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Diffstat (limited to 'compat-28.el')
| -rw-r--r-- | compat-28.el | 31 |
1 files changed, 31 insertions, 0 deletions
diff --git a/compat-28.el b/compat-28.el index dec2d8c..55e0b75 100644 --- a/compat-28.el +++ b/compat-28.el @@ -635,6 +635,37 @@ is included in the return value." default))) ": ")) +;;;; Defined in faces.el + +(compat-defvar color-luminance-dark-limit 0.325 ;; <compat-tests:color-dark-p> + "The relative luminance below which a color is considered \"dark\". +A \"dark\" color in this sense provides better contrast with white +than with black; see `color-dark-p'. +This value was determined experimentally." + :constant t) + +(compat-defun color-dark-p (rgb) ;; <compat-tests:color-dark-p> + "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]. +This predicate can be used both for determining a suitable (black or white) +contrast color with RGB as background and as foreground." + (unless (<= 0 (apply #'min rgb) (apply #'max rgb) 1) + (error "RGB components %S not in [0,1]" rgb)) + ;; Compute the relative luminance after gamma-correcting (assuming sRGB), + ;; and compare to a cut-off value determined experimentally. + ;; See https://en.wikipedia.org/wiki/Relative_luminance for details. + (let* ((sr (nth 0 rgb)) + (sg (nth 1 rgb)) + (sb (nth 2 rgb)) + ;; Gamma-correct the RGB components to linear values. + ;; Use the power 2.2 as an approximation to sRGB gamma; + ;; it should be good enough for the purpose of this function. + (r (expt sr 2.2)) + (g (expt sg 2.2)) + (b (expt sb 2.2)) + (y (+ (* r 0.2126) (* g 0.7152) (* b 0.0722)))) + (< y color-luminance-dark-limit))) + ;;;; Defined in windows.el (compat-defun count-windows (&optional minibuf all-frames) ;; <compat-tests:count-windows> |