Comp tricks?

Hi guys, quick one about this shot. It’s a screen replacement on the out of focus bg.
I’m fairly confident I could get there with a bunch of probably fairly unsophisticated methods - pixel spreading, yada yada - but is this an instance for the additive keyer hack? I’ve watched the videos but it seems they only apply when the screen is green? I was on set for this, I didn’t want to have a green cast over everything and I figured I’d just smash it with a heap of luma keying, and reverse pixel spreading on the plate and comp over the top. Any other ideas?

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Seems pretty straight forward. You could use min/darken as the blend mode.

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Yeah. Just start by grading your screen comp into the image.

Multiply would probably be a good start but if you can do that then you basically have your additive key.


The heart of any good screen comp is basically multiplying the plate over the tracked image. Track the image and pipe it out of action over black. Blur the image and its matte to match the focus of the plate. Divide the image by its matte to unpremultiply it. Pull out a color correct and in the front sample select a part of the screen on the plate without any reflections, usually near the center of the screen. This is important because the brightness of most screens falls off as you move from the center. Then gain up until the front sample is about 1 in the result. Multiply that color corrected plate over the unpremultiplied image. Then pull a key off the plate. I only use the 3d keyer and the masterkeyer but your plate might work with a luma key from the 2d histo. You want to push the key a little further than you might be used to. Don’t worry about anything in the key outside of the edges of the screen frame. You can negate the matte of the image, use a matte edge and combine it with the key to fill in the areas outside of the screen. Then use that matte to comp the plate back over your unpremultiplied image with the mult layer. Lastly add a color correct to your image at the beginning of the comp. View the result in context and slowly raise the offset in the color correct until the edges of the screen comp blend into the edges of the plate. Not sure if that makes any sense but its how I would approach this shot.


The background behind the screen is very clean. Therefore I would remove all of the bright ref. screen first, then add your insert. With that way you won’t have to struggle around matching 100% of the matte blur and the optical borders. Way faster than additive keying.

The peoples’ focus is not to blurry, so Pixelspread on stretch might be enough. (Depends on their movements)

Ugh, this is making my head hurt! Can you guys be a little more specific? “grade my screen comp into the image”, what does that even mean?

Ryland, I think i’m missing a vital step in your suggestion because it isn’t working… :frowning:

Applying a simple multiply comp gets great edges on their hair but of course wherever the insert is dark is no good. Are you suggesting roto?

Sorry, I know this is 101 but after 20 years I still struggle with blending functions and when to properly use them.

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Yeah that was my first approach, it hasn’t worked out :frowning:

No I was suggesting think less about trying to comp the image into the screen.

Use the footage to grade the bright grey screen to look like it has the footage inside of it.

Yes there will be roto. Use a luma key to get somewhere but don’t kill those good hair edges. That is the beauty of additive keying.

Now your FG already has light grey in it so roto to finish is your only hope.

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I was in the middle of writing this when Bettsy replied.

For posterity: When I grade stuff in I basically do something like this. It’s a bit complicated and more or less does the same as grading up bg and multiplying or grade up fg and multiply. Grade up bg and you get bright edges. Grade up fg sbd you get dark edges. But is useful sometimes.


Ooh. Nice approach @johnt

Works well for logos and pictures on walls. Better use for that than screen replacements.

Thanks for that guys, now I see what you’re actually trying to do, all these workflows are making sense :+1:

Am a bit late here, so you may have solved your shot, Unclextacy, but when I look at that plate it looks like a prime candidate for the Spotlight or Spotlight Blend transfer mode in Action (forgive me as it’s been a while since I’ve had a shot like this).

Spotlight (or Spotlight Blend) returns 100% of the foreground (to be inserted screen comp) surface when the background image is exactly 50% grey (i.e. 128/128/128 in 8-bit etc.)

Background values above 50% grey will progressively lighten the foreground plate and background values below 50% grey will progressively darken the plate.

This transfer mode is a great tool to use when you’re looking to retain fine luminance detail (hair and contact shadows) as long as it’s possible to equalize the blue/green screen to a roughly 50% luminance value which I do by making the image monochrome and using a Color Correct node to set the base value of the backing to 50% grey (and I will add additional CC nodes + Gmasks to counteract lighting falloff or lens vignetting in the interest of making a best-possible Spotlight / Spotlight Blend plate).

Once I get fine detail with a Spotlight / Spotlight Blend surface I use a 3DKeyer node to pull a tighter matte and Blend that on top of the Spotlight / Spotlight Blend surface to fill the composite.


Spotlight is interesting. Not one i use nearly enough but i will give it some more attention.
I have a couple of phone comps like this coming up. I usually give Overlay a crack but I’ll see what Spotlight gives me.

I tried this today and it worked so, so well! Thank you!!