text: /* See LICENSE file for copyright and license details. */
  #ifndef TYPE
  #include "common.h"
  
  USAGE("luma-stream")
  
  #define FILE "blind-set-luma.c"
  #include "define-functions.h"
  
  int
  main(int argc, char *argv[])
  {
  	struct stream colour, luma;
  	void (*process)(struct stream *colour, struct stream *luma, size_t n);
  
  	UNOFLAGS(argc != 1);
  
  	eopen_stream(&colour, NULL);
  	eopen_stream(&luma, argv[0]);
  
  	SELECT_PROCESS_FUNCTION(&colour);
  	CHECK_CHANS(&colour, == 3, == 1);
  	CHECK_COLOUR_SPACE(&colour, CIEXYZ);
  
  	fprint_stream_head(stdout, &colour);
  	efflush(stdout, "<stdout>");
  	process_two_streams(&colour, &luma, STDOUT_FILENO, "<stdout>", process);
  	return 0;
  }
  
  #else
  
  static void
  PROCESS(struct stream *colour, struct stream *luma, size_t n)
  {\
  	size_t i;
  	TYPE a, y;
  	for (i = 0; i < n; i += colour->pixel_size) {
  		a = ((TYPE *)(luma->buf + i))[1];
  		a *= ((TYPE *)(luma->buf + i))[3];
  		y = ((TYPE *)(colour->buf + i))[1];
  		((TYPE *)(colour->buf + i))[0] += y * a - y;
  		((TYPE *)(colour->buf + i))[1]  = y * a;
  		((TYPE *)(colour->buf + i))[2] += y * a - y;
  		/*
  		 * Note, this changes the luma only, not the saturation,
  		 * so the result may look a bit weird. To change both
  		 * you can use `blind-arithm mul`.
  		 * 
  		 * 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
  		 *   
  		 *      ⎛0 1   0⎞
  		 *      ⎜1 −1  0⎟
  		 *      ⎝0  1 −1⎠
  		 *   
  		 *   (X Y Z)' gives a colour model similar to
  		 *   CIE L*a*b*: a model where each parameter is
  		 *   a linear transformation of the corresponding
  		 *   parameter in CIE L*a*b*. The inverse of that
  		 *   matrix is
  		 *   
  		 *      ⎛1 1  0⎞
  		 *      ⎜1 0  0⎟
  		 *      ⎝0 0 −1⎠
  		 *   
  		 *   and
  		 *   
  		 *      ⎛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.
  		 */
  	}
  }
  
  #endif
  