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Bizarre render bug when overlaying footage with alpha channel
I did a cursory search of the forums but didn’t see a relevant post.
FCP 6 running on Leopard.
I’m doing a simple edit in FCP in which several scenes on V1 are separated by a superimposed clip on V2 of a curtain closing, then opening. The curtain footage was shot on a green screen, keyed/rotoscoped in Combustion, then rendered with the Quicktime Animation codec (millions + colors) with an alpha channel. The underlying footage (and sequence) is 720p24 (23.976) ProRes 422.
Once I render the sequence in FCP, at the frame in which the curtain starts to fall over the underlying footage, the underlying footage skips back 2-4 frames and repeats, causing a visible stutter. The bizarre thing is, if I step through frame by frame, everything looks fine. When I export the sequence as a reference or self-contained Quicktime, the stutter is present on both playback and single-frame step-through. The footage with the alpha channel is identical in pixel dimensions and frame rate to the ProRes footage on V1 – the only difference is the codec.
I found a workaround, which is to put a cut point on the V1 track where the V2 track begins, trim 4 frames from V1 and slide the underlying V1 footage back 4 frames. While this works, I’m incredulous as to why this is even necessary. It seems to be a render bug in FCP 6. I’ve tried trashing my render files and re-rendering, but no dice. I’m curious to see if using ProRes 4444 instead of Animation would solve the issue, but I haven’t upgraded to FCP7 yet.
Any thoughts/clues? I could simply do the composited transitions in Combustion, but then I have to deal with the associated Quicktime gamma shift issues so I’m hoping to just keep it all in FCP.
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3 Replies
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Now that I think of it, I should add that I recently had another issue with Animation-codec supers in a recent project. The supers were simply a color bar graphic with a name that wiped in over interview subjects.
I noticed that after rendering, any supers placed over footage that contained the 3-way CC filter would alter the look of the CC slightly for the duration of the super (they knocked down the highlights slightly). The funny thing is, the shift wasn’t apparent until the sequence was rendered.
I’m starting to think that FCP just doesn’t like Quicktime Animation footage that contains an alpha channel.
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PRoRes 4444 will most likely solve the color issue because unlike Animation, which is RGB, 4444 is YCbCr, the same as video.
The first part may be a frame rate issue. What frame rate exactly is the Animation clip and what frame rate is the sequence?
All the best,
Tom
Class on Demand DVDs “Complete Training for FCP6,” “Basic Training for FCS2” and “Final Cut Express Made Easy”
Author: “Final Cut Pro 5 Editing Essentials” and “Final Cut Express 4 Editing Workshop” -
[Tom Wolsky] “PRoRes 4444 will most likely solve the color issue because unlike Animation, which is RGB, 4444 is YCbCr, the same as video. “
the ProRes 4444 codec supports both RGB and YUV colorspace formats.
444 material as dual link is captured by default from the source as RGB since that is what the vast majority of cameras deliver 444 out as, while some cameras can convert to YUV on output the default recording is RGB.
gary adcock
Studio37
HD & Film Consultation
Post and Production Workflows for the Digitally Inclined
Chicago, ILhttps://library.creativecow.net/articles/adcock_gary/AJAIOHD.php
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