Pframe

Pframe, short for predicted frame, is a type of video frame used in many block-based video compression standards. A P-frame encodes only the parts of the image that differ from a previously decoded frame, typically the most recent I- or P-frame. The encoder performs motion estimation to find how blocks in the current frame can be predicted from blocks in a reference frame, records motion vectors describing those displacements, and encodes the residual difference between the actual and predicted blocks.

In practice, P-frames rely on temporal redundancy to achieve compression. The residual is generally transformed and quantized, then entropy-coded to reduce bitrate. This approach contrasts with I-frames, which are intra-coded and contain a self-sufficient image, and with B-frames, which can use reference frames from both past and future positions for prediction.

Within a group of pictures (GOP), I-frames provide random access and reset accumulated error, while P-frames contribute the majority of temporal compression by reusing data from previous frames. The exact rules for P-frames vary across standards such as MPEG-1/2/4, H.261/263, H.264/AVC, and HEVC, including how many reference frames may be used and how motion is estimated.

Advantages of P-frames include substantially reduced bitrate for moving content and efficient use of temporal redundancy. Limitations include dependency on reference frames, which can complicate error propagation if frames are lost, and higher computational load due to motion estimation. P-frames remain a fundamental element of modern video codecs alongside I-frames and, in some codecs, B-frames.