Chroma key – problem with shimmering in hair

[Steve Bentley]

Been looking at the footage.

I think you are suffering from the quality of the camera and the low bandwidth of the codec.
Even though the footage appears to be 50fps progressive, there seems to be an interlace “cover up” going on there. Some cameras only shoot interlaced video or a goofy pulldown to give it more of a cinema look, and then do a wonky pulldown maneuver to simulate progressive. If you watch the highlights in the eyeballs they are clean for a few frames and then there is this duplication that happens for a few frames (the highlight suddenly has two iterations of itself in two slightly different positions) and then the pattern repeats. So even before you get your file out of the camera, the camera is already mangling the image. To be fair, if this is whats happening, its a pretty good conversion, but its going to be part of the jitter problem along the hair. It would have been better though to shoot with pulldown, take the pulldown out manually, do the key and then put the pulldown back in if needed.

The footage is pretty noisy too, there’s the sensor noise but there is also the codec noise which makes details kind of jump around (the noisy matte along the hair line).

Still playing with it here.

[Steve Bentley]

If you don’t need the final at 50fps (which I doubt you do) bringing it down to 25fps will half the “boiling” of the hair (yes, that is the technical term for the problem, really!)
Still playing

[Steve Bentley]

Here’s a go at it with native effects.
First, in this image you can see the problem with lighting. There is a halo where the keyer has been tuned to the color right around the head but then another key must be done for the area near the frame edges where the green is darker. I have not bothered to do that in this comp and will leave that for you.

We’ve tried all sorts of tricks but it comes down to simply crushing the matte to get rid of the of the jittering edges. Noise is noise (especially compression noise). As helmet heads go this one’s not too bad, but its not quite there in my books.
Here’s the comp (AE2017)and a final output of 5 seconds worth.

The other issues with crushing the key are that you get what we call keyholes: areas where two objects come close together and the matte tends to close up before they actually touch (his hand on the photocopier for instance) or you get rounded corners instead of sharp ones (his armpit). This is solved by doing one key for the head that gets crushed as I’ve done here, and then another for areas that are not as noisy but with less key crushing, and then another key for the outer darker screen areas.

12398_cowgreenscreenproblem.aep.zip

12399_test2264.mov.zip

[Karen Pouye]

I was going to go for white after I saw how clean it looked, but a colored background may be smarter in this instance to maybe hide some of the weakness. I did another video previous to this, where the hair caused the same problem and I used a background to distract, but it didn’t really achieve much.
I’ll find a background for you anyway.

[Karen Pouye]

I used a Sony VG20 which is a semi-pro DSLR video camera. I agree that there is a fair amount of noise. I found that when I was shooting, I had to flood the subject with considerable light because the camera is very poor in low light and the lens that is provided is a slow f4 zoom lens. I even had to resort to using gain because the 4 lights I used on the foreground were not enough (3 X 1200 LEDs + a red head with softbox). The green screen was independently lit with fluros.
Sony claims that because the sensor is so big, that it makes up for the slow lens…but I beg to differ. I can resort to using my fast prime lenses but they have no zoom.

[Karen Pouye]

Thank you Steve for all your work. You managed to get the hair tamed down a lot more than I did. The rest of the issues are easily fixed… as you said by separate keys.

I think it comes down to the quality of the footage. I never thought about the 50fps….good thinking. I usually shoot in 25fps which may have helped avoid this previously.
Now that I know that it isn’t so much a keying issue I’ll focus on the capture again & try to improve that. I can resort to direct capture in 4:2:2, but it makes the process a bit more complicated & the files much bigger to handle.

One other possibility…my green screen lights are blue & my foreground lights red…could this cause weird effects do you think?
Again…thanks so much Steve for your help. I have spent days on this so your input & assistance is greatly appreciated.

[Steve Bentley]

Perhaps sony means that the bigger chip lets them get more tech in there, but on the surface: a bigger area to fill means you need more light to fill it (unless I missed the current administration cancelling the inverse square law).
If you shine a projector at the wall, the bigger you make the image, the dimmer that image gets.

But the problem remains: using that DSLR for regular shooting may be fine, but when it comes to FX work, often you need a better solution. Even in the film world we use a special stock (different from what they used in principle photog) when doing fx work to reduce grain so we can pull out the details we need. And when budget allows, even a special camera that lets us have a larger film frame, and therefore a smaller grain structure per square inch of image area when transferred to digital.

Then there’s the file fomat the camera spits out. I don’t think the camera noise is killing you (and we have noise killers that can overcome that), its the compression artifacts. When you get right down to it, compression artifacts are little areas of the image that are approximations of what should be there. If you can get that compression “block” small enough you end up with pixels: as close as you can get to an accurate rendering of the original. But as the blocks get bigger (and to keep bandwidth low the blocks are always bigger) they just average what they think is important. And that averaging happens every frame (or worse every, keyframe). The idea is: don’t change the pixels between keyframes if nothing in that area is moving. This is a great way to save drive space but a lousy way to have a clean image. So you end up with details jumping around as the system re averages what its seeing.
Often these cameras will have a digital out: you can plug in your computer or drive and capture or record a higher bandwidth master for VFX work. Even the big sony cameras compress the image to a tape. But the feed off the back is a glorious, nearly uncompressed, 10-16 bit version, far superior to what’s on the tape from the very same camera.
Rule of thumb: don’t digitize after the fact; capture live as uncompressed as you can.

[Steve Bentley]

My last post got delayed somehow but was composed before your last.
Re red and blue lights. Do you mean actually red and actually blue? or just their color temp (3500 k vs 6500k perhaps?)
The green screen looks good to me (save the uneven lighting, but often that is unavoidable, and easily dealt with as we have discussed, especially with no action at the edges and good garbage matte will fix the lighting flags etc).
Having more red on the talent is a good thing because that will increase the separation required from the green and also helps to offset the green spill from the screen itself. If the lights were used the other way around it might not turn out as well.
Double check for noise in the reflective surfaces like the photocopier on the left. Normally we would use the green sheen to push in color from the background but with the noise problem I would just crush the matte in these areas and let it be.
Large files are the cross we VFX types have to bear. The larger the file, the easier the key, so the time spent transferring files is saved in the composting.
The main issue is that compression. All other considerations seem to be well within specs for pulling a decent key.
More light though will reduce the voltage injected into the camera chip -which is what causes the noise – more light=less noise. Less noise coming from the camera also reduces the compression artifacts – the more noise the codec sees the more averaging it has to do.
But even though you might be able to get more lights, don’t overexpose – shoot for that color of green in camera – it’s pretty much spot on.
There are also some fallacies out there that the talent should be over or under exposed. Neither is true. The talent should be perfectly “exposed” as should the background screen. This old optical technician’s wife’s tale is probably from that optical era, when every step of the process added noise and contrast. (ah I remember it well – I love the smell of developer in the morning!)