Both converters can output \pos and deal with font sizes, so they assume
a specific script resolution (PlayResX/PlayResY). The implicit
assumption was that a specific resolution was guaranteed. The
MP_ASS_FONT_PLAYRESY constant is connected to this.
Better make it explicit, so that the implicit dependency on
MP_ASS_FONT_PLAYRESY is removed. (Unfortunately, libavcodec sub
converters still don't set PlayResX/PlayResY explicitly, so the value
set by that constant can't be declared as arbitrary yet.)
PlayResY=288 is most likely the SSA natural script resolution (or
something like this?), as well as the libass and VSFilter default.
PlayResX=384 is the fallback value set by libass if PlayResY is set to
288, and PlayResX is unset.
wm4
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