Fix errors

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

1 files changed, 37 insertions, 2 deletions

diff --git a/src/blind-set-luma.c b/src/blind-set-luma.c
index 4500645..71bd59c 100644
--- a/src/blind-set-luma.c
+++ b/src/blind-set-luma.c
@@ -13,11 +13,44 @@ static void
 process_xyza(struct stream *colour, struct stream *luma, size_t n)
 {
 	size_t i;
-	double a;
+	double a, y;
 	for (i = 0; i < n; i += colour->pixel_size) {
 		a = ((double *)(luma->buf + i))[1];
 		a *= ((double *)(luma->buf + i))[3];
-		((double *)(colour->buf + i))[1] *= a;
+		y = ((double *)(colour->buf + i))[1];
+		((double *)(colour->buf + i))[0] += y * a - y;
+		((double *)(colour->buf + i))[1]  = y * a;
+		((double *)(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:
+		 *   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⎠
+		 */
 	}
 }
 
@@ -42,6 +75,8 @@ main(int argc, char *argv[])
 	else
 		eprintf("pixel format %s is not supported, try xyza\n", colour.pixfmt);
 
+	fprint_stream_head(stdout, &colour);
+	efflush(stdout, "<stdout>");
 	process_two_streams(&colour, &luma, STDOUT_FILENO, "<stdout>", process);
 	return 0;
 }