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Canon 7D H264s: Codecs and green keying

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Luke Botteron
Canon 7D H264s: Codecs and green keying
on Dec 2, 2010 at 8:23:10 pm

I'm working with 7D 1080p footage of actors in front of a green screen. Normally when working with H264s, I convert to ProRes LT or ProRes for a stress free editing workflow.

This time, I will need to take the footage into AE to pull keys and I'm worried about color shifts, data loss, noise, and other things that could make the keying harder/messy.

I converted the H264 to ProRes Lt, ProRes, ProRes HQ, FCP Uncompressed 10bit, Apple Animation and Apple XDCAM EX (since the B camera is an EX1). Brought them all into after effects and found that the orginal H264 was brighter than all of them. XDCAM EX and Animation had crushed the blacks, but the prores family had nearly the same blacks as the H264 but the mids were lowered.

On further inspection, the ProRes family and FCP 10bit looked the same and were a solid black screen when compared using a Difference composite mode.


So really what I want to know is... When going from H264 into an editing/compositing codec, is there a right codec? If everything is darker than the source file, is the source file the problem or is it the conversions?


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Luke Botteron
Re: Canon 7D H264s: Codecs and green keying
on Dec 2, 2010 at 9:23:24 pm

The difference in color between the H264 or the ProRes family is due to Mpeg Streamclip. Compressor conversions were perfect.


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Noah Kadner
Re: Canon 7D H264s: Codecs and green keying
on Dec 2, 2010 at 9:27:36 pm

If I were doing effects work I'd pick ProRes 4444 which is the only format other than Animation to have an alpha channel. Also most VFX folks prefer to receive source footage as image sequences- TIF, TGA, Cineon or DPX typically.

Noah

Unlock the secrets of 24p, HD and Final Cut Studio with Call Box Training. Featuring the Canon 5D Mark II and 7D.


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Luke Botteron
Re: Canon 7D H264s: Codecs and green keying
on Dec 2, 2010 at 9:35:39 pm

Update: The color shift was cause by Mpeg Streamclip. Compressor and FCP log and transfer (just got the plug in) showed no differences in picture from the original h264


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Dave LaRonde
Re: Canon 7D H264s: Codecs and green keying
on Dec 3, 2010 at 2:55:27 am

I disagree with Noah on one point: you don't have to convert the footage to ProRes 4444 immediately; ProRes 422 is more than adequate for your footage, and you will suffer no problems from using it. You gain no benefit from 4444 at this particular point. You simply eat up more storage than necessary, with no gain in image quality.

If you choose to pull your key with NO background, creating the need for an alpha channel, it is at this point that you would want to use ProRes 4444; you would need the alpha channel, which this codec supports.

If you choose to include the background at the same time, you don't need ProRes 422 AT ALL: simply work in ProRes 422, and life will be good; you have both foreground and background in a codec that far exceeds the abilities of your original capture codec, and that is a very good thing indeed.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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