Offspeed playback in a video editing system of video data compressed using long groups of pictures

A motion video editing system, comprising:

• storage for making available data files containing video data encoded using long groups of pictures;
• an editing interface enabling a user to specify a sequence of clips of video data from the data files in the storage;
• a compressed data buffer into which video data from the storage is read and temporarily stored before playback;
• a streaming decoder having an input for receiving video data encoded using long groups of pictures from the compressed data buffer and an output for providing a stream of decompressed data for a sequence of images from the received video data;
• a stateless, single frame decoder, having inputs for receiving uncompressed data for at least one reference frame from video data encoded using long groups of pictures from a decompressed reference frame cache, compressed data for a single frame from the encoded video data from the compressed data buffer, and state information for decompressing the encoded video data and an output for providing decompressed data for the single image, wherein the stateless, single frame decoder does not retain reference frames extracted from the encoded video data from one decompression operation to the next;
• wherein the decompressed reference frame cache maintains uncompressed data for most recently used reference frames in a form usable by the decoder;
• wherein, in response to a request through the editing interface for playback of a plurality of images at frill frame rates, the streaming decoder is used to playback the plurality of images from the compressed data buffer;
• and
• wherein, in response to a request through the editing interface for playback of one or more images at an offspeed rate, the stateless, single frame decoder is used to separately decode and playback each of the one or more images from the compressed data buffer and using the decompressed reference frame cache;
• wherein the decoder determines if uncompressed data for a reference frame is in the cache, and the decoder decompresses compressed data from the buffer for the reference frame and updates the cache to include the uncompressed data if the uncompressed data is not in the cache.