Finding Precise Frames

A frame directory can help locate data in a compressed file because it can indicate a suitable key frame image to act as a starting point. However, there still might be a significant delay until a requested image is displayed. For example, a file might have a large number of interframe encoded images the device must decode before it can display them. In an extreme case, a motion video file might be entirely interframe encoded, with the exception of its initial image.

To handle delays associated with interframe encoding, the digital video command set includes the seek exactly flag for the set command (for example, set device name seek exactly on). With seek exactly enabled, a seek command, or a play command with the from flag, will go to the precise frame specified by the command. This can cause a significant delay as the device locates and decompresses the frame. With seek exactly disabled, the file will be positioned to a (optimally) nearby, previous key frame image. An application can determine the approximate location of the nearest preceding key frame image by using the status device name reference command. This is only approximate, because a frame directory might be required to efficiently determine the precise location of this image. The exact location can be determined by using the status device name position command after using the seek command. Because implementation of the seek exactly mode is tricky, especially with out of order algorithms such as MPEG, the default is device dependent.