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Sunday, August 18, 2013

••◊ C100 AVCHD versus Atomos Samura Prores for Green Screen Work

Scott, Eva, and I went over to Video Gear to do some green screen tests with the C100, the initial results of which are published over on their blog.  Surprisingly I was able to pull a reasonable key with the AVCHD native codec on the C100.  What we initially intended to do was to compare the C100 footage to the Atomos Samurai footage to see the difference between codec quality.  The Atomos Samurai encodes to Prores HQ 4:2:2, which should obviously be "better" for green screen work, but we wanted to find out how much better is "better", really?

To compare the keyed footage I recommend you click on the photo below and view it on a full HD monitor.  The image is 1920x1080.  Immediately I see a difference in Eva's hair flying in the wind.  Notice how the key breaks up just to the right of her neck on the C100 footage?  The left side body outline is a bit less defined on the C100 footage as well.  Is this really a deal breaker?  Dunno.  Depends on who is paying the crew and what they want.

The AVCHD codec is 8-bit 4:2:0 24Mbit/s and the Prores footage is 8-bit (because the camera only outputs 8-bits) 4:2:2 at about 146Mb/s.  The doubling of chroma sub-sampling along with the 6x increase in bit rate should pay off with a better key, and it does.

An issue I ran into, which we are still working on solving is trying to make the Samurai do 3:2 pull down on the Canon's 60i output.  I was working on a production with a Canon rep yesterday and he suggested using 24PF mode in the camera.  There is also some talk of a technique in section 10 of the Atomos manual on how to enable 24p 3:2 pull down from a 60i camera.  I have yet to get this to work, but hopefully we can figure it out by next weekend.

Until then, I recorded the C100 output at 29.97p into the Atomos Samurai.  This means that three of every five frames are encoded properly as 23.976p (24p) frames.  We have to reconstruct that fourth 23.976p frame from two adjacent 29.97p frames using pull down.  For that, I employed After Effects and the "Interpret Footage" menu item - you can Google how to get to it.

I selected 23.976 as the frame rate, clicked on "Guess 3:2 Pulldown" and chose a square pixel aspect ratio.  After Effects was then able to do the final 3:2 pull down and reconstruct the fourth frame from two of the adjacent 29.97p frames.  I obviously want the recorder to do this automatically, so we're still working on making that happen.

Hope this information helps others out there.  If you have any experience with the C100 and Samurai in conjunction with 24p recording I would like to hear about it.  

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