6.5 Loop filter

6.5 Loop filter

At low bit rates the quantiser step size is normally large. Larger step sizes can force many DCT coefficients to zero. If only the DC and a few AC coefficients remain, then the reconstructed picture appears blocky. When the positions of blocky areas vary from one frame to another, it appears as a high frequency noise, commonly referred to as mosquito noise. The blockiness degradations at the slant edges of the image appear as staircase noise. Figure 6.13 illustrates single shots of a CIF size Claire test image sequence and its coded version at 256 kbit/s. The sequence is in colour, with 352 pixels by 288 lines at 30 Hz, but only the luminance is shown. The colour components have a quarter resolution of luminance (176 pixels by 144 lines). As can be seen at this bit rate the coded image quality is very good with no visible distortions.

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Figure 6.13: Picture of Claire (a original) (b H.261 coded at 256 kbit/s)

At lower bit rates, artefacts begin to appear. This is shown in Figure 6.14, where there are more severe distortions at 64 kbit/s than at 128 kbit/s. When the sequence is displayed at its normal rate (30 Hz), the positions of the distortions move in different directions over the picture, and the appearance of mosquito noise is quite visible.

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Figure 6.14: H.261 coded at (a 128 kbit/s) (b 64 kbit/s)

Coarse quantisation of the coefficients which results in the loss of high frequency components implies that compression can be modelled as a lowpass filtering process [3,4]. These artefacts are to some extent reduced by using the loop filter (see position of the loop filter in Figure 6.3). The lowpass filter removes the high frequency and block boundary distortions. The same pictures with the use of a loop filter are shown in Figure 6.15.

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Figure 6.15: Coded pictures with loop filter (a 128 kbit/s) (b 64 kbit/s)

Loop filtering is introduced after the motion compensator to improve the prediction. It should be noted that the loop filter has a picture blurring effect. It should be activated only for blocks with motion, otherwise nonmoving parts of the pictures are repeatedly filtered in the following frames, blurring the picture. Since it is motion based, loop filtering is thus carried out on a macroblock basis and it has an impulse response given by:

(6.2) 

for pixels well inside the picture. For pixels at the image border, or corners, another function may be used. Figure 6.16 shows an example of the filter response in these areas.


Figure 6.16: Loop filter impulse response in various parts of the image

The loop filter is only defined for H.261 (no other video codecs use it) and is activated for all six DCT blocks of a macroblock. The filtering should be applied for coding rates of less than 6 × 64 kbit/s = 386 kbit/s and switched off otherwise. At higher bit rates the filter does not improve the subjective quality of the picture [3]. MPEG-1 does not specify the requirement of a loop filter, because pictures coded with MPEG-1 are at much higher bit rates than 386 kbit/s.



Standard Codecs(c) Image Compression to Advanced Video Coding
Standard Codecs: Image Compression to Advanced Video Coding (IET Telecommunications Series)
ISBN: 0852967101
EAN: 2147483647
Year: 2005
Pages: 148
Authors: M. Ghanbari

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