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) is a complete image, like a JPG or BMP 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. 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 preceding 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, one preceding and one succeeding.
In the H.264/MPEG-4 AVC standard, the granularity of prediction types is brought down to the "slice level." A slice is a spatially distinct region of a frame that is encoded separately from any other region in the same frame. I-slices, P-slices, and B-slices take the place of I, P, and B frames.
Typically, pictures (frames) are segmented into macroblocks, and individual prediction types can be selected on a macroblock basis rather than being the same for the entire picture, as follows:
Furthermore, in the H.264 video coding standard, the frame can be segmented into sequences of macroblocks called slices, and instead of using I, B and P-frame type selections, the encoder can choose the prediction style distinctly on each individual slice. Also in H.264 are found several additional types of frames/slices:
Multi‑frame motion estimation increases the quality of the video, while allowing the same compression ratio. SI and SP frames (defined for the Extended Profile) improve error correction. When such frames are used along with a smart decoder, it is possible to recover the broadcast streams of damaged DVDs.
See also: Key frame (animation) and Intra-frame
Often, I‑frames are used for random access and are used as references for the decoding of other pictures. Intra refresh periods of a half-second are common on such applications as digital television broadcast and DVD storage. Longer refresh periods may be used in some environments. For example, in videoconferencing systems it is common to send I-frames very infrequently.