[FFmpeg-trac] #4746(swscale:closed): prores alpha blending results in opaque rectangle

FFmpeg trac at avcodec.org
Mon Aug 31 12:17:47 CEST 2015

#4746: prores alpha blending results in opaque rectangle
             Reporter:               |                    Owner:
  projectsymphony                    |                   Status:  closed
                 Type:  defect       |                Component:  swscale
             Priority:  normal       |               Resolution:  fixed
              Version:  git-master   |               Blocked By:
             Keywords:  prores       |  Reproduced by developer:  1
  alpha                              |
             Blocking:               |
Analyzed by developer:  0            |

Comment (by iive):

 @projectsymphony, FFmpeg provides you with the options to get your desired
 behavior, please use them.

 1. '''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.
  If you wanted that background to be black, you should have blended it
 before encoding and not using alpha channel.

 2. There is no any solid proof that blend to black is standard behavior.
 Your player does that, but it is not the only reference player for FFmpeg.
 If there is no written standard, quicktime behavior could be changed in
 next version.

 3. If you can find or demonstrate that the default desired color is
 explicitly stored in the file, then you could request feature that allow
 it to be passed as metadata.

 You should understand that blend-to-black might work for you, but setting
 it as default is as wrong as any of the other possible behaviors.

 - There are encodes that signal existence of alpha channel, but it
 contains random garbage. These files would suddenly break in spectacular

 - There are samples (laraShadow_dl.flv) that assume blend-to-white as
 correct and standard behavior.

 - There are samples (your prores encode) that assume blend-to-black as
 correct and standard behavior.

 All these behaviors are wrong. None of them could be enabled without
 breaking the other two (in quite obscure way).

 Now, it might be possible to implement format specific defaults, by
 passing metadata from the demuxer to the filter chain. I have no idea how
 hard that task is. [http://ffmpeg.org/pipermail/ffmpeg-
 devel/2015-February/168545.html This] might be relevant.

 If @gjdfgh (I guess that is @kierank) wants to implement it, then I would
 ask Carl to assign this bug to him and to leave it open.
 Until then I guess the status of the bug is "closed - won't fix".

Ticket URL: <https://trac.ffmpeg.org/ticket/4746#comment:42>
FFmpeg <https://ffmpeg.org>
FFmpeg issue tracker

More information about the FFmpeg-trac mailing list