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get correct ::distance() and ::delta() implementations for cases involving achromatics

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This commit is contained in:
Paul Davis 2014-11-20 14:01:41 -05:00
parent 137425faf1
commit 6edd81e4f6
1 changed files with 34 additions and 4 deletions
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38
libs/canvas/colors.cc
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@ -19,6 +19,7 @@
#include <algorithm>
#include <cmath>
#include <stdint.h>
#include <cfloat>
#include "canvas/colors.h"
#include "canvas/colorspace.h"
@ -365,9 +366,16 @@ HSV
HSV::delta (const HSV& other) const
{
HSV d;
d.h = h - other.h;
d.s = s - other.s;
d.v = v - other.v;
if (is_gray() && other.is_gray()) {
d.h = 0.0;
d.s = 0.0;
d.v = v - other.v;
} else {
d.h = h - other.h;
d.s = s - other.s;
d.v = v - other.v;
}
/* do not clamp - we are returning a delta */
return d;
}
@ -375,6 +383,28 @@ HSV::delta (const HSV& other) const
double
HSV::distance (const HSV& other) const
{
if (is_gray() && other.is_gray()) {
/* human color perception of achromatics generates about 450
distinct colors. By contrast, CIE94 could give a maximal
perceptual distance of sqrt ((360^2) + 1 + 1) = 360. The 450
are not evenly spread (Webers Law), so lets use 360 as an
approximation of the number of distinct achromatics.
So, scale up the achromatic difference to give about
a maximal distance between v = 1.0 and v = 0.0 of 360.
A difference of about 0.0055 will generate a return value of
2, which is roughly the limit of human perceptual
discrimination for chromatics.
*/
return fabs (360.0 * (v - other.v));
}
if (is_gray() != other.is_gray()) {
/* no comparison possible */
return DBL_MAX;
}
/* Use CIE94 definition for now */
double sL, sA, sB;
@ -454,7 +484,7 @@ void
HSV::print (std::ostream& o) const
{
if (!is_gray()) {
o << '(' << s << ',' << v << ',' << a << ')';
o << '(' << h << ',' << s << ',' << v << ',' << a << ')';
} else {
o << "gray(" << v << ')';
}