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Make color-distance symmetric and more accurate

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* src/xfaces.c (color_distance): Don't throw away the low 8 bits of
the colours, and make the function symmetric (bug41544)
(Fcolor_distance): Add caution about this not being a true metric.
* test/src/xfaces-tests.el: New file.
This commit is contained in:
Mattias Engdegård 2020-06-02 22:31:14 +02:00
parent 74966904b0
commit 7e8c1a6718
2 changed files with 39 additions and 10 deletions
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22
src/xfaces.c
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@ -4356,15 +4356,15 @@ color_distance (Emacs_Color *x, Emacs_Color *y)
See <https://www.compuphase.com/cmetric.htm> for more info. */
long r = (x->red - y->red) >> 8;
long g = (x->green - y->green) >> 8;
long b = (x->blue - y->blue) >> 8;
long r_mean = (x->red + y->red) >> 9;
long long r = x->red - y->red;
long long g = x->green - y->green;
long long b = x->blue - y->blue;
long long r_mean = (x->red + y->red) >> 1;
return
(((512 + r_mean) * r * r) >> 8)
+ 4 * g * g
+ (((767 - r_mean) * b * b) >> 8);
return (((((2 * 65536 + r_mean) * r * r) >> 16)
+ 4 * g * g
+ (((2 * 65536 + 65535 - r_mean) * b * b) >> 16))
>> 16);
}
@ -4374,7 +4374,9 @@ COLOR1 and COLOR2 may be either strings containing the color name,
or lists of the form (RED GREEN BLUE), each in the range 0 to 65535 inclusive.
If FRAME is unspecified or nil, the current frame is used.
If METRIC is specified, it should be a function that accepts
two lists of the form (RED GREEN BLUE) aforementioned. */)
two lists of the form (RED GREEN BLUE) aforementioned.
Despite the name, this is not a true distance metric as it does not satisfy
the triangle inequality. */)
(Lisp_Object color1, Lisp_Object color2, Lisp_Object frame,
Lisp_Object metric)
{
@ -4931,7 +4933,7 @@ DEFUN ("face-attributes-as-vector", Fface_attributes_as_vector,
/* If the distance (as returned by color_distance) between two colors is
less than this, then they are considered the same, for determining
whether a color is supported or not. The range of values is 0-65535. */
whether a color is supported or not. */
#define TTY_SAME_COLOR_THRESHOLD 10000

27
test/src/xfaces-tests.el Normal file
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@ -0,0 +1,27 @@
;;; xfaces-tests.el --- tests for xfaces.c -*- lexical-binding: t -*-
;; Copyright (C) 2020 Free Software Foundation, Inc.
;; This file is part of GNU Emacs.
;; GNU Emacs is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.
;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
;; GNU General Public License for more details.
;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs. If not, see <https://www.gnu.org/licenses/>.
(require 'ert)
(ert-deftest xfaces-color-distance ()
;; Check symmetry (bug#51455).
(should (equal (color-distance "#222222" "#ffffff")
(color-distance "#ffffff" "#222222"))))
(provide 'xfaces-tests)