diff options
| author | Hugo Hörnquist <hugo@lysator.liu.se> | 2022-02-02 02:36:43 +0100 |
|---|---|---|
| committer | Hugo Hörnquist <hugo@lysator.liu.se> | 2022-02-02 02:36:43 +0100 |
| commit | cf692903b848d7b02a1966da671f5c653212a25c (patch) | |
| tree | c2eadcfc96a0f533c09bf2fea30aebb900045104 /module/calp/html/util.scm | |
| parent | Documentation of (hnh util ...) (diff) | |
| download | calp-cf692903b848d7b02a1966da671f5c653212a25c.tar.gz calp-cf692903b848d7b02a1966da671f5c653212a25c.tar.xz | |
Move calculate-fg-color into calp subtree.
While the algorithm was general, the packing of colors, along with the
error handling made it to non-portable.
Diffstat (limited to 'module/calp/html/util.scm')
| -rw-r--r-- | module/calp/html/util.scm | 20 |
1 files changed, 20 insertions, 0 deletions
diff --git a/module/calp/html/util.scm b/module/calp/html/util.scm index ecb54198..75137d4e 100644 --- a/module/calp/html/util.scm +++ b/module/calp/html/util.scm @@ -13,3 +13,23 @@ (lambda (ev) (or (id ev) (set/r! (id ev) (symbol->string (gensym "__html_id_"))))))) + +;; Returns a color with good contrast to the given background color. +;; https://stackoverflow.com/questions/1855884/determine-font-color-based-on-background-color/1855903#1855903 +(define-public (calculate-fg-color c) + (catch #t + (lambda () + (define (str->num c n) (string->number (substring/shared c n (+ n 2)) 16)) + ;; (format (current-error-port) "COLOR = ~s~%" c) + (let ((r (str->num c 1)) + (g (str->num c 3)) + (b (str->num c 5))) + (if (< 1/2 (/ (+ (* 0.299 r) + (* 0.587 g) + (* 0.114 b)) + #xFF)) + "#000000" "#FFFFFF"))) + (lambda args + (format (current-error-port) "Error calculating foreground color?~%~s~%" args) + "#FF0000" + ))) |