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Extracting From A Green Screen – The LAB Trick

If youre shooting in a studio for compositing with Photoshop then you may be using a grey or green backdrop. I was spending a long time making masks until I saw this cool tip from Photoshop trainer Dave Cross, the time its saved me, thanks Dave!

Open the images

For this example Ive got two images, one of a wall and then one of a young lady listening to music, if youd like to work along theyre both available from Fotolia.

The wall is a black and white image, but Im looking to have the image in greyscale so this shouldn’t be an issue. It should be mentioned that if the final piece was going to be in colour I would choose a background with a little more colour in it.

Making the Mask

A Brief Visit To LAB

I saw Dave Cross do this a couple of years ago and its one of those tips that will alter your workflow once you know it.

With both layers in the same document hide the background by clicking on the eye icon next to it;

then head up to the menu and choose Image > Mode > LAB Color

This may not be a colour space that youre comfortable with but hang in there, were just briefly visiting. When youve got more than one layer Photoshop will ask if you want to merge them, well keep then un merged.

In the Channels panel we now have our Lab, a, b and Lightness channels. If youre unfamiliar with this don’t worry I think Ive got an idea for my next post!

All we need here is a copy of the a channel, so select it, right click it, and choose Duplicate Channel;

and rename it Mask

(You could drag it to the New Channel icon but youd get a an a Copy channel, good housekeeping is to name your layers and channels)

Back to RGB

Were done in LAB now so back to the menu and choose Image > Mode > RGB Color;

Again, we don’t want to merge

Were now back in familiar territory with a Red, Green and Blue channel, plus our Mask channel.

Lighten the Lights and Darken the Darks

We have a great starting point for the mask now but it needs a little refining.

With the Mask channel selected press Shift + Backspace to bring up the Fill dialogue box;

Choose White and Overlay from the drop down menus;

And press OK

Already you can see were getting a great mask!

Next press Shift + Backspace to bring up the Fill dialogue box again, this time choose black and Overlay from the drop down menus

Repeat the black one or two more times to get a good black and white mask.

Select the Mask

Once youre happy put your mouse over the thumbnail of the Mask channel and press the Ctrl (Pc) or Cmd (Mac) key. This will change the pointer to a finger with a dotted square and indicates that youre about to select the pixels on this channel.

Click to load the selection.

Add The Mask

To apply this to the image first click the RGB channel to bring the RGB layer back into play;

The Mask channel should be deselected, if it doesn’t you may need to manually do this.

Head back to the Layers panel and with the layer selected click the New Layer Mask icon at the bottom of the Layer panel;

Turning on the visibility of the Background layer shows us how well this worked, but theres still a little to do. I have some colour spill on her hair from the green screen, but thats where Mask Edge (just another name for Refine Edge) comes into play.

To access Mask Edge / Refine Edge open the Properties panel;

And click the Mask Edge button. (Double clicking the mask will bring the Properties Panel Up)

So there we are, a really quick and helpful LAB masking trick. Again, thanks to Dave Cross for first introducing me to this.

Finishing off

To finish this image Ive converted both layers to Smart Objects, I ran a Field Blur on the background to defocus it a little. For Layer 1 I ran Camera Raw filter to adjust the Exposure, Contrast and Highlights then tweaked this a little with a Levels Smart Filter.

Finally I added a 50% grey layer in Overlay and added a transparent to black radial gradient to make a vignette.

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