AfterDawn: Glossary

Group Of Pictures

In order to allow high Compression rates for digital video it may be encoded into GOPs, or Groups of Pictures, which consist of keyframes and delta frames. Keyframes are normal frames that consist of a complete picture, similar to a still image file format like JPEG. Keyframes may be followed by delta frames, which only include changes from the previous keyframe or even a previous Delta Frame.

Since delta frames are significantly smaller than keyframes, they typically make up the majority of a GOP. While this is helpful for keeping stream (file) sizes down, it makes editing more difficult since you're limited to cutting at particular frames to ensure every Frame kept can be decoded properly.

Frame Accurate Editing
Alternatively you can combine editing with re-encoding, or just re-encoding 1-2 GOPs to compensate for the missing information. Some video editors include either built-in encoding features or the capability to use an external encoder for this purpose. Others have no encoding capabilities and are limited in which frames must be kept.

The most common GOP-based video formats are MPEG-1, MPEG-2, and MPEG-4. Each uses the same convention for the three types of frames used. Keyframes are called I or Intra-Coded frames, in reference to the fact that they're self contained and require no other frames to be properly decoded. There are two types of Delta frames, P and B, which stand for Predictive-Coded and Bidirectional-Predictive Coded respectively. P frames rely on information from the prior I or P Frame to be decoded. B Frames are also delta frames, but use information from the previous I or P frame, as well as the following I or P frame, making them smaller than P frames.

GOPs With Multiple Keyframes
Some video formats allow GOPs to contain more than one Keyframe. While this doesn't affect the encoding quality, it does minimize the GOP header information, which is typically written only at the beginning of each GOP. It may also restrict the frames you can cut at with a GOP accurate editor.

GOPs With No Delta Frames
Although GOP based video isn't generally well suited to editing, some encoders can create video streams with only I frames. Although they will still be divided into GOPs, since each frame is self contained no re-encoding is necessary no matter where edits are made. However, the resulting file will be significantly larger than one encoded with delta frames as well.


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