[pull] master from FFmpeg:master#243
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pull[bot] merged 7 commits intojing207:masterfrom Dec 4, 2022
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There is no v4 ASS. There are versions 1 to 4 of SSA (with only v4 being supported by softsub renderers), ASS which declares itself as v4+ or v4.00+, and the rarely used v4++/v4.00++, which isn't fully supported by libass. As reflected by the [V4+ Styles] section header, FFmpeg uses ASS, not SSA v4, so adjust the comment accordingly.
Colour values used in ASS files without a "YCbCr Matrix" header set to "None" are subject to colour mangling, due to how ASS was historically conceived. A more in-depth description can be found in the documetation inside libass' public ass_types.h header. The important part is, if this header is not set to "None", the final output colours can deviate from the literal value specified in the file. When converting from non-ASS formats we do not want any colour shift to happen, so let's set the appropiate header. NB: ffmpeg's subtitle filter, does not follow libass' documentation regarding colour mangling, thus hiding the bug. Anything based on VSFilter, XySubFilter or e.g. mpv do and might show the issue. (Of course native ASS subs, which _do_ rely on colour mangling won't work properly with the subtitle filter, but this can be fixed another time)
The Encoding field (and the \fe tag) allows to limit font selection to only those fonts declaring support for the specified codepage in their OS/2's table "Code Page Character Range" field. Particularly, Encoding=0 means only font's declaring support for "ANSI", or rather "Latin (Western European)", are allowed to be selected. Specifying Encoding=1 allows all fonts to be considered. We do not want to limit font selection, so specify Encoding=1. NB: at the time of writing libass only partially supports this field, thus hiding the issue in any libass-based renderer. A VSFilter-based DirectShow filter or XySubFilter will reveal the issue when a font not declaring support for latin characters is specified in a style.
Fixes overreads and artifacts for some heights not multiple of 16.
We didn't do this, because it's zero anyway, but it prevents users from using uninitialized memory in calculations.
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