I have been experimenting to find a rendering codec supporting alpha
transparency. This is an important intermediate step to prevent
projects getting too complex. I used YUVA as the project format.
After much failure I succeeded rendering to component Y'CbCrA8-bit
4:4:4:4 in a Quicktime container. Alpha info was preserved fine. The
file size was 6.2GB for 2 1/2 minutes of PAL DV footage, about 41
MB/second. This turned out to be the best of the working options in
terms of file size and the convienience of rendering an audio track
alongside.
To be thorough I went through all the eligible formats. Working formats
were:
TIFF sequence with the appropriate color space selected. 50 MB/second.
TGA sequence with the appropriate color space selected. 40 MB/second.
Non-working formats were as follows:
Render to EXR with alpha channels/ all types of compression (including
uncompressed) crashed cinelerra.
Render motion jpeg-a in a Quicktime container - results did not contain
any alpha info.
Render motion jpeg-a in avi container, as for quicktime container, a
video track with no alpha information
Render to png-a in a Quicktime container format appeared to work but the
playback of the rendered file did not work. Normal playback appeared to
work at first and the timeline thumbnails showed up fine but stepping
through image by image there was no picture at all. When another track
was put under this alpha track and play was pressed you could only the
see the track meant to be underneath - the rendered alpha track was
completely invisible.
Render to uncompressed RGBA in a Quicktime container format produced
same result as above.
I tried the render to png-a in a quicktime container format again with a
much simpler project. This project had all the alpha information
already rendered to a single track and no effects or masks to apply.
The render worked as far as preserving alpha information went but seemed
to drop a color channel. The rendered image was cyan and magenta only.
Render to a png sequence with alpha selected produced no alpha info.
Render to Plain png with alpha selected produced no alpha info.
Render to an avi with uncompressed RGBA appended a garbled video track
to the project. When I opened the rendered track it appeared empty.
It would be nice to have one of the more compressed formats working,
like motion jpeg-a for instance. But for short spurts Quicktime and
component Y'CbCrA8-bit 4:4:4:4 was useful for me.
Graham
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