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master ba450b6: Define the dark luminance limit as a named constant
From: |
Mattias Engdegård |
Subject: |
master ba450b6: Define the dark luminance limit as a named constant |
Date: |
Thu, 18 Jun 2020 15:22:09 -0400 (EDT) |
branch: master
commit ba450b6f462e278fcd3bc96c88f154fce219f5fc
Author: Mattias Engdegård <mattiase@acm.org>
Commit: Mattias Engdegård <mattiase@acm.org>
Define the dark luminance limit as a named constant
To make the meaning of the color-dark-p cutoff luminance clear,
define it as a named constant. (We no longer use the somewhat
obscure 0.6^2.2 definition since it doesn't really make sense
to define the limit in gamma-compressed space.)
* lisp/faces.el (color-luminance-dark-limit): New constant.
(color-dark-p): Use color-luminance-dark-limit.
---
lisp/faces.el | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/lisp/faces.el b/lisp/faces.el
index f6b9593..2480aaa 100644
--- a/lisp/faces.el
+++ b/lisp/faces.el
@@ -1794,6 +1794,11 @@ on which one provides better contrast with COLOR."
(color-values color)))
"#ffffff" "black"))
+(defconst color-luminance-dark-limit 0.325
+ "The relative luminance below which a color is considered 'dark',
+in the sense that white text is more readable than black with the
+color as background. This value was determined experimentally.")
+
(defun color-dark-p (rgb)
"Whether RGB is more readable against white than black.
RGB is a 3-element list (R G B), each component in the range [0,1].
@@ -1814,7 +1819,7 @@ contrast colour with RGB as background and as foreground."
(g (expt sg 2.2))
(b (expt sb 2.2))
(y (+ (* r 0.2126) (* g 0.7152) (* b 0.0722))))
- (< y (eval-when-compile (expt 0.6 2.2)))))
+ (< y color-luminance-dark-limit)))
(declare-function xw-color-defined-p "xfns.c" (color &optional frame))
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