Flash Video Encoder User Guide

Flash Video Encoder User Guide > Understanding video standards and terminology > Keyframes

Keyframes

Keyframes are complete video frames (or images) that are inserted at consistent intervals in a video clip. The frames between the keyframes contain information on movement and scene changes that occurs between keyframes. For example, if a video depicts a person walking past a doorway, the keyframes contain the complete image of the person and the door in the background, and the interval frames contain information describing the person's movement as they walk in front of the door.

By default, Flash Video Encoder automatically determines the keyframe interval to use based on the video clip's frame rate. The keyframe interval value tells the encoder how often to re-evaluate the video image and record a full frame, or keyframe, into an FLV file. In Flash Video Encoder, this setting is the keyframe interval value, which represents the number of frames between keyframes. Flash Video Encoder approximates frames that reside between keyframes by estimating the full value of all pixels on the screen by comparing multiple frames and eliminating redundant information.

The keyframe interval value can be any number up to 100. When you select Automatic as an interval setting, Flash Video Encoder places a keyframe every two seconds of playback time. For example, if the video you're encoding has a frame rate of 30 fps, a keyframe will be inserted every 60 frames. In general, the default keyframe interval value provides a reasonable level of control when seeking within a video clip. If you need to select a custom keyframe placement value, be aware that the smaller the keyframe interval, the larger the file size.

If your footage has a lot of scene changes or rapidly moving motion or animation, then the overall image quality may benefit from a lower keyframe interval. In general, a higher keyframe interval produces better image quality because data is not wasted describing the areas of an image that remain unchanged from frame to frame.

An important dependency of the keyframe interval is that it affects the ability of Flash Player to seek (fast-forward or rewind) through an FLV file. Flash Player can only advance from keyframe to keyframe, so if you want to skip to different places and pause the frames, you must use a lower keyframe interval value. If you want to advance to every frame in an FLV file, use a keyframe interval value of 1. When you reduce the keyframe interval value, you must raise the data rate for the Flash video to maintain comparable image quality.

Flash CS3