Creating Perfect Alpha Channels

When do you make an alpha channel? Whenever you want to embed the transparency data into the flattened file. You worked hard for that transparency. Why throw it away? Think of an alpha channel as a stencil or mask. Most nonlinear editing systems support some form of real-time alpha keying. It's a good idea to use it.

Now you're probably thinking, "My NLE supports Photoshop layers and transparency. I don't need an alpha channel." Wrong. Support for layers and transparency is great, but you will not get a perfect import. Even if your layer effects, grouping, and blend modes make it in (they won't, by the way), do you really want a five-layer lower third? Talk about hogging tracks in the timeline.

I'm happy to have layer support, but I still pick a single, streamlined file 90% of the time. Sometimes you will want to animate or manipulate layers within the NLE, but that's why you always keep a layered design file to go back to. Proper use of alpha channels lets you change part of a composition without having to redo all of your work.

What are Alpha Channels?

Alpha channels grew out of work done at the New York Institute of Technology back in 1977. The goal was to embed transparency data directly into each file to cut down on rendering. The name alpha was chosen because it is the part of a mathematical equation that represents blending between composited images. The embedded alpha channel eliminated the need for a separate traveling matte. After Effects users should consider embedding alpha channels like Photoshop users do. Embedded mattes reduce the need for two-step rendering and eliminate the possibility of a misaligned matte.

Video #9 Perfect Alpha Channels

For more on alpha channels, don't miss the video tutorial on the DVD-ROM.

Creating a "Perfect" Alpha Channel

Making a perfect alpha channel starts with having an active selection loaded. One easy way to create a perfect selection is to use a "targeted merge" approach. If you have not already extracted objects to their own layers, use the techniques described earlier in the chapter. Once everything is on extracted layers, you need to create a composite layer to generate the selection.

Step 1. Open the file Ch04_Lower_Third.psd from the chapter folder on the DVD-ROM.

Step 2. Turn visibility icons off (eyeballs) for all layers that should not be part of the alpha channel.

Step 3. Create a new layer by clicking on the new layer icon at the bottom of the Layers palette and highlight it.

Step 4. Hold down the (A ) and choose Merge Visible from the Layers palette submenu.

Step 5. Now you have only one layer to load. B-a+click (^0+click) on the composite layer's thumbnail to create the marching ants. You can now throw this layer away.

Step 6. Switch to the Channels palette and click the Save Selection as Channel icon (second from left). Make sure you have only one alpha channel for your document (if you have more than four channels in an RGB+ Alpha image, you have more than one alpha channel).

Step 7. Save your native .psd file.

Step 8. Choose File>Save As..., and choose a format supported by your NLE. The most common formats are PICT and Targa. Make sure the Alpha Channels and As a Copy boxes are checked.

Photoshop used to have the Save a Copy menu option. This is actually the preferred method for saving a flattened file, and still saving and preserving your .psd file. The keyboard command still works. Choose (X+O+© (L+©+©) and you will automatically be in Save a Copy mode. Save a Copy is the preferred a good alpha channel starts with a good method because it allows you to save any changes you make, first selection. With the selection active, into the flattened production file, and then in the layered .psd click on the Save Selection as Channel design file (upon closing the document). Save the flattened file button in the Channels palette. Save to the appropriate location. Then, when you close your layered your flattened file with the alpha channel .psd file, tell it to save changes. That's it; no voodoo magic. Have a attached and you're done! selection made, switch to the Channels palette, and save it.

Premultiplied versus Straight Alpha Channels

You have to make some choices in Photoshop if you are saving out flattened files with alpha channels. It's important to decide between premultiplied or straight alpha channels. This becomes an issue when you have soft edge effects, especially drop shadows and glows.

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The alpha channel stored the transparency information as an 8-bit or 16-bit file. This grayscale file contains levels of opacity. The stars in the accompanying figure have a yellow glow applied to them. The star on the left is over black, a common background color used by video pros when building graphics. The star on the right (it's really there) has been placed over a background that is the same color as the glow.

Even Faster with Actions

^ If you load the Video Actions that come with the built-find the alpha channel action saved for you. I created this for Adobe and it automates the above steps. Just open the Actions palette and click on the palette submenu to load them. The instructions are built in when you run the action.

The star on the left is premultiplied; the glow is composited over black. The star on the right cannot be seen without enabling the alpha channel. The right star will generate better results when keyed over a video source.

When we create an alpha channel for these files, the shape is identical. The star over black, however, is considered premulti-plied, a benefit when working in Photoshop so that we can see our work. The star on the right contains a straight alpha—that is, the alpha contains the glow info, but the background is a solid color that will become the glow when the image is keyed.

Premultiplied alpha channels will often pick up noise from the background layer. This is evident in the left star by the black fringe in the outer glow.

Notice how the premultiplied alpha picks up black around the edges. This is a subtle but nasty problem. It will show up when you have light-colored glows or drop shadows. The second star, with a straight alpha, keys perfectly with no color contamination. To create a straight alpha channel you must do one of the following:

• Place a solid color behind the objects that matches your glow or shadow.

• Place a copy of the photo behind the masked layer. This will allow the soft edges of the image to blend with the colors in the photo.

It is a limitation that Photoshop cannot generate a straight alpha channel when saving. While some programs such as Adobe After Effects and Apple Final Cut Pro will try to automatically compensate for a premultiplied alpha channel over a black or white background, it is not a perfect process. While it is an extra step to create a straight alpha, it's worth doing before saving your flattened files for video.

For Best Results with Alpha

Channels

• Have only one alpha channel per document; delete unused channels before saving.

• Choose a 32-bits-per-pixel alpha channel.

• Do not use any compression when saving your file.

• Do not check the Save Transparency box in the TIFF dialog box as it cancels out your Embedded Alpha channel.

Understanding Adobe Photoshop Features You Will Use

Understanding Adobe Photoshop Features You Will Use

Adobe Photoshop can be a complex tool only because you can do so much with it, however for in this video series, we're going to keep it as simple as possible. In fact, in this video you'll see an overview of the few tools and Adobe Photoshop features we will use. When you see this video, you'll see how you can do so much with so few features, but you'll learn how to use them in depth in the future videos.

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  • Leah Fried
    How to make Embedded alpha channel in video?
    8 years ago

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