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	blind-set-luma.c - blind - suckless command-line video editing utility
	git clone git://git.suckless.org/blind
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	blind-set-luma.c (3592B)
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	1 /* See LICENSE file for copyright and license details. */
	2 #ifndef TYPE
	3 #include "common.h"
	4
	5 USAGE("luma-stream")
	6
	7 #define FILE "blind-set-luma.c"
	8 #include "define-functions.h"
	9
	10 int
	11 main(int argc, char *argv[])
	12 {
	13 struct stream colour, luma;
	14 void (process)(struct stream colour, struct stream *luma, size…
	15
	16 UNOFLAGS(argc != 1);
	17
	18 eopen_stream(&colour, NULL);
	19 eopen_stream(&luma, argv[0]);
	20
	21 SELECT_PROCESS_FUNCTION(&colour);
	22 CHECK_CHANS(&colour, == 3, == 1);
	23 CHECK_COLOUR_SPACE(&colour, CIEXYZ);
	24
	25 fprint_stream_head(stdout, &colour);
	26 efflush(stdout, "<stdout>");
	27 process_two_streams(&colour, &luma, STDOUT_FILENO, "<stdout>", p…
	28 return 0;
	29 }
	30
	31 #else
	32
	33 static void
	34 PROCESS(struct stream colour, struct stream luma, size_t n)
	35 {\
	36 size_t i;
	37 TYPE a, y;
	38 for (i = 0; i < n; i += colour->pixel_size) {
	39 a = ((TYPE *)(luma->buf + i))[1];
	40 a = ((TYPE )(luma->buf + i))[3];
	41 y = ((TYPE *)(colour->buf + i))[1];
	42 ((TYPE )(colour->buf + i))[0] += y a - y;
	43 ((TYPE )(colour->buf + i))[1] = y a;
	44 ((TYPE )(colour->buf + i))[2] += y a - y;
	45 /*
	46 * Note, this changes the luma only, not the saturation,
	47 * so the result may look a bit weird. To change both
	48 * you can use `blind-arithm mul`.
	49 *
	50 * Explaination of algorithm:
	51 *
	52 * Y is the luma, but (X, Z) is not the chroma,
	53 * but in CIELAB, L* is the luma and (a, b) is
	54 * the chroma. Multiplying
	55 *
	56 * ⎛0 1 0⎞
	57 * ⎜1 −1 0⎟
	58 * ⎝0 1 −1⎠
	59 *
	60 * (X Y Z)' gives a colour model similar to
	61 * CIE Lab*: a model where each parameter is
	62 * a linear transformation of the corresponding
	63 * parameter in CIE Lab*. The inverse of that
	64 * matrix is
	65 *
	66 * ⎛1 1 0⎞
	67 * ⎜1 0 0⎟
	68 * ⎝0 0 −1⎠
	69 *
	70 * and
	71 *
	72 * ⎛1 1 0⎞⎛a 0 0⎞⎛0 1 0⎞ ⎛1 a−…
	73 * ⎜1 0 0⎟⎜0 1 0⎟⎜1 −1 0⎟ = ⎜0 a…
	74 * ⎝0 0 −1⎠⎝0 0 1⎠⎝0 1 −1⎠ ⎝0 …
	75 *
	76 * Explanation of why changing only the luma looks weird:
	77 *
	78 * Consider when you are workings with colours,
	79 * when you want to change the brightness of a
	80 * colour, you multiply all parameters: red, green,
	81 * and blue, with the same value (this is however
	82 * only an approximation in most cases, since you
	83 * are usually usally working with colours that
	84 * have the sRGB transfer function applied to their
	85 * parameters). This action is the same in all
	86 * colour models and colour spaces that are a
	87 * linear transformation of the sRGB colour spaces
	88 * (sans transfer function); this is simply because
	89 * of the properties of linear transformations.
	90 *
	91 * The reason you change brightness this way can
	92 * be explained by how objects reflect colour.
	93 * Objects can only reject colours that are present
	94 * in the light source. A ideal white object will look
	95 * pure red if the light sources is ideal red, and a
	96 * a ideal blue object will pure black in the same
	97 * light source. An object can also not reflect
	98 * colours brighter than the source. When the brightne…
	99 * of a light source is changed, the intensity of all
	100 * colours (by wavelength) it emits is multiplied by
	101 * one value. Therefore, when changing the brightness
	102 * it looks most natural when all primaries (red, gree…
	103 * and blue) are multiplied by one value, or all
	104 * parameters of the used colour spaces is a linear
	105 * transformation of sRGB, such as CIE XYZ.
	106 */
	107 }
	108 }
	109
	110 #endif