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try a LUT for atan2

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This commit is contained in:
John Cupitt 2018-03-14 17:19:17 +00:00
parent 754a1e5419
commit ae82877e66
1 changed files with 75 additions and 13 deletions
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88
libvips/convolution/canny.c
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@ -31,17 +31,17 @@
*/
/*
#define DEBUG
*/
/* TODO
* - verify that our interpolating max edge works
* - 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?
*/
/*
#define DEBUG
*/
#ifdef HAVE_CONFIG_H
@ -55,6 +55,10 @@
#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;
@ -136,9 +140,9 @@ vips_canny_polar_generate( VipsRegion *or,
for( x = 0; x < r->width; x++ ) {
for( band = 0; band < Gx->Bands; band++ ) {
int x = p1[band] - 128;
int y = p2[band] - 128;
int a = VIPS_DEG( atan2( x, y ) ) + 360;
int gx = p1[band] - 128;
int gy = p2[band] - 128;
int i = ((gx >> 4) | (gy & 0xf0)) & 0xff;
/* We should calculate
* 0.5 * sqrt( x * x + y * y )
@ -147,8 +151,8 @@ vips_canny_polar_generate( VipsRegion *or,
* magnitude, so we can skip the sqrt and just
* shift down 9 bits.
*/
q[0] = (x * x + y * y + 256) >> 9;
q[1] = 256 * a / 360;
q[0] = (gx * gx + gy * gy + 256) >> 9;
q[1] = vips__atan2[i];
q += 2;
}
@ -162,8 +166,8 @@ vips_canny_polar_generate( VipsRegion *or,
}
/* 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.
* code theta as below. Scale G down by 0.5 so that we don't clip on hard
* edges.
*
* For a white disc on a black background, theta is 0 at the bottom, 64 on the
* right, 128 at the top and 192 on the left edge.
@ -349,7 +353,42 @@ vips_canny_nonmaxi_generate( VipsRegion *or,
int highb = p[offset[(high_theta + 4) & 0x7]];
int high = (higha * (32 - residual) +
highb * residual) / 32;
#ifdef DEBUG
if( G > 0 ) {
int i, j;
printf( "Gradient:\n" );
for( j = 0; j < 3; j++ ) {
for( i = 0; i < 3; i++ )
printf( "%3d ", p[j * lsk + i * psk] );
printf("\n");
}
printf( "Theta:\n" );
for( j = 0; j < 3; j++ ) {
for( i = 0; i < 3; i++ )
printf( "%3d ", p[j * lsk + i * psk + 1] );
printf("\n");
}
printf( "G = %d\n", G );
printf( "theta = %d\n", theta );
printf( "low_theta = %d\n", low_theta );
printf( "high_theta = %d\n", high_theta );
printf( "residual = %d\n", residual );
printf( "lowa = %d\n", lowa );
printf( "lowb = %d\n", lowb );
printf( "low = %d\n", low );
printf( "higha = %d\n", higha );
printf( "highb = %d\n", highb );
printf( "high = %d\n", high );
if( G <= low || G < high )
printf( "thin\n" );
printf( "\n" );
}
#endif /*DEBUG*/
/* Set G to 0 if it's not the local maxima in
* the direction of the gradient.
*/
@ -447,9 +486,30 @@ 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 );
@ -457,6 +517,8 @@ 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 );