Explaination of why blind-set-luma may look weird

Signed-off-by: Mattias Andrée <maandree@kth.se>

1 files changed, 33 insertions, 1 deletions

diff --git a/src/blind-set-luma.c b/src/blind-set-luma.c
index 71bd59c..49cbc08 100644
--- a/src/blind-set-luma.c
+++ b/src/blind-set-luma.c
@@ -26,7 +26,8 @@ process_xyza(struct stream *colour, struct stream *luma, size_t n)
 		 * so the result may look a bit weird. To change both
 		 * you can use `blind-arithm mul`.
 		 * 
-		 * Explaination:
+		 * Explaination of algorithm:
+		 * 
 		 *   Y is the luma, but (X, Z) is not the chroma,
 		 *   but in CIELAB, L* is the luma and (a*, *b) is
 		 *   the chroma. Multiplying
@@ -50,6 +51,37 @@ process_xyza(struct stream *colour, struct stream *luma, size_t n)
 		 *      ⎛1 1  0⎞⎛a 0 0⎞⎛0 1   0⎞   ⎛1 a−1 0⎞
 		 *      ⎜1 0  0⎟⎜0 1 0⎟⎜1 −1  0⎟ = ⎜0  a  0⎟.
 		 *      ⎝0 0 −1⎠⎝0 0 1⎠⎝0  1 −1⎠   ⎝0 a−1 1⎠
+		 * 
+		 * Explanation of why changing only the luma looks weird:
+		 * 
+		 *   Consider when you are workings with colours,
+		 *   when you want to change the brightness of a
+		 *   colour, you multiply all parameters: red, green,
+		 *   and blue, with the same value (this is however
+		 *   only an approximation in most cases, since you
+		 *   are usually usally working with colours that
+		 *   have the sRGB transfer function applied to their
+		 *   parameters). This action is the same in all
+		 *   colour models and colour spaces that are a
+		 *   linear transformation of the sRGB colour spaces
+		 *   (sans transfer function); this is simply because
+		 *   of the properties of linear transformations.
+		 * 
+		 *   The reason you change brightness this way can
+		 *   be explained by how objects reflect colour.
+		 *   Objects can only reject colours that are present
+		 *   in the light source. A ideal white object will look
+		 *   pure red if the light sources is ideal red, and a
+		 *   a ideal blue object will pure black in the same
+		 *   light source. An object can also not reflect
+		 *   colours brighter than the source. When the brightness
+		 *   of a light source is changed, the intensity of all
+		 *   colours (by wavelength) it emits is multiplied by
+		 *   one value. Therefore, when changing the brightness
+		 *   it looks most natural when all primaries (red, green,
+		 *   and blue) are multiplied by one value, or all
+		 *   parameters of the used colour spaces is a linear
+		 *   transformation of sRGB, such as CIE XYZ.
 		 */
 	}
 }