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fix atan2 lut in canny

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This commit is contained in:
John Cupitt 2018-03-16 16:29:52 +00:00
parent ae82877e66
commit 7328f47333
1 changed files with 40 additions and 32 deletions
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72
libvips/convolution/canny.c
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@ -32,12 +32,11 @@
*/
/* TODO
* - does it actually help much?
* - support other image types
* - swap atan2 for a LUT with perhaps +/- 2 or 4 bits
* - add autothreshold with otsu's method
* - leave blob analysis to a separate pass
* - add sobel option?
* - support other image types
*
* - leave blob analysis to a separate pass
*/
/*
@ -55,10 +54,6 @@
#include <vips/vips.h>
/* LUT for calculating atan2() with +/- 4 bits of precision in each axis.
*/
static VipsPel vips__atan2[256];
typedef struct _VipsCanny {
VipsOperation parent_instance;
@ -117,6 +112,10 @@ vips_canny_gradient_simple( VipsImage *in, VipsImage **Gx, VipsImage **Gy )
return( 0 );
}
/* LUT for calculating atan2() with +/- 4 bits of precision in each axis.
*/
static VipsPel vips_canny_polar_atan2[256];
static int
vips_canny_polar_generate( VipsRegion *or,
void *vseq, void *a, void *b, gboolean *stop )
@ -142,7 +141,11 @@ vips_canny_polar_generate( VipsRegion *or,
for( band = 0; band < Gx->Bands; band++ ) {
int gx = p1[band] - 128;
int gy = p2[band] - 128;
int i = ((gx >> 4) | (gy & 0xf0)) & 0xff;
/* gx/gy are -128 to +127, we need -8 to +7
* for the atan2 LUT.
*/
int i = ((gx >> 4) & 0xf) | (gy & 0xf0);
/* We should calculate
* 0.5 * sqrt( x * x + y * y )
@ -150,9 +153,12 @@ vips_canny_polar_generate( VipsRegion *or,
* clipping. We are only interested in relative
* magnitude, so we can skip the sqrt and just
* shift down 9 bits.
*
* Try "256 * (VIPS_DEG( atan2( gx, gy ) ) +
* 360) / 360;" to test LUT.
*/
q[0] = (gx * gx + gy * gy + 256) >> 9;
q[1] = vips__atan2[i];
q[1] = vips_canny_polar_atan2[i];
q += 2;
}
@ -165,6 +171,26 @@ vips_canny_polar_generate( VipsRegion *or,
return( 0 );
}
static void *
vips_atan2_init( void *null )
{
int i;
for( i = 0; i < 256; i++ ) {
/* Use the bottom 4 bits for x, the top 4 for y. The double
* shift does sign extension, assuming 2s complement.
*/
int bits = sizeof( int ) * 8 - 4;
int x = ((i & 0xf) << bits) >> bits;
int y = ((i >> 4) & 0x0f) << bits >> bits;
double theta = VIPS_DEG( atan2( x, y ) ) + 360;
vips_canny_polar_atan2[i] = 256 * theta / 360;
}
return( NULL );
}
/* Calculate G/theta from Gx/Gy -- rather like rect -> polar, except that we
* code theta as below. Scale G down by 0.5 so that we don't clip on hard
* edges.
@ -175,6 +201,10 @@ vips_canny_polar_generate( VipsRegion *or,
static int
vips_canny_polar( VipsImage **args, VipsImage **out )
{
static GOnce once = G_ONCE_INIT;
g_once( &once, (GThreadFunc) vips_atan2_init, NULL );
*out = vips_image_new();
if( vips_image_pipeline_array( *out,
VIPS_DEMAND_STYLE_THINSTRIP, args ) )
@ -486,30 +516,10 @@ vips_canny_thresh( VipsCanny *canny, VipsImage *in, VipsImage **out )
return( 0 );
}
static void *
vips_atan2_init( void *null )
{
int i;
for( i = 0; i < 256; i++ ) {
/* Use the bottom 4 bits for x, the top 4 for y.
*/
int x = (i & 0xf) - 8;
int y = ((i >> 4) & 0xf) - 8;
double theta = VIPS_DEG( atan2( x, y ) ) + 360;
vips__atan2[i] = 256 * theta / 360;
}
return( NULL );
}
static int
vips_canny_build( VipsObject *object )
{
static GOnce once = G_ONCE_INIT;
VipsCanny *canny = (VipsCanny *) object;
VipsImage **t = (VipsImage **) vips_object_local_array( object, 20 );
@ -517,8 +527,6 @@ vips_canny_build( VipsObject *object )
VipsImage *Gx;
VipsImage *Gy;
g_once( &once, (GThreadFunc) vips_atan2_init, NULL );
if( VIPS_OBJECT_CLASS( vips_canny_parent_class )->build( object ) )
return( -1 );