Friday, August 12, 2011

Alpha Channels and Chroma Keys

For all my test animations and test stop motions...I have been using the chroma keyer in Final Cut Pro. I use my video camera to shoot my toys in front of a green screen, then I use software I dial out the green. But, I think I can get a better key if I shoot with my still camera, then delete the green background using photoshop. It's easy to get rid of the green using the magic wand. Then you have an image with a transparent background. The trick is keeping that background transparent once you save it. The only way I knew how to do this was to make animated GIFs. These take a lot of processing power, and I gave up on that a long time ago. But I remembered something about alpha channels that might work. I am going to write it down here so I can come back if I forget. I feel like I re-invent the wheel every couple of years when it comes to some of the obscure tricks in photoshop.

The way you maintain a transparent background in Photoshop using alpha channels:
1. double click on your layer in the layer palette to unlock it
2. click the "channels" tab next to the layer tab
3. click on the "new channel" button at the bottom (this adds the alpha channel)
4. turn the eyeball on all the channel layers
5. do a "save as" and make sure the "alpha channels" button is checked
6. import into Final Cut Pro

It seems like there should be a more intuitive way to do this. I would love to hear if anyone has a simpler method.

Here's a frame grab of my alpha channel test. There is still a lot of green around the pigs, but I did not light this correctly. Once I shoot everything in the studio, there shouldn't be any green outline.

I also taught myself how to export flash movies with transparent backgrounds. To do this using Flash:
1. Have an animation ready and select "export" then "export movie"
2. Choose the save as a QuickTime format
3. Click Save
4. make sure the "Ignore stage color (generate alpha channel)" is checked
5. Click export
6. Import file into Final Cut Pro

Here's a test of this technique:

My Thesis Film