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app: MyBrush: Improve blend math

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This commit is contained in:
Daniel Sabo 2015-12-19 15:53:42 -08:00
parent 44d5e7adcd
commit bf161e7b14
1 changed files with 11 additions and 5 deletions
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16
app/paint/gimpmybrushsurface.c
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@ -373,14 +373,20 @@ gimp_mypaint_surface_draw_dab (MyPaintSurface *base_surface,
rr = calculate_rr (ix, iy, x, y, aspect_ratio, sn, cs, one_over_radius2);
alpha = calculate_alpha_for_rr (rr, hardness, segment1_slope, segment2_slope) * opaque;
dst_alpha = pixel[ALPHA];
a = alpha * color_a + dst_alpha * (1.0f - alpha);
/* a = alpha * color_a + dst_alpha * (1.0f - alpha);
* which converts to: */
a = alpha * (color_a - dst_alpha) + dst_alpha;
if (a > 0.0f)
{
float a_term = dst_alpha * (1.0f - alpha);
dst_pixel[RED] = (color_r * alpha * color_a + pixel[RED] * a_term) / a;
dst_pixel[GREEN] = (color_g * alpha * color_a + pixel[GREEN] * a_term) / a;
dst_pixel[BLUE] = (color_b * alpha * color_a + pixel[BLUE] * a_term) / a;
/* By definition the ratio between each color[] and pixel[] component in a non-pre-multipled blend always sums to 1.0f.
* Originaly this would have been "(color[n] * alpha * color_a + pixel[n] * dst_alpha * (1.0f - alpha)) / a",
* instead we only calculate the cheaper term. */
float src_term = (alpha * color_a) / a;
float dst_term = 1.0f - src_term;
dst_pixel[RED] = color_r * src_term + pixel[RED] * dst_term;
dst_pixel[GREEN] = color_g * src_term + pixel[GREEN] * dst_term;
dst_pixel[BLUE] = color_b * src_term + pixel[BLUE] * dst_term;
}
dst_pixel[ALPHA] = a;