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AfterEffects 0003 - Using Alpha Channel Masks

This tutorial will take you through the steps of creating a movie with an Alpha channel mask in AfterEffects.

You will need to download this small archive containing a PICT with an Alpha Channel and a small movie clip.

ae0003-archive.sit --- 1.1MB

1. Open AfterEffects and create a new composition. Set the dimensions to 320 x 240 and the fps to 15. Set the duration to 5 seconds.

2. Go to the FILE menu and choose IMPORT > Footage Files. Locate the ae0003-archive folder and import leaves.mov and mask.pct. A dialog will appear when you import the mask file. Make sure that the alpha setting is set to "Treat as Straight (Unmatted)".

3. Drag leaves.mov from the project list to the comp window.

4. Drag mask.pct to the comp window. You should be able to "see-through" the mask image. The color black is set as transparent in the Alpha Channel. You can invert the image to make other areas appear translucent.

5. Save your file. Preview the movie to see the result. Pretty cool, huh?

6. Now we need to make our movie. Go to the COMPOSITION menu and choose Make Movie. Name your movie "ae0003.mov". Click the OK button and you will get the Render Queue window. Click the item marked "Lossless". This opens the Output Settings window. Click the Format Options button and set the compressor to Video. Click the OK button in that window, and the OK button in the Output Settings window.

7. Click the Render button to render your movie. When it is done, open the movie in the QuickTime Movie Player. Very cool!

The variations to this are endless, depending on the mask image you choose! Try a bunch of different images over this same movie clip.

To create an image with an alpha channel in Photoshop:

1. Open the image.

2. Select All > Copy.

3. Open the Channels palette (usually the second tab in the layers palette). Create a new channel by clicking the New icon on the bottom of the palette.

4. Paste the image into the channel.

5. Save the file as a PICT.

You can now import it with its alpha channel into both AfterEffects and Premiere.