MPEG 1&2 Video Compression Idea: Predict the current frame - - PowerPoint PPT Presentation

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MPEG 1&2 Video Compression

Idea:

• Predict the current frame based on what was in the previous frame

(or possibly what is in the frames to either side).

• Subtract the predicted pixels from the actual values.
• Employ (essentially) JPEG compression on what is left.

Useful textbooks:

• B. Haskell, A. Puri, and A. N. Netravali [1997]. Digital Video: An Introduction to MPEG-2,

Kluwer Academic Publishers.

• J. Mitchell, W. Pennebaker, C. Fogg, and D. LeGall [1997]. MPEG Video Compression Standard,

Chapman and Hall.

• V. Bhaskaran and K. Konstantinides [1995]. Image and Video Compression Standards, Kluwer

Academic Press.

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Step 1

• Use a YUV or equivalent color space.

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Step 2

• Divide the current frame into macroblocks of 16 by 16 pixels.

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Step 3

• "Down sample" the two color planes by a factor of two in each

direction.

• So now we have a gray scale plane of n by n pixels and two color

component planes of dimension n/2 by n/2.

• That is, each macroblock has four 8 by 8 blocks for the gray scale

plane and one 8 by 8 block for each color plane.

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Step 4

• The first frame to be encoded is always an I-frame ("intra-frame"),

where its 8x8 blocks are encoded in a fashion similar to JPEG.

• P-frames ("predicted frames") can be encoded as follows:

For each macroblock X of the current frame, determine the macroblock Y of the previous frame that is most similar to X (e.g., least mean squared error) and send a pair of numbers to the encoder for the difference between its coordinates and that of the macroblock being encoded.

• B-frames ("bidirectional" frames) are encoded by predicting from

the left, the right, or averaging predictions from both the frame to the left and the right. Note: B-frames cannot be predicted from other B-frames; that is, a sequence of B-frames must be preceded and followed by and I or P frame. P-frames are always predicted from the closest previous P-frame or I-frame.

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Step 4 continued Typically I-frames appear at regular intervals in a pattern such as IBBPBBPBBI..., where B frames transmitted slightly out of order:

(Figure 8.6 of the Haskell, Puri, Netravali book)

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Step 5 For each block of an I frame and each predicted block in a P-frame or B- frame, compute the difference block X between the prediction and the

• riginal block and encode X in a fashion similar to the JPEG standard

(DCT followed by quantization followed by a combination of run-length and Huffman coding). The degree of quantization can be varied from frame to frame to accommodate a desired channel rate.

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MPEG1 vs. MPEG2: We have described what is common to MPEG1 and MPEG2. MPEG2 adds many new features, including:

• Interlaced video (frames can be divided into two fields consisting
• f the odd rows and even rows of pixels).
• A number of new prediction modes that involve fields (and in

some cases 8 by 16 blocks).

• Prediction at the 1/2 pixel resolution (by interpolation).
• A rich system level syntax that allows several video packet streams

mixed together.