Loads of you have been asking about shooting on green screen so here's a quick tutorial covering the basics of chroma keying with After Effects. I'll be showing you how to 'pull a matte' from your green screen shot with Keylight, then replace the background with another still image or video.
First things first: setting up the project.
Open up After Effects and click on "New Composition" on the Welcome screen
On the Composition Settings window, set up the resolution and frame rate to match your video. If you're using the JVC HM100 with our standard settings, you'll want to set this to HDV 1280 by 720 at 25 frames per second
After that click OK then import your video from the Film menu.
Next you will need to drag your video onto the viewer of your Composition. You'll see an outline of the frame to help you line up the footage.
The box will snap to the edge of the Composition to make sure its centered properly.
Now the Comp is set up, you're ready to add a keying effect to the video. Click open the Keying section of the Effects & Presets panel and find KeyLight (1.2)
Drag it onto your video strip in the timeline and let go. You should get the Keylight controls appear on the left of the screen.
Find the setting called "Screen Color" and click on the eyedropper next to the Black colour swatch. Use the dropper to select the green colour of the background. You need to pick a part of the screen with an average green tone, not over-lit or under-lit and not too close to the foreground subject otherwise you'll risk picking an off green colour which doesn't match the actual colour of the backing.
It looks like nothing has happened, but if you go to the View setting and pick "Screen Matte" from the menu, you'll see the alpha matte you've just made from the green screen plate.
This grey-scale image is what will make your video semi-transparent. Solid Black is completely transparent and lets the background show through, Solid White is fully opaque and shows the foreground image.
There are a couple of problems with the matte right now. Firstly there is some grey noise in the background especially in the corners. This is going to make the green screen show through a bit and discolour the background. Second, there are patches of grey on the actress particularly on her dress. Remember, anything but solid white is going to be slightly transparent so in the final shot you'll see the background showing through her body. To fix this, you'll need to increase the contrast of the matte image until the background is solid black and the actress' body is solid white.
Her shawl is trickier because its naturally transparent and needs to stay a shade of grey.
Keylight provides settings to control the matte's contrast. Click on "Screen Matte" to open the controls, then click open "Clip Black" and "Clip White"
Increasing the Clip Black will start to 'eat away' at the noise in the background. Turning down Clip White will start to fill in the grey regions of the dress. Its REALLY important that you adjust the sliders by the absolute minimal amount that gives a good result. If you over do it, you'll damage the matte image and end up with an ugly final composite.
Finally you should test the matte to check its giving good results. The best way to do this is by putting a coloured solid behind your subject. To add a solid, click on the matte image, then go to Layer -> New -> Solid
Make sure the size matches your original video resolution then pick a bright red with the colour swatch.
Click OK and the new Solid will appear on the timeline. You may have to re-order it in the layer stack to get it behind the video by using the Layer -> Arrange menu. Finally you'll need to set the View property of Keylight back to "Final Result".
Hopefully you should have a nice clean foreground subject on a solid red background. If not, go back to Keylight and adjust the settings until you get a good, clean key. Once you've done that, you can swap the Red Solid for a still image or video and you're done!
That's it for now. If you're trying a green screen shot and get stuck any of the DMW technicians will be able to help you out.
"Govida Close" Green Screen Plates are copyright Hollywood Camera Work
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