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Picture types

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.

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Picture types

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  1. 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. tMyn

  2. 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. tMyn

  3. 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. tMyn

  4. 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 tMyn

  5. 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 tMyn

  6. 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 tMyn

  7. 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. tMyn

  8. FORWARD PREDICTION PICTURE DISPLAY ORDER 1 2 6 3 5 7 4 4 2 6 1 3 7 5 PICTURE TRANSMIT ORDER BI-DIRECTIONAL PREDICTION BI-DIRECTIONAL (B) PICTURE PREDICTED (P) PICTURE INTRA (I) PICTURE Figure 1. MPEG-2 I-, P and B pictures. tMyn

  9. 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. tMyn

  10. 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. tMyn

  11. 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. tMyn

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