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Concept
Video compression picture types
Summary
In the field of video compression a video frame is compressed using different algorithms with different advantages and disadvantages, centered mainly around amount of data compression. These different algorithms for video frames are called picture types or frame types. The three major picture types used in the different video algorithms are I, P and B. They are different in the following characteristics:
I‐frames are the least compressible but don't require other video frames to decode.
P‐frames can use data from previous frames to decompress and are more compressible than I‐frames.
B‐frames can use both previous and forward frames for data reference to get the highest amount of data compression.
Three types of pictures (or frames) are used in video compression: I, P, and B frames.
An I‐frame (Intra-coded picture) is a complete image, like a JPG or image file.
A P‐frame (Predicted picture) holds only the changes in the image from the previous frame. For example, in a scene where a car moves across a stationary background, only the car's movements need to be encoded. The encoder does not need to store the unchanging background pixels in the P‐frame, thus saving space. P‐frames are also known as delta‐frames.
A B‐frame (Bidirectional predicted picture) saves even more space by using differences between the current frame and both the preceding and following frames to specify its content.
P and B frames are also called Inter frames. The order in which the I, P and B frames are arranged is called the Group of pictures.
While the terms "frame" and "picture" are often used interchangeably, the term picture is a more general notion, as a picture can be either a frame or a field. A frame is a complete image, and a field is the set of odd-numbered or even-numbered scan lines composing a partial image. For example, an HD 1080 picture has 1080 lines (rows) of pixels. An odd field consists of pixel information for lines 1, 3, 5...1079. An even field has pixel information for lines 2, 4, 6...1080.
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