From cf899e0439cf4fbfbda8138c3152d540c7838e5f Mon Sep 17 00:00:00 2001
From: Thomas White <taw@physics.org>
Date: Sat, 6 Nov 2021 13:28:45 +0100
Subject: Add colour-as-rgbw

---
 guile/starlet/colours.scm | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/guile/starlet/colours.scm b/guile/starlet/colours.scm
index c7d1de0..f18b986 100644
--- a/guile/starlet/colours.scm
+++ b/guile/starlet/colours.scm
@@ -27,6 +27,7 @@
              make-colour-rgb
              colour-as-cmy
              colour-as-rgb
+             colour-as-rgbw
 
              cyan
              magenta
@@ -117,6 +118,15 @@
                            (make-exception-with-irritants (colour-type col))))))))
 
 
+(define (colour-as-rgbw col)
+  (let ((rgb (colour-as-rgb col)))
+    (let ((w (apply min rgb)))
+      (list (- (red rgb) w)
+            (- (green rgb) w)
+            (- (blue rgb) w)
+            w))))
+
+
 (define (colour-as-cmy col)
   (let ((val (colour-value col)))
     (case (colour-type col)
-- 
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