## MMPM/2 Device Driver Reference

[prev][next][parent][TOC]

### Playback Characteristics

While Ultimotion is a runtime scalable data stream, the magnitude that the data stream will "scale" is determined by the amount of information that is encoded in the data stream when it was created. That is, the amount of data placed in the stream at creation time determines the "maximum" playback characteristics that a particular stream can achieve. In turn, the processing capabilities of the playback system determine how much of the data can be processed and presented during playback. Therefore, the playback characteristics of a given video are a function of the data put into the video by the author and the processing capabilities of the playback system.

The factors affecting the data stream at creation time are:

Resolution - width and height of video
Frame duration - frequency at which frames are to be displayed
I-frame rate - frequency at which reference frames are to occur
Data rate - average amount of data allowed for an interval of video

Factors affecting the playback of a video are:

Processing power of the CPU
Throughput of data storage (for example, CD-ROM, hard disk, and LAN)
Efficiency of the display subsystem

Resolution Scalability

Resolution of a video determines how much spatial information is in a video image, as well as the "normal" playback size. Ultimotion video compression enables the output image size to be efficiently scaled up or down as it is decompressed, based on user preference or application requirements. Scaling down the output image size enables more efficient playback on slower systems.

Frame Rate Scalability

Frame duration for the frames in a video is usually referred to as frame rate. A frame compressed without regard to a prior frame uses intraframe compression and is called an I-frame. A frame compressed by considering only what has changed from a prior frame uses temporal compression and is called a delta frame. I-frames are a snapshot in time of the video. Delta frames show only what changed. The I-frame rate of a video is the frequency that I-frames occur in the data stream.

The I-frame rate affects several characteristics of a video stream:

Data rate
Quality of motion during scanning
Performance of seek operations
Ability to maintain smooth motion when frames are dropped to maintain audio synchronization
Image re-paint time in a windowing system where no backup saving is used during clipping

Since I-frames contain the compressed data of the entire image, they are typically larger than delta frames. Ultimotion data streams that mix I- frames and delta frames:

Reduce throughput requirements with smaller delta frames
Provide points in the stream at which the entire image exists (good for seeking and scanning)
Provide points in the video stream at which artifacts are repaired (with I -frames)

Generally, an I-frame rate of one I-frame every one or two seconds provides the best results.

Color Scalability

Ultimotion compression algorithms store images in a YUV direct color space. This color space is mapped at playback time to the number of colors available on the playback system. This avoids the need to have different versions of video content to fully exploit varying hardware capabilities.