In an MPEG video stream, an I Frame is a Full Frame. This Frame does not need any information from the frame before it or after it to be played back properly.

While most of the time each I frame is the beginning of a new GOP, in some cases GOPs can contain multiple I frames. This is rare, and only done to reduce the number of GOP headers written. Since the size of GOP headers is usually minimal compared to the video this isn't a common practice, especially since most standard video formats place limits on the size of a GOP.

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