Inside Macintosh: QuickTime Components

Types of Video Digitizer Components

Video digitizer components fall into four categories, distinguished by their support for clipping a digitized video image:

basic digitizers, which do not support clipping
alpha channel digitizers, which clip by means of an alpha channel
mask plane digitizers, which clip by means of a mask plane
key color digitizers, which clip by means of key colors

Basic video digitizer components are capable of placing the digitized video into memory, but they do not support any graphics overlay or video blending. If you want to perform these operations, you must do so in your application. For example, you can stop the digitizer after each frame and do the work necessary to blend the digitized video with a graphics image that is already being displayed. Unfortunately, this may cause jerkiness or discontinuity in the video stream. Other types of digitizers that support clipping make this operation much easier for your application.

Alpha channel digitizer components use a portion of each display pixel to represent the blending of video and graphical image data. This part of each pixel is referred to as an alpha channel. The size of the alpha channel differs depending upon the number of bits used to represent each pixel. For 32 bits per pixel modes, the alpha channel is represented in the 8 high-order bits of each 32-bit pixel. These 8 bits can define up to 256 levels of blend. For 16 bits per pixel modes, the alpha channel is represented in the high-order bit of the pixel and defines one level of blend (on or off).

Mask plane digitizer components use a pixel map to define blending. Values in this mask correspond to pixels on the screen, and they define the level of blend between video and graphical image data.

Key color digitizer components determine where to display video data based upon the color currently being displayed on the output device. These digitizers reserve one or more colors in the color table; these colors define where to display video. For example, if blue is reserved as the key color, the digitizer replaces all blue pixels in the display rectangle with the corresponding pixels of video from the input video source.

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