Understanding Keyframes

Question: What are the differences between Indeo® video interactive and Indeo video Release 3.2?

The main differences are higher quality video, processor scalability, and new features designed for interactive multimedia applications. Indeo video interactive software uses a completely new compression algorithm, including advantageous features such as bidirectional prediction, to yield better image quality than Release 3.2, comparable to software MPEG. Indeo video interactive also encodes data at lower rates than Release 3.2, so more video of a given quality can fit onto a CD­ROM. Indeo video interactive also allows image quality to scale according to the processor power available. Finally, Indeo video interactive supports the development of interactive multimedia applications with features unavailable in Release 3.2. You can specify that portions of a video frame be rendered as transparent, or that only a portion of the frame be decoded. Users can change the brightness, contrast, and saturation of video files during playback. Key frames can be placed wherever they are needed, at arbitrary intervals. And video files can be protected from unauthorized access.

Question: What are the requirements for playback using local decode?

You can change the position of the viewport only at key frames. You can make the viewport smaller but not larger until the next key frame.

Question: What is the benefit of aperiodic keyframe access?

Aperiodic keyframe access improves compression and image quality by allowing keyframes to be placed at scene changes or edit boundaries. It also improves user access, as you can place key frames at any point in the file to which you wish users to be able to jump.

Question: What is bidirectional prediction used for?

Compression can be of two kinds: intraframe and interframe. Intraframe compression compresses the data within one frame relative only to itself. Key frames are compressed with intraframe compression because they must reconstruct an entire image without reference to other frames. Interframe compression compresses the data in one frame relative to others. These relative frames are called delta frames. If the information in delta frames is encoded relative only to previous frames, then that is unidirectional compression--the codec always looks back in order to decompress the video image. Bidirectional prediction is a form of compression in which the codec uses information not only from frames that have already been decompressed, but also from frames yet to come. The codec looks in two directions: ahead as well as back. This helps avoid large spikes in data rate caused by scene changes or fast movement, improving image quality. Bidirectional prediction adds a certain amount of playback overhead; for a clip without much movement, such as a talking head, it may not significantly improve visual quality.

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