Intra-frame coding


Intra-frame coding is used in video coding. It is part of an intra-frame codec like ProRes: a group of pictures codec without inter frames.
Intra-frame prediction exploits spatial redundancy, i.e. correlation among pixels within one frame, by calculating prediction values through extrapolation from already coded pixels for effective delta coding. It is one of the two classes of predictive coding methods in video coding. Its counterpart is inter-frame prediction which exploits temporal redundancy. Temporally independently coded so-called intra frames use only intra coding. The temporally coded predicted frames may use intra- as well as inter-frame prediction.
Usually only few of the spatially closest known samples are used for the extrapolation. Formats that operate sample by sample like Portable Network Graphics can usually use one of four adjacent pixels or some function of them like e.g. their average. Block-based formats prefill whole blocks with prediction values extrapolated from usually one or two straight lines of pixels that run along their top and left borders.
The term intra-frame coding refers to the fact that the various lossless and lossy compression techniques are performed relative to information that is contained only within the current frame, and not relative to any other frame in the video sequence. In other words, no temporal processing is performed outside of the current picture or frame. Non-intra coding techniques are extensions to these basics. It turns out that this block diagram is very similar to that of a JPEG still image video encoder, with only slight implementation detail differences.
Inter frame has been specified by the CCITT in 1988–1990 by H.261 for the first time. H.261 was meant for teleconferencing and ISDN telephoning.

Coding process

Data is usually read from a video camera or a video card in the YCbCr data format. The coding process varies greatly depending on which type of encoder is used, but the most common steps usually include: partitioning into macroblocks, transformation, quantization and entropy encoding.