Optical flow background artifacts
by Alfred Guzzetti
on
Jun 30, 2008 at 9:20:31 pm
I exported an HDV clip from FCP to Motion to slow it down 25 per cent using Optical Flow. When a foreground figure crosses in front of a bright vertical pole in the background, the pole distorts. Is there a solution to this kind of artifacting?
Alfred Guzzetti
Mac Pro Quad 2.66GHz 4GB RAM
Intensity Pro card
OS 10.4.9
FCP 6.0
Re: Optical flow background artifacts by Alfred Guzzetti on Jul 2, 2008 at 12:21:33 am
Thanks again. I tried this but it led to the same artifacting. I'll try capturing the HDV clip using ProRes via HDMI (rather than doing a software conversion) and report back.
Alfred Guzzetti
Mac Pro Quad 2.66GHz 4GB RAM
Intensity Pro card
OS 10.4.9
FCP 6.0
Re: Optical flow background artifacts by Adam Wilder on Jul 9, 2008 at 11:14:15 am
I don't think this is a codec problem. What's most likely happening is that your subject is crossing this pole fairly quickly and what you're seeing is the result of the blending created by the Optical Flow algorithm. For example, take a look at this clip showing the results of optical flow.
There are two areas where I think you'll see the type of artifact you are describing. The first is at about 52 seconds. Look at the arm. The second is at about 1 minute, when the frisbee crosses over the logo on the t shirt. In both cases you have significant changes between a short number of frames, and now you're trying to blend and stretch the data.
Lets say that in real time, there are 3 frames for the frisbee to cross the logo on the shirt. Frame 1, before it crosses, Frame 2, covering the logo, and frame 3. When you slow that down 25%. you're now generating 12 frames and only three of those are "real" the rest are interpolated and since you don't have a lot transition information (example: 2 more frame where the frisbee partially covers the logo) in those areas you sometimes get these artifacts. Optical flow is working correctly with the information it has.
The only way you could maybe minimize the results would be to use a different speed, faster or slower (oddly enough sometime slower fixes the problem). You could also try duplicating the clip, use a different frame blending and mask it over the optical flow clip.
