Edge details turns black on key

Ive got to key in a dark BG into a fairly well lit GS shot. A problem i am having are the edges on some of the hair and car seat seem to subtract rather than add over the BG if that makes sense?

Ive tried many different keyers even in Nuke and it seems to be the same issue.

Any ideas how i could solve it?

soft matte/paint in a colour over the offending areas

burn, multiply offending areas


Im really stuck on these keys. I dont suppose anybody is available for a fee to just give me a little bit of help? Once i get one sorted im sure it will be ok


There’s so many issues that could be contributing to this. One thing that I find helps with edge issues is to use a luminance curves-based comp node for the problem area. In Flame, it’s called the Matte Curves node and in Nuke it’s called Addmix. There’s some directions on using the Flame one that I’ll link below but, frankly, the way I use it to just connect the inputs and just play with the curves until I have something that looks ok.

Aside from that, some other ways to deal with key issues: use your key as a matte to grade up/down the background in the area where you’re having an issue. Make sure the problem you’re having is actually in your comp node and not from some other distortion/lighting effect that’s part of your setup.

Possibly, using IBK might help.

Also, if you’re doing something like keying and comping everything with denoised plates (as you should be), the method you’re using to put grain back might be causing the issue.


This is the issue ibseem to be getting. It recognises the hair but subtracts when compee.

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ah, I see. That does look pretty nasty.

I would suggest keeping your key and front image separate. Just take the matte output from your key and use that as the matte input in your comp node. And then take your image and pipe that into the front. You’re going to need to do some grading, despill and edge extension on the image, it looks like.

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Try comping log not acescg?


Two possible reasons.

  1. You have negative values in your matte.

  2. Using IBK your green subtraction is making the super saturated green hair black as it subtracts. Try another green suppression method.


I’ve been using the MK for suppression but even without it I seem to be getting the exact same issues.

This was from the IBK. Generated a clean screen but the difference was minimal.

I denoised the plates in neat video but the noise is very heavy in the blue channel compared to Red and green, could this be causing issues I wonder?

I don’t know if NeatVideo is causing this problem. I doubt it but it is interesting that you noticed the blue channel has more noise. This is common and one of the excuses I use for shooting against green rather than blue (sometimes).
It is caused by the uneven distribution of Red, Green and Blue photosensors on a camera image sensor.

This will probably not help you with this black edge but it is mildly interesting. When you solve the edge problem look up Bayer Filter

The advantage of testing this out using a log colourspace (Arri Wide Gamut Log C) as @randy mentioned is that it should (ACEScc and ACEScct still have negative values) remove all negative values.

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U tried the crok additive keyer, enable Joel’s version? It’s out of focus enough it could work

Hi Joel

I did try the additive but can’t seem to pull the dark BG through. I always thought it was more for brighter plates?

not sure what color space you’re using but it looks like negative values what happens when you put a clamp node on the front and back see what that looks like , this will prevent negative values, but its just a test and don’t keep it that way but it will let you know if that’s what’s causing the problem, from the look of the front back it appears to be two separate color spaces and you have a transform happening creating negative values.


The traditional one yes, the Joel version just loves out of focus no difference to light or dark

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They are both ACES. The shots i took were exposed up to show the issue a little more

I’ll give it another go but it didn’t seem to come through at all.

So have we ruled out negative values? Like if you take the color sampler and put it over those regions in your matte (and/or your rgb of your comp) and everything is above 0?

Maybe seeing a screenshot of just the matte output would help as well?

Hi Joel

Is there a specific way i need to set up the inputs as i just done get anything coming through unfortunately.

All you need is FG original/input 1, FG despilled /input 2 and back / input 3.
After you enable Joel’s version play around with the 3 sliders to get something going