink; const double xp = (i - half) / shrink;
const double axp = VIPS_FABS( xp ); const double axp = VIPS_FABS( xp );
const double axp2 = axp * axp; const double axp2 = axp * axp;
const double axp3 = axp2 * axp; const double axp3 = axp2 * axp;
@ -406,14 +408,15 @@ calculate_coefficients_lanczos( double *c,
{ {
/* Needs to be in sync with vips_reduce_get_points(). /* Needs to be in sync with vips_reduce_get_points().
*/ */
const int n_points = rint( 2 * a * shrink ) + 1; const int n_points = 2 * rint( a * shrink ) + 1;
const double half = x + n_points / 2.0 - 1;
int i; int i;
double sum; double sum;
sum = 0; sum = 0;
for( i = 0; i < n_points; i++ ) { for( i = 0; i < n_points; i++ ) {
double xp = (i - (n_points - 2) / 2 - x) / shrink; double xp = (i - half) / shrink;
double l; double l;