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Flash Video 101: Using Alpha Video

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Flash 8 now supports alpha transparency, which lets you encode video with the background removed so you can overlay the subject of the video on top of other Flash content. In this article, author James Gonzalez gives you the lowdown on this new Flash video capability. If you're a Flash developer interested in gaining a basic knowledge about adding an alpha transparency layer to your video and incorporating it into your applications, this article is for you.
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Flash’s video capabilities have improved dramatically with the last several versions of the application. This is particularly true with Flash Professional 8. Two new Flash Professional 8 features (in particular, the FLVPlayback component and support for an 8-bit alpha channel), have opened up a wealth of new possibilities for Flash animators and developers. In the past, any effort to add Flash "alpha" transparency to your video clips had to be hacked together—with less-than-satisfactory results.

However, now with Flash Professional 8, all you have to do is link to a Flash Video (FLV) file that has an alpha channel already applied to it, and objects or animations can be made to show through the background of the video. The end result is vastly improved user experiences in your video-based Flash applications (see Figure 1).

Figure 1

Figure 1 An application of alpha video on the Adobe web site. The seated figure is alpha channel video composited in front of Flash animation, graphics, text, and a photograph.

In this article, I review this new alpha video technology. I discuss the basics of the technology, show you some simple applications, and also direct you to some additional resources and tutorials for learning how to create alpha channels in your source video clips. If you are a Flash developer interested in gaining a basic knowledge about adding an alpha transparency layer to your video and incorporating it into your applications, this article is for you.

About Alpha Channels

Color information in a video is contained in three channels: red, green, and blue. The video can also include an additional invisible fourth channel, called an alpha channel (See Figure 2). This additional channel contains transparency information.

Figure 2

Figure 2 Channels at a glance: A. Separated color channels; B. Alpha channel; C. All channels viewed together

The great thing about alpha channels is that they provide a way to store both images and their transparency information in a single file without disturbing the color channels.

When you view the alpha channel in After Effects or the Adobe Premiere Pro Monitor panel, white indicates complete opacity, black indicates complete transparency, and shades of gray indicate partial transparency.

This video alpha channel support in Flash 8 is now possible because of the incorporation of the On2 VP6 video codec in Flash Player 8. The On2 VP6 codec not only provides higher quality video when compared with the Sorenson Spark codec but it also enables you to encode an 8-bit alpha channel directly into FLV files.

To support better-quality video at the same data rate, the On2 VP6 codec requires noticeably more time to encode and, more importantly, also requires more processor power on the client computer to decode and play it back.

If you have done any video editing, you already know that creating alpha channels is a fairly straightforward process. All the popular video-editing applications, such as Adobe Premiere Pro and After Effects, Apple Final Cut Pro, and the Avid suite of products can create alpha channels. With the Flash Video Encoder (which works with any of these products), creating alpha channels ready for inclusion into a Flash project is as simple as selecting a check box during export.

I will show you this shortly, but let me first give you a better theoretical grounding of this technology.

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