Draft: Fix handling of alpha when using luma file - #1276
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Hmm don't know how but it's only now that I realize that there is an issue with my change, bottom left part of the blue triangle is missing.. |
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Pull request overview
This PR updates the core luma transition implementation to make alpha handling consistent when a luma file is used, by ensuring the luma-based compositing path respects the alpha_over setting instead of implicitly behaving as if alpha_over were enabled.
Changes:
- Add an
alpha_overparameter to the luma compositing functions (YUV422/RGBA32/RGBA64) and thread it throughtransition_get_image. - Gate the “alpha-over” normalization/alpha-channel updates in the luma compositing loops behind
alpha_over.
Comments suppressed due to low confidence (1)
src/modules/core/transition_luma.c:438
- In the translucent+YUV422 luma path, the alpha pointers are advanced linearly across pixels (immediately after this block) but are not re-based per row/field or adjusted for differing src/dst row strides. If frames are interlaced (
field_count=2) or src/dst dimensions differ, alpha reads/writes can become misaligned and produce incorrect alpha output.
}
*q = sample_mix(*q, *p++, mix_b);
q++;
Comment on lines
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| mix_a = calculate_mix(1.0f - value, alpha_dest ? *alpha_dest : 255); | ||
| if (invert && alpha_src) { | ||
| float mix2 = mix_b + mix_a; | ||
| *alpha_src = 255 * mix2; | ||
| if (mix2 != 0.f) | ||
| mix_b /= mix2; | ||
| } else if (!invert && alpha_dest) { | ||
| float mix2 = mix_b + mix_a; | ||
| *alpha_dest = 255 * mix2; | ||
| if (mix2 != 0.f) | ||
| mix_b /= mix2; | ||
| } |
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Currently, the luma transition behaves differently when using a luma file or not on images with an alpha channel. When a luma file is used, it always behaves as if alpha_over was enabled. Here are some screenshot comparing a transition between 2 images of triangles with a transparent background. Each image below contains 4 different cases:
Frame 0: only first image is visible (red triangle in top right corner):
Last frame of the transition, you can see the behavior difference when a luma is used or not (bottom left should be similar to top left):
With the changes in this merge request, the result is now as expected:
The only question is do we want to maintain backwards compatibility, and how ?
Feedback is welcome.