Video compression picture types

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:

Three types of pictures (or frames) are used in video compression: I, P, and B frames.

An I‑frame (Intra-coded picture), a complete image, like a JPG or BMP image file. P and B frames hold only part of the image information (the part that changes between frames), so they need less space in the output file than an I‑frame.

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.

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. When video is sent in interlaced-scan format, each frame is sent in two fields, the field of odd-numbered lines followed by the field of even-numbered lines.

A frame used as a reference for predicting other frames is called a reference frame.

Frames encoded without information from other frames are called I-frames. Frames that use prediction from a single reference frame (or a single frame for prediction of each region) are called P-frames. B-frames use prediction from a (possibly weighted) average of two reference frames.

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