[FFmpeg-trac] #4814(undetermined:new): swscale default alpha blending drops alpha
FFmpeg
trac at avcodec.org
Mon Aug 31 12:47:23 CEST 2015
#4814: swscale default alpha blending drops alpha
-------------------------------------+-------------------------------------
Reporter: michael | Owner:
Type: enhancement | Status: new
Priority: normal | Component:
Version: unspecified | undetermined
Keywords: swscale, | Resolution:
alpha | Blocked By:
Blocking: | Reproduced by developer: 0
Analyzed by developer: 0 |
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Comment (by iive):
Using the alpha channel means that you allow the player and/or editing
software to put any kind of background there.
This is the only reasonable assumption.
Black and white chessboard pattern is used by all editing software. We
should do the same.
----
So far three default behaviors have been argued:
- Alpha channel should be ignored/dropped.
There are encodes that signal existence of alpha channel, but it contains
random garbage. Also this is the fastest way to handle the conversion.
- The image should be alpha blended with white background. Sample
"laraShadow_dl.flv" looks correctly only with that assumption.
- The image should be alpha blended with black background. Sample from
#4746 (prores) looks correctly only with that assumption.
All these behaviors are wrong. None of them could be enabled without
breaking the other two. On top of that they would be breaking in quite
obscure way.
----
The optimal solution would be to have format specific workarounds.
For the general case, I think that the default should be black&white
chessboard pattern:
- It is already supported as mode for alpha blending.
- It is used by all editing software to signal the presence of alpha
channel.
- The users would immediately recognize it and will know that the problem
is related to alpha channels. Then they can set the desired behavior on
their own and eventually demand format specific workaround from us.
--
Ticket URL: <https://trac.ffmpeg.org/ticket/4814#comment:2>
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