A few hacky color-related functions

color-utils.el

(defun format-color (rgb)
  "Convert a triplet of floating point (0.0-1.0) RGB values into
a hex triplet"
  (apply 'format "#%02x%02x%02x"
         (mapcar (lambda (x) (* 255 x)) rgb)))



(defconst colour-triplet-regex
  "\\([0-9A-Fa-f]\\{2\\}\\)\\([0-9A-Fa-f]\\{2\\}\\)\\([0-9A-Fa-f]\\{2\\}\\)")

(defun parse-colour (colour)
  "Parse a hex colour string into a triplet of floating point (0.0-1.0) RGB values."
  (when (string-match colour-triplet-regex colour)
    (mapcar (lambda (x) (/ (float (string-to-int (match-string x colour) 16)) 255.0)) '(1 2 3))))


(defun matrix-multiply (m1 m2)
  (mapcar m1))

;; http://en.wikipedia.org/wiki/SRGB#The_forward_transformation_.28CIE_xyY_or_CIE_XYZ_to_sRGB.29
;; (matrix-multiply '((0.4124 0.3576 0.1805)
;;                      (0.2126 0.7152 0.0722)
;;                      (0.0193 0.1192 0.9505))
;;                    srgb)
(defun srgb->cie-xyz (srgb)
  (let ((r (nth 0 srgb))
        (g (nth 1 srgb))
        (b (nth 2 srgb)))
    (list (+ (* r 0.4124) (* g 0.3576) (* b 0.1805))
          (+ (* r 0.2126) (* g 0.7512) (* b 0.0722))
          (+ (* r 0.0193) (* g 0.1192) (* b 0.9505)))))

(defun srgb->linear-rgb (srgb)
  (mapcar (lambda (c)
            (if (> c 0.04045)
                (expt (/ (+ c 0.055) 1.055) 2.4)
              (/ c 12.92)))
          srgb))


;; SRGB: 002b36
;; 0  43  54
;; Generic RGB 042028
(parse-colour "ffffff")
(format-color (srgb->linear-rgb (parse-colour "")))


(defun apply-gamma (v gamma)
  "Apply gamma to a color value v in the range "
  (expt v gamma))

;; x = v ** gamma

(defun change-gamma (v old-gamma new-gamma)
  (apply-gamma (/ 1 (log v old-gamma)) new-gamma))


(defvar default-gamma 0.4545)

(defun adjust-gamma (hexcolor old-gamma new-gamma)
  (let* ((rgb (parse-colour hexcolor))
         (new-rgb (mapcar (lambda (v) (change-gamma v old-gamma new-gamma)) rgb)))
    (format-color new-rgb)))


(format-color '(0.0159 0.1265 0.1597))