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Picture types

• There are three types of coded pictures.• I (intra) pictures are fields or frames coded as a

stand-alone still image.• These are coded without reference to other pictures.• Moderate compression is achieved by reducing

spatial redundancy but not temporal redundancy.• They allow random access points within the video

stream.

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• As such, I pictures should occur about two times a second.

• I pictures also should be used where scene cuts occur.

• P (predicted) pictures are fields or frames coded relative to the nearest previous I or P picture, resulting in forward prediction processing.

• P pictures provide more compression than I pictures, through the use of motion compression, and are also a reference for B pictures and future P pictures.

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• B (bidirectional) pictures are fields or frames that use the closest past and future I or P picture as a reference, resulting in bidirectional prediction.

• B pictures provide the most compression, and decrease noise by averaging two pictures.

• Typically, there are two B pictures separating I or P pictures.

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• Compression performance could be something like:

Type I, Compression 7:1

Type P, Compression 20:1

Type B, Compression 50:1

An an average, Compression 27:1

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• Macroblocks in P pictures are coded using the closest previous I or P picture as a reference, resulting in two possible codings:

intra coding– no motion compensation

forward prediction– closest previous I or P picture is the reference

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• Macroblocks in B pictures are coded using the closest previous and/or future I or P picture as a reference, resulting in four possible codings:

intra coding– no motion compression

forward prediction– closest previous I or P picture is the reference

backward prediction– closest future I or P picture is the reference

bi-directional prediction– two pictures used as the reference: the closest previous I or

P picture and the closest future I or P picture

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• To enable backward prediction from a future frame, the coder re-orders the pictures from the natural display order to a ”transmission” (or ”bit-stream”) order so that the B-picture is transmitted after the past and future pictures which it references, Figure 1.

• This introduces a delay which depends upon the number of consecutive B-pictures.

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INTRA (I) PICTUREPREDICTED (P) PICTURE

BI-DIRECTIONAL (B) PICTURE

PICTURE DISPLAYORDER

PICTURE TRANSMITORDER

FORWARD PREDICTION

BI-DIRECTIONAL PREDICTION

Figure 1. MPEG-2 I-, P and B pictures.

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• A group of pictures (GOP) is a series of one or more coded pictures intended to assist in random accessing and editing.

• The GOP value is configurable during the encoding process.

• The smaller the GOP value, the better the response to movement (since the I pictures are closer together), but the lower the compression.

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• In the coded bitstream, a GOP must start with an I picture and may be followed by any number of I, P or B pictures in any order.

• In display order, a GOP must start with an I or B picture and end with an I or P picture.

• Thus, the smallest GOP size is a single I picture, with the largest size unlimited.

• Each GOP should be coded independently of any other GOP.

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• However, this is not true unless no B pictures precede the first I picture, or if they do, they use only backward motion compensation.

• This results in both open and closed GOP formats.• A closed GOP is a GOP that can be decoded without

using pictures of the previous GOP for motion compensation.