Best/Most efficient way of chromakeying long 4K source for 720p output?

Question

I'm doing keying work for a video with a man in front of a green screen. The source is 4K (3840x2160@25 fps), just over 46 minutes in duration but only needs to be output at 720p.

It seems to key reasonably well (using the colour range keyer) except at various points the man keeps moving his hand/arm outside of the greenscreen background. This will need rotoscoping. I created a couple of mattes and scaled the video a bit (to get rid of things on the sides too), and have keyframed the matte around the hand/arm in some places. It's far from perfect though around the hand (though it's so quick when he moves his hands that I don't think it will matter all that much), but I'm wondering what's the best/most efficient way. He doesn't seem to move his hands outside the greenscreen all that much. Should I be using seperate masks for the hand? Is it simpler to use rotobrush or something else?

What about the resolution and file format? Since the output for the client will be only 720p and the source is 3840x2160@ 25 fps and is 46 mins long approx, should I convert to a 720p I-frame only format for more efficient working with it? Or to 1080p (I-frame only?)? What will be the best, most efficient way of doing this that will give reasonable results at 720p?

edit: I've created a separate mask for the hand, to hopefully make thing easier (so I can just keyframe that mask so it separate from the other 'garbage' masks). I've also added a spill suppressor and that's improved things so it looks quite good at 720p. I'm still wondering if there's a better/more efficient way though. Also, the keyed hand even though it's not exact (the mask is more basic than the hand outline, for speed), because of the composited background and a bit because of the hand is moving, the discrepancies don't stand out very much.

edit2: I'm currently pre-composing everything (actually doing 2 pre-composes) to a 720p resolution comp since it only needs to be output at 720p. But I'm wondering if there are better ways that will render everything in approx the same quality (looks about the same) but rendered faster (it uses over 80% of RAM which is good).

I'm currently using the old CS5.5 version of AE with Windows 10.

Dave_LaRonde · Accepted Answer

A.I.1 wroteSo you think render the source video to I-frame only, still 4K, then replace the source footage in the comp, and that will give quite big increase in render time (and probably faster if I need to do more mask keyframes too) when I render to the 720p output (with all the chroma keys and chroma suppression effect and masks (with keyframes))?Just as I suspected -- 4K video with the bejeezus compressed out of it. Mp4 and avc-intra are pretty much death on AE CS 5. Frankly, I'm surprised you've been able to do any work at all! You'll have a LOT easier time of this chore by transcoding.So what to transcode TO? Personally, I like Quicktime Movies in JPEG 2000, PNG, Photo JPEG or Animation codecs. If you have access to ProRes, use it. Be prepared for astoundingly - huge file sizes compared to the source.

Dave_LaRonde · Answer

You're doing a forty-six-minute-long chroma key? Ugh.

What's the media container & codec of this 4K source footage?

You may find renders will go a lot faster if you transcode to a different codec. If you would benefit from the transcode, keep it in the same dimensions -- you wouldn't have to do the masking work over again.

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