dnrops
/
gimp
mirror of https://github.com/GNOME/gimp.git
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

libgimpcolor: implement CIEDE2000 distance algorithm to compare colors. 

This is used for the gimp_color_is_perceptually_identical() function,
because the Euclidean distance in LCH is extremely limited, if not wrong
in many cases. Indeed LCH is not perfectly perceptually uniform, and for
this exact reason, the CIE defined the specific Delta E algorithms.
Later versions are also based on LCH values, so my intuition to use it
for distance was on a good start, yet these algorithms were refined a
few times to allow for corrections in perceptual uniformity
imperfections.

This was in particular needed to verify if a color is out of a CMYK
space gamut. The idea is to compare the distance of the RGB (or other)
and the CMYK version, since we cannot just check if the CMYK color is
out of the [0; 1] range (it never is). Instead if both colors are
perceptually identical, then we consider that the RGB color was inside
the CMYK space gamut.
The naive algorithm was giving any (or nearly) color as out-of-gamut.
Now using CIEDE2000, I get a much nicer results.
This commit is contained in:
Jehan 2023-12-19 22:59:01 +09:00
parent 4d2acac506
commit 12b2abce7c
3 changed files with 98 additions and 20 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

114
libgimpcolor/gimpcolor.c
View File

@ -23,6 +23,7 @@
#include <cairo.h>
#include <gdk-pixbuf/gdk-pixbuf.h>
#include <math.h>
#include <babl/babl.h>
#include <gegl.h>
@ -41,6 +42,8 @@
static const Babl * gimp_babl_format_get_with_alpha (const Babl *format);
static gfloat gimp_color_get_CIE2000_distance (GeglColor *color1,
GeglColor *color2);
/**
@ -119,29 +122,16 @@ gboolean
gimp_color_is_perceptually_identical (GeglColor *color1,
GeglColor *color2)
{
gfloat pixel1[4];
gfloat pixel2[4];
g_return_val_if_fail (GEGL_IS_COLOR (color1), FALSE);
g_return_val_if_fail (GEGL_IS_COLOR (color2), FALSE);
/* CIE LCh space is considered quite perceptually uniform, a bit better than
* Lab on this aspect, AFAIU.
/* All CIE deltaE distances were designed with a 1.0 JND (Just Noticeable
* Difference), though there was some revision to 2.3 for the CIE76 version.
* I could not find a reliable source about whether such a revision happened
* for the CIE2000 algorithm. My own tests though seemed to lean towards
* using ~0.6 for the JND. That's what I'm using for the time being.
*/
gegl_color_get_pixel (color1, babl_format ("CIE LCH(ab) alpha float"), pixel1);
gegl_color_get_pixel (color2, babl_format ("CIE LCH(ab) alpha float"), pixel2);
/* This is not a proper distance computation, but is acceptable for our use
* case while being simpler. While we used to use 1e-6 as threshold with float
* RGB, LCh is not in [0, 1] range, and some channels can reach over 300 with
* wide gamut spaces. This is why use the threshold is 1e-4 right now.
*/
#define SQR(x) ((x) * (x))
return (ABS (pixel1[3] - pixel2[3]) <= 1e-4 &&
(SQR (pixel1[0] - pixel2[0]) +
SQR (pixel1[1] - pixel2[1]) +
SQR (pixel1[2] - pixel2[2]) <= 1e-4));
#undef SQR
return (gimp_color_get_CIE2000_distance (color1, color2) < 0.6);
}
/**
@ -398,3 +388,89 @@ gimp_babl_format_get_with_alpha (const Babl *format)
return new_format;
}
/**
* gimp_color_get_CIE2000_distance:
* @color1: a [class@Gegl.Color]
* @color2: a [class@Gegl.Color]
*
* Compute the CIEDE2000 distance between @color1 and @color2 which tries to
* measure visual difference in the CIELAB color space while correcting the
* computation to take into account the space being not perfectly perceptual
* uniform.
*
* This function does not take into account any transparency channel.
*
* Returns: the distance computed using the CIEDE2000 algorithm.
*
* Since: 3.0
**/
static gfloat
gimp_color_get_CIE2000_distance (GeglColor *color1,
GeglColor *color2)
{
gfloat lab1[3];
gfloat lab2[3];
gfloat dL;
gfloat C_prime;
gfloat dC;
gfloat dh;
gfloat dH;
gfloat h_prime;
gfloat T;
gfloat L_50_2;
gfloat SL;
gfloat SC;
gfloat SH;
gfloat C_prime7;
gfloat RT;
gfloat dE00;
gfloat RC;
gfloat d0;
g_return_val_if_fail (GEGL_IS_COLOR (color1), FALSE);
g_return_val_if_fail (GEGL_IS_COLOR (color2), FALSE);
gegl_color_get_pixel (color1, babl_format ("CIE LCH(ab) float"), lab1);
gegl_color_get_pixel (color2, babl_format ("CIE LCH(ab) float"), lab2);
dL = lab2[0] - lab1[0];
dC = lab2[1] - lab1[1];
dh = lab2[2] - lab1[2];
dH = 2.f * sqrtf (lab1[1] * lab2[1]) * sinf (dh / 2.0f * M_PI / 180.f);
h_prime = lab1[2] + lab2[2] ;
if (lab1[1] * lab2[1] != 0.f)
{
if (fabsf (dh) <= 180.0f)
{
h_prime /= 2.0f;
}
else
{
if (h_prime < 360.f)
h_prime = (h_prime + 360.f) / 2.f;
else
h_prime = (h_prime - 360.f) / 2.f;
}
}
T = 1.f - 0.17f * cosf ((h_prime - 30.f) * M_PI / 180.f) + 0.24f * cosf (2.f * h_prime * M_PI / 180.f) +
0.32f * cosf ((3.f * h_prime + 6.f) * M_PI / 180.f) - 0.2f * cosf ((4.f * h_prime - 63.f) * M_PI / 180.f);
C_prime = (lab1[1] + lab2[1]) / 2.f;
L_50_2 = (((lab1[0] + lab2[0]) / 2.f) - 50.f);
L_50_2 *= L_50_2;
SL = 1.f + 0.015f * L_50_2 / sqrtf (20.f + L_50_2);
SC = 1.f + 0.045f * C_prime;
SH = 1.f + 0.015f * C_prime * T;
C_prime7 = powf (C_prime, 7.f);
d0 = 30.f * expf (- powf ((h_prime - 275.f) / 25.f, 2.f));
#define CONST_25_POWER_7 6103515625.0f
RC = 2.f * sqrtf (C_prime7 / (C_prime7 + CONST_25_POWER_7));
#undef CONST_25_POWER_7
RT = - sinf (2.f * d0 * M_PI / 180.f) * RC;
dE00 = sqrtf (powf (dL / SL, 2.f) + powf (dC / SC, 2.f) + powf (dH / SH, 2.f) +
RT * dC * dH / SC / SH);
return dE00;
}

1
libgimpcolor/gimpcolor.h
View File

@ -64,6 +64,7 @@ gboolean gimp_color_is_out_of_self_gamut (GeglColor *color);
gboolean gimp_color_is_out_of_gamut (GeglColor *color,
const Babl *space);
G_END_DECLS
#endif /* __GIMP_COLOR_H__ */

3
libgimpcolor/meson.build
View File

@ -31,11 +31,12 @@ libgimpcolor_headers_introspectable = files(
'gimphsv.h',
'gimppixbuf.h',
'gimprgb.h',
'gimpcolor.h',
)
libgimpcolor_headers = [
libgimpcolor_headers_introspectable,
'gimpcolor.h',
]
libgimpcolor_introspectable = [