Intra-coding works in three dimensions on the horizontal and vertical spatial axes and on the sample values. Analysis of typical television pictures reveals that whilst there is a high spatial frequency content due to detailed areas of the picture, there is a relatively small amount of energy at such frequencies. Often pictures contain sizeable areas in which the same or similar pixel values exist. This gives rise to low spatial frequencies. The average brightness of the picture results in a substantial zero frequency component. Simply omitting the highfrequencycomponents is unacceptable as this causes an obvious softening of the picture.
A coding gain can be obtained by taking advantage of the fact that the amplitude of the spatial components falls with frequency. It is also possible to take advantage of the eye’s reduced sensitivity to noise in high spatial frequencies. If the spatial frequency spectrum is divided into frequency bands the high-frequency bands can be described by fewer bits not only because their amplitudes are smaller but also because more noise can be tolerated. The wavelet transform (MPEG-4 only) and the discrete cosine transform used in JPEG and MPEG-1, MPEG-2 and MPEG-4 allow two-dimensional pictures to be described in the frequency domain.
Inter-coding takes further advantage of the similarities between successive pictures in real material. Instead of sending information for each picture separately, inter-coders will send the difference between the previous picture and the current picture in a form of differential coding.
Figure 1.8 shows the principle. A picture store is required at the coder to allow comparison to be made between successive pictures and a similar store is required at the decoder to make the previous picture available.
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Figure 1.8: An inter-coded system (a) uses a delay to calculate the pixel differences between successive pictures.
To prevent error propagation, intra-coded pictures (b) may be used periodically.
The difference data may be treated as a picture itself and subjected to some form of transform-based spatial compression.
The simple system of Figure 1.8(a) is of limited use as in the case of a transmission error, every subsequent picture would be affected. Channel switching in a television set would also be impossible. In practical systems a modification is required. One approach is the so-called ‘leaky predictor’ in which the next picture is predicted from a limited number of previous pictures rather than from an indefinite number. As a result errors cannot propagate indefinitely. The approach used in MPEG is that periodically some absolute picture data are transmitted in place of difference data.
Figure 1.8(b) shows that absolute picture data, known as I or intra pictures are interleaved with pictures which are created using difference data, known as P or predicted pictures. The I pictures require a large amount of data, whereas the P pictures require fewer data. As a result the instantaneous data rate varies dramatically and buffering has to be used to allow a constant transmission rate. The leaky predictor needs less buffering as the compression factor does not change so much from picture to picture.
The I picture and all of the P pictures prior to the next I picture are called a group of pictures (GOP). For a high compression factor, a large number of P pictures should be present between I pictures, making a long GOP.
However, a long GOP delays recovery from a transmission error.
The compressed bitstream can only be edited at I pictures as shown.
In the case of moving objects, although their appearance may not change greatly from picture to picture, the data representing them on a fixed sampling grid will change and so large differences will be generated between successive pictures. It is a great advantage if the effect of motion can be removed from difference data so that they only reflect the changes in appearance of a moving object since a much greater coding gain can then be obtained.
This is the objective of motion compensation.
A coding gain can be obtained by taking advantage of the fact that the amplitude of the spatial components falls with frequency. It is also possible to take advantage of the eye’s reduced sensitivity to noise in high spatial frequencies. If the spatial frequency spectrum is divided into frequency bands the high-frequency bands can be described by fewer bits not only because their amplitudes are smaller but also because more noise can be tolerated. The wavelet transform (MPEG-4 only) and the discrete cosine transform used in JPEG and MPEG-1, MPEG-2 and MPEG-4 allow two-dimensional pictures to be described in the frequency domain.
Inter-coding takes further advantage of the similarities between successive pictures in real material. Instead of sending information for each picture separately, inter-coders will send the difference between the previous picture and the current picture in a form of differential coding.
Figure 1.8 shows the principle. A picture store is required at the coder to allow comparison to be made between successive pictures and a similar store is required at the decoder to make the previous picture available.
Figure 1.8: An inter-coded system (a) uses a delay to calculate the pixel differences between successive pictures.To prevent error propagation, intra-coded pictures (b) may be used periodically.
The difference data may be treated as a picture itself and subjected to some form of transform-based spatial compression.
The simple system of Figure 1.8(a) is of limited use as in the case of a transmission error, every subsequent picture would be affected. Channel switching in a television set would also be impossible. In practical systems a modification is required. One approach is the so-called ‘leaky predictor’ in which the next picture is predicted from a limited number of previous pictures rather than from an indefinite number. As a result errors cannot propagate indefinitely. The approach used in MPEG is that periodically some absolute picture data are transmitted in place of difference data.
Figure 1.8(b) shows that absolute picture data, known as I or intra pictures are interleaved with pictures which are created using difference data, known as P or predicted pictures. The I pictures require a large amount of data, whereas the P pictures require fewer data. As a result the instantaneous data rate varies dramatically and buffering has to be used to allow a constant transmission rate. The leaky predictor needs less buffering as the compression factor does not change so much from picture to picture.
The I picture and all of the P pictures prior to the next I picture are called a group of pictures (GOP). For a high compression factor, a large number of P pictures should be present between I pictures, making a long GOP.
However, a long GOP delays recovery from a transmission error.
The compressed bitstream can only be edited at I pictures as shown.
In the case of moving objects, although their appearance may not change greatly from picture to picture, the data representing them on a fixed sampling grid will change and so large differences will be generated between successive pictures. It is a great advantage if the effect of motion can be removed from difference data so that they only reflect the changes in appearance of a moving object since a much greater coding gain can then be obtained.
This is the objective of motion compensation.
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