Keying out a static object in a static background with a moving subject

[dimyanah]

Alright so I'm still at a beginner level with after effects, but have a pretty decent understanding of it. 

I have a talking head bit where there are doorknobs in the background that are distracting. The subjects hands pass over it a quite a few times, otherwise I would just use a clone stamp or rotoscope it. 

I tried a difference key and no success. I'm not really familiar with any other keying besides extract, but  it doesn't really help me here. I would really appreciate any suggestions of if someone can point me in the right direction. 

[Mylenium]

Not really. Too much motion blur and unfortunately not enough color contrast. Everything is pale-pink-ish, so extracting a matte e.g. based on the skin tones would be extremely hard and result in frizzy edges due to having to clip the thresholds aggressively. Conversely, any masking and tracking would likely not really help much because the feathering and shape would change on every frame. Rotoscoping such a nightmare shot by hand is something they only do for very expensive blockbuster movies. The only realistic halfway automated approach to this IMO would be a "motion (difference) key". That's not a functionality included in AE and would require buying plug-ins. The basic drill would be to extract the pixel motion vectors, manipulate their thresholds to clip out the unwanted parts and then also use this info to accumulate a clean version of the obscured pixels. Pretty advanced stuff and a costly proposition of buying RevisionFX' RSMB Pro and some of their other plug-ins or using an advanced compositing tool like Nuke. So you see, all you options are kind of bad. Therefore the only sane advice would be to just leave it as is. Perhaps you can disguise it with some graphical fancy like e.g. a gradient overlay fading from left to right with a suitable color correction. Likewise, a bit of scaling the image and repositioning it might do wonders.

Mylenium

[dimyanah]

Ah, I see. Thank you so much for your detailed response. - I learned a lot.

Quick follow up question. If I had a grey logo to cover the doorknobs, but
I still wanted it behind the talent, what would be the appropriate keying
method for that? Is that possible?
--
Dimyana Marie

[Rick Gerard]

What you want to do with that scene is called Rotoscope. AE has a tool called Rotobursh but I don't think it will work very well with that shot because of the motion blur, but it may be worth a try.

The technique involves layers. You need to create a matte that is just large enough to cover the foreground object, the actor's hand when it passes in front of the object you want to remove, the doorknob. When you get the matte created all you have to do is use the matte to cut a hole in the replacement image or poke a hole in the original footage so you can see the replacement image behind the original.

I would start by drawing a shape layer above the original footage using the pen tool. Pick Fill only and a color that complements the flesh tones so you can set the blend mode to something like screen and see through the layer. Something like this:

For this shot, the green is really easy to see through if you set the blend mode to Color. There is a nice contrast with the doorknobs. The path must be accurate only when the hands cross the doorknobs and for the rest of the time, the mask should be wider. Motion blur is required to match the motion blur of the hands. From your screenshot I needed about a 300º shutter in the Comp Settings Advanced Tab to get enough motion blur to match the hands. You don't have to keyframe every frame, just check the top and bottom movement and don't worry about anything where the doorknobbs are going to be replaced.

The next step is to create a patch over the doorknobs and insert it between the shape layer track matte and the footage. Just exporting a frame to a PSD when the doorknobs are uncovered is plenty. Put that replacement layer between the two and set it to take an alpha inverted track matte from the shape layer and you're done. That's the only reasonable way I know of to clean up the background. It's going to take a lot of careful keyframing to get it working really well and RotoBezier is probably the best choice for the path type. Set as few keyframes as you can, the doorknob patch should be just slightly bigger than the doorknobs, and only judge the final result when the footage is playing back. You'll never get every frame perfect but it should be doable.

 

In the future, if you think you are going to have to do some roto, use a much higher shutter speed. Motion blur is really hard to deal with in post but it's really easy to add back into a shot.