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GreenScreen_JokerPulling a key is easy; pulling a good key that will hold up to color correction and serious compositing takes more than a single click.

You want a matte with zero transparent pixels in the holdout matte and a great blend of semi-transparent edges around hair or sheer clothing under all gamma and exposure conditions.

If you ever had to pull a gradient screen and a tarp and a green cloth together, while keeping the shadow and utilizing core mattes, this is the place for you. Quick tips to make sure you get a solid matte – here we go!

Essential Options:

1) Screen Colour: Click somewhere close to your subject to sample the hue you want to make transparent. Keeping your sample close to your subject negates any screen lighting discrepancies which can be garbage matted later. Sample a few areas with the Alpha Preview on to judge the best sample (most black pixels around edges). Keylight_SampleRange

2. Screen Gain: Used to remove more values around the screen color sample. This is a sledgehammer. Use sparingly – I never go beyond 1.10. This is the fastest way to destroy your edges. I start with a very small amount to begin the edge seperation. The real tweaking is handled after this. Exception: if you are pulling a Core Matte, then far larger values should be used (See 6. Source Alpha below).

3. Clip Black and 4. Click White: These sliders will remap your back and white point in the alpha which gives you far more granular control compared to the Screen Gain. As with any keyer, any adjustment destroys existing pixel values – only use as much as minimally needed and no more. Naturally, only edge integrity should be considered – not garbage or holdout matte areas.

5. Screen Dilate: To grow or shrink your matte – I only use this when pulling a core matte to be added in later to an edge matte. Beware: adjusting this will destroy the aliasing in your edges! See 6. Source Alpha.

6. Source Alpha: Pulling multiple keys at the same time for hair or trouble spots is a great way to go. After pulling a core matte (strong Screen Gain, strong White Point), add to another Keylight keyer to isolate the focus and use a subtle approach on the second. Make sure your Source Alpha is set to Add to Inside Mask and your core key is set to Intermediate Result.

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7. InM Component: Rotoscope any matte holes (reflections, etc) as a holdout matte and plug in here. Make sure to set to Alpha and if you’re in Nuke, set your Project Settings appropriately! Rotoscope nodes default to the Clip to Format, not the size of your keying footage!

8. OutM Component: Rotoscope garbage matte elements here (lights, rigs, uneven screen defects, etc). See the important roto notes in 7 above.

Extra Tips:

What about spill? Notice I didn’t touch the Alpha Bias to suppress green spill or contamination.

If you’re using Nuke, the HueCorrect is a much better choice.Keylight_SpillSupression Sample the spill value, set your HueCorrection to g_sup (green suppression, not green), and bring your curve down to the 0 (transparent) range. You’ll find a much stronger suppression and will do little damage to other color ranges in your source footage.

Keylight_ColorCorrection

Always shoot in a higher resolution than the final result will need, and as high a color depth as you can (16bit or HDRI). For low light or desaturated footage, do an initial color adjustment before you start to help the Keyer.

Don’t worry what your key is doing to your RGB pixel source values – you’re only concerned about generating a great matte channel. I always shuffle my new alpha back into the original RGB pipe after the key anyway (this will also serve to negate any temporary color correction used earlier.

And of course never leave a background plugged into the node and leave the View on Composite. That leaves no room for additional color correction/animation/compositing on the individual elements if you merge it back to another background right away! The final Keylight should be left at Final Result in the View dropdown.

For many more tips on green screen shooting and keying, see my other pages at Visual Effects > Green Screen Studio Tips!

Watch for the advanced Keylight information pages coming soon!


Ciao!