Gracefully handle negative Encoding values [VSFilter] #28
Conversation
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Back in 2012 libass added a custom Encoding=-1 extension to enable automatic BiDi base direction instead of the usual LTR default. The assumption/hope presumably was that since all valid font encodings are positive VSFilter would just gracefully ignore them. However, this is unfortuantely not the case. When passed to GDI the charSet is cast to a byte and will wrap around to a valid value, severely restricting which fonts can be used. Searching through a large pile of subtitles the only hits for negative values in the Encoding field were from generally corrupted files. It thus appears so far fortunately nobody used the extension in released fiels (only transient conversion from other formats during playback) and also nobody is relying on the wraparound in VSFilter. To minimise future problems treat all negative Encodings as the default charset. This matches how libass does fontselection for -1 albeit it doesn't reproduce the BiDi base direction. It also conveniently carves out the [-2, INT_MIN] range for potential future extensions common to VSFilters and libass, e.g. Encoding=-2 for a RTL base direction (which should be easier to implement in VSFilter than auto base direction)
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Back in 2012 libass added a custom Encoding=-1 extension to enable automatic BiDi base direction instead of the usual LTR default. The assumption/hope presumably was that since all valid font encodings are positive VSFilter would just gracefully ignore them. However, this is unfortuantely not the case.
When passed to GDI the charSet is cast to a byte and will wrap around to a valid value, severely restricting which fonts can be used.
Searching through a large pile of subtitles the only hits for negative values in the Encoding field were from generally corrupted files. It thus appears so far fortunately nobody used the extension in released fiels (only transient conversion from other formats during playback) and also nobody is relying on the wraparound in VSFilter.
To minimise future problems treat all negative Encodings as the default charset. This matches how libass does fontselection for -1 albeit it doesn't reproduce the BiDi base direction.
It also conveniently carves out the [-2, INT_MIN] range for potential future extensions common to VSFilters and libass, e.g. Encoding=-2 for a RTL base direction (which should be easier to implement in VSFilter than auto base direction)