P-frame
The use of differentially coded single images ( predictive-coded picture ) is a method in digital video compression . These are individual images (English frame ) that are described by the difference to previous individual images (a so-called delta coding ), a type of so-called inter-images . In contrast to intra-images , which can be decoded independently, a part of the image is described by reference to image data already present in another individual image, which results in a reduction in the amount of data required, which ultimately enables better compression rates to be achieved. A P-picture thus consists partly of intra-macroblocks (independent, complete picture parts) and partly of reconstructed macroblocks that are based on a predicted picture signal. For the prediction, the video to be encoded is analyzed and broken down into objects with directions and speeds of movement (movement vectors), which can then be used to predict images. The use of picture prediction results in groups of interdependent coded pictures each with at least one independently decodable picture ( I picture ) - the so-called group of pictures (GoP). Newer methods such as H.264 can also select the prediction type of the image differently for partial images. In the H.264 example, this results in so-called P-slices on a macroblock basis.
P-pictures are used in most major video codecs.
Image prediction classifies each image in a series of images into different image types:
- Intra Coded Picture (I-Frame) is an independent picture that does not need a reference to the other pictures in order to be decoded. It is the first picture in the series.
- Predictive Coded Picture (P-Frame) use information from previous I- or P-Frames for coding
- Bidirectional Coded Picture (B-Frame) uses information from both the previous and the following pictures (frames).
disadvantage
When decoding , the required previous images must be kept. This results in the need for larger buffers . To decode a P-picture, the entire chain of pictures that build on each other since the last I-picture has to be decoded, which causes problems or delays when jumping to any position in a file or entering a stream. Often at least a minimum frequency of I-pictures is therefore enforced.