Sail fails to decode some APNGs

[qbnu]

Sail fails to decode Animated PNG files which use a Color Type other than 6 and APNG_BLEND_OP_OVER. This is because png_private_blend_over only supports RGBA, and doesn't check the tRNS chunk. Even if the PNG doesn't have any transparency information, blend_source should be used instead (equivalent to blend_over assuming all pixels are opaque).

Examples from https://philip.html5.org/tests/apng/tests.html:
8-bit greyscale and alpha, with blending
1-bit palette and alpha, with blending

const char* filename = "038.png";
sail_image* image = NULL;
const sail_codec_info* codec_info = NULL;
void* state = NULL;
sail_codec_info_from_extension("png", &codec_info); // SAIL_OK
sail_start_loading_from_file(filename, codec_info, &state); // SAIL_OK
sail_load_next_frame(state, &image); // SAIL_OK
sail_load_next_frame(state, &image); // SAIL_ERROR_UNSUPPORTED_BIT_DEPTH

And here's a 16-bit grayscale+alpha APNG that will incorrectly be blended as if it's 8-bit RGBA:

[HappySeaFox]

I agree there are some blending issues. I've just fixed a memory corruption issue (at least on MSVC 2019) on free() and allowed blending for BPP16-GRAYSCALE-ALPHA and BPP32-GRAYSCALE-ALPHA pixel formats. I will probably continue looking into it (sorry cannot guarantee).

Regarding the 8-bit greyscale and alpha, with blending image: it has three frames:

1. The first frame is hidden.
2. The second frame is black (gray component: 0, alpha: 255), blending mode: over.
3. The 3rd frame consists of semi-transparent white pixels which will look like gray after blending with black pixels (gray component: 255, alpha: 128), blending mode: over.

https://philip.html5.org/tests/apng/tests.html specifies that the image should be rendered as This should be solid grey. but I have a question why 🤔 The second frame is black. The third (blended) frame is gray. I would expect flashing black and gray frames and this is what I see currently in the local fixed SAIL version.

[qbnu]

If the image viewer respects num_plays in the acTL chunk then it should just play once and pause on the last frame.

[HappySeaFox]

Oh, I see. This explains a lot. The next rc will have blending of grayscale frames and I'll add num_plays as a special property for those who are interested.

[HappySeaFox]

Did you install sail through vcpkg?

[qbnu]

No, I built it from source.

[HappySeaFox]

You can try the latest master with blending of grayscale images.

[HappySeaFox]

Please reopen if needed