Correct dmnsn_color_illuminate().

1 files changed, 18 insertions, 10 deletions

diff --git a/libdimension/color.c b/libdimension/color.c
index 4589824..2089326 100644
--- a/libdimension/color.c
+++ b/libdimension/color.c
@@ -301,15 +301,23 @@ dmnsn_color_illuminate(dmnsn_color light, dmnsn_color color)
   dmnsn_CIE_Lab Lab1 = dmnsn_Lab_from_color(light, dmnsn_whitepoint);
   dmnsn_CIE_Lab Lab2 = dmnsn_Lab_from_color(color, dmnsn_whitepoint);
 
-  double dot  = Lab1.L*Lab2.L + Lab1.a*Lab2.a + Lab1.b*Lab2.b;
-  double norm = sqrt(Lab2.L*Lab2.L + Lab2.a*Lab2.a + Lab2.b*Lab2.b);
-
-  if (norm == 0.0)
-    return dmnsn_black;
-
-  dmnsn_CIE_Lab Lab = { .L = dot*Lab2.L/norm/100.0,
-                        .a = dot*Lab2.a/norm,
-                        .b = dot*Lab2.b/norm };
+  /*
+   * This is derived from the simple illumination equation for RGB color:
+   *   RGB' = (R_light*R_color, G_light*G_color, B_light*B_color)
+   *
+   * CIE L*a*b* may be transformed to an RGB colorspace by considering `a'
+   * to be the green -> magenta opponent process (from negative to positive),
+   * and `b' to be the blue -> yellow opponent process:
+   *   RGB  = (L + a + b, L - a + b, L + a - b)
+   *
+   * The reverse transformation is:
+   *   Lab  = ((G + B)/2, (R - G)/2, (R - B)/2)
+   */
+  dmnsn_CIE_Lab Lab = {
+    .L = (Lab1.L*Lab2.L + (Lab1.b - Lab1.a)*(Lab2.b - Lab2.a))/100.0,
+    .a = (Lab1.a*(Lab2.L + Lab2.b) + Lab2.a*(Lab1.L + Lab1.b))/100.0,
+    .b = (Lab1.b*(Lab2.L + Lab2.a) + Lab2.b*(Lab1.L + Lab1.a))/100.0
+  };
 
   dmnsn_color ret = dmnsn_color_from_Lab(Lab, dmnsn_whitepoint);
   ret.filter = color.filter;
@@ -318,7 +326,7 @@ dmnsn_color_illuminate(dmnsn_color light, dmnsn_color color)
   return ret;
 }
 
-/* Find the perceptual difference between two colors, using CIE L*a*b*. */
+/* Find the perceptual difference between two colors, using CIE L*a*b* */
 double
 dmnsn_color_difference(dmnsn_color color1, dmnsn_color color2)
 {