Video compression picture types

The use of difference -coded frames ( predictive - coded picture ) is a method in digital video compression. There are individual pictures (English frame ), which are described on the difference to the previous frames ( a so-called delta encoding ), a type of so-called Inter- images. Unlike intra- frames to which are independently decodable, then a part of the image by reference to in another frame is described existing image data, thus resulting in a reduction of the required amount of data, which ultimately better compression rates can be achieved. A P-frame is thus part of intra- macroblocks (stand-alone, complete parts of the image ) and partly of reconstructed macroblocks, which are based on a predicted image signal. To predict to be coded video is analyzed and decomposed into objects with motion directions and speeds ( motion vectors ), which can then be predicted images. The use of prediction to groups result of interdependent images encoded with at least one independently decodable picture (I- picture) - the so-called group of pictures or English Group of Pictures ( GoP ). Newer methods such as H.264 can choose the prediction type of the image to partial images differently. Thus, the so-called H.264 P slices Example result on macroblock basis.

P pictures found in most major video codecs use.

Disadvantages

When decoding the required previous images must be kept. This results in the need for larger buffer. For the decoding of a P- picture, the whole chain coherent set of images since the last I-frame has to be decoded, which prepares the jump to a desired position in a file or the entry into a stream or problems leading to delays. Often, therefore, a minimum frequency of I-frames is enforced, at least.