Sunday, 17 February 2013

GoPro 4K and Twixtor - not a viable solution

The most cost effective way of shooting 4K footage today is with a GoPro Hero3 Black edition. With a name like that it should be shaped like a Stealth bomber or Darth Vader's helmet but in fact it looks like this.
The GoPro Hero3 Black
So not much to look at but it does record 4K footage. Actually not quite as the resolution is 3840x2160 not the 4096x2160 (or 4096x2304) generally accepted as 4K, but close enough. Not quite close enough either is the frame rate it records at, which in PAL setting is 12.5 frames per second, half of what is generally accepted and the result is (if the footage is played at normal speed) a pretty jumpy video. But a San Francisco company called Re:Vision Effects Inc. has produced a piece of software called Twixtor which can re-time your video and insert the missing frame through very clever interpolation. It is designed to allow normal speed footage to be slowed down but I thought I would shoot a 4K clip on my GoPro Hero3 Black and see whether Twixtor could make a nice smooth 25fps real speed version of it.

The test footage was shot on a tripod so most of the image was not changing from frame to frame. The girl on the swing is passing across a complex background of leafy trees and a solid tree trunk. The rope tied to the tire is quite thin and had a tendency to warp with some settings. I experimented with a number of settings using Twixtor in After Effects (this is the trial Twixtor software hence the red cross) and this is the best solution I could come up with frame interpolation set to nearest.
Twixtor settings

Here is the treated footage and a link to the 1080p YouTube version.

So to me it is quite smooth and at first look, fairly acceptable, but looking at it more carefully the girl looks like she has been rotoscoped into the video and around her border there is a lot of blurry and incorrect pixels. Here are two screen grabs of an original frame followed by an interpolated Twixtor frame. 
Original frame
Twixtor interpolated frame
And here are crops of the above images
Original frame cropped

Twixtor processed frame cropped
So unfortunately using Twixtor to interpolate the 12.5 frames per second of the GoPro Hero3 in 4K is not going to be the solution to videographers dreams. But all is not lost, because the Sony sensor within the GoPro Hero3 is capable of proper full frame (4096x2160) 4K recording at frame rates of up to 60fps in 10 bit mode and 48fps in 12 bit mode. So GoPro may just be teasing us with the current "gelded" version and the next release may make the Canon 1DX C look like an over priced paper weight. Well a man can dream can't he?
Sony IMX117CQT sensor details

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