5.11 Intra-coding

Intra-coding or spatial compression in MPEG is used in I pictures on actual picture data and in P and B pictures on prediction error data. MPEG-1 and MPEG-2 use the discrete cosine transform described in section 3.13. In still pictures, MPEG-4 may also use the wavelet transform described in section 3.14.

Entering the spatial frequency domain has two main advantages. It allows dominant spatial frequencies which occur in real images to be efficiently coded, and it allows noise shaping to be used. As the eye is not uniformly sensitive to noise at all spatial frequencies, dividing the information up into frequency bands allows a different noise level to be produced in each.

The DCT works on blocks and in MPEG these are 8 x 8 pixels. Section 5.7 showed how the macroblocks of the motion compensation structure are designed so they can be broken down into 8 x 8 DCT blocks. In a 4:2:0 macroblock there will be six DCT blocks whereas in a 4:2:2 macroblock there will be eight.

Figure 5.29 shows the table of basis functions or wave table for an 8 x 8 DCT. Adding these two-dimensional waveforms together in different proportions will give any original 8 x 8 pixel block. The coefficients of the DCT simply control the proportion of each wave which is added in the inverse transform. The top-left wave has no modulation at all because it conveys the DC component of the block. This coefficient will be...