H.264/MPEG-4 AVC - Decoded Picture Buffering

Decoded Picture Buffering

Previously encoded pictures are used by H.264/AVC encoders to provide predictions of the values of samples in other pictures. This allows the encoder to make efficient decisions on the best way to encode a given picture. At the decoder, such pictures are stored in a virtual decoded picture buffer (DPB). The maximum capacity of the DPB is in units of frames (or pairs of fields), as shown in parentheses in the right column of the table above, can be computed as follows:

Where MaxDpbMbs is a constant value provided in the table below as a function of level number, and PicWidthInMbs and FrameHeightInMbs are the picture width and frame height for the coded video data, expressed in units of macroblocks (rounded up to integer values and accounting for cropping and macroblock pairing when applicable). This formula is specified in sections A.3.1.h and A.3.2.f of the 2009 edition of the standard.

For example, for an HDTV picture that is 1920 samples wide (PicWidthInMbs = 120) and 1080 samples high (FrameHeightInMbs = 68), a Level 4 decoder has a maximum DPB storage capacity of Floor(32768/(120*68)) = 4 frames (or 8 fields) when encoded with minimal cropping parameter values. Thus, the value 4 is shown in parentheses in the table above in the right column of the row for Level 4 with the frame size 1920×1080.

It is important to note that the current picture being decoded is not included in the computation of DPB fullness (unless the encoder has indicated for it to be stored for use as a reference for decoding other pictures or for delayed output timing). Thus, a decoder needs to actually have sufficient memory to handle (at least) one frame more than the maximum capacity of the DPB as calculated above.