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Appendix-Filter-Intra.md

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Filter Intra Prediction

1. Description of the algorithm

Filter intra mode involves generating intra sample prediction in a raster scan order based on a set of neighboring predicted samples. As an illustration of the basic idea consider the sample P shown in Figure 1 below. The intra prediction for sample P is the weighted sum of samples A, B and C. The latter could be reference pixels for the block and/or already predicted (i.e. filtered) samples within the same block.

filter_intra_fig0

Figure 1. Prediction of the sample P using the neighboring samples A, B and C.

The prediction procedure is outlined as follows:

  • The predicted block is divided into 4x2 sub-blocks. Example: An 8x8 block is divided into 8 4x2 blocks. See the example of the 8x8 block in Figure 2 below.

filter_intra_fig1

Figure 2. Splitting a block into 4x2 blocks.

  • For each 4x2 sub-block:

    • An array of 7 neighboring samples is prepared. For example, as shown in Figure 3, the 7 neighboring samples for Block_0 are reference samples for the 8x8 block. For Block_1, the above 5 neighboring samples are reference sample for the 8x8 block, the left neighboring samples are filtered samples from Block_0.

filter_intra_fig2

Figure 3. Example of reference samples for 4x2 blocks.
  • The predicted pixels in the 4x2 sub-block are obtained by filtering the array of 7 neighboring pixels using the filtering methods. To illustrate the procedure, consider the 4x2 block shown in Figure 4 below:

filter_intra_fig3

Figure 4. Reference sample around a 4x2 block.

The predicted kth pixels, k=0,…,7, in the 4x2 block is given by:

av1_filter_intra_taps[mode][k][0] * P0 + av1_filter_intra_taps[mode][k][1] * P1 +
av1_filter_intra_taps[mode][k][2] * P2 + av1_filter_intra_taps[mode][k][3] * P3 +
av1_filter_intra_taps[mode][k][4] * P4 + av1_filter_intra_taps[mode][k][5] * P5 +
av1_filter_intra_taps[mode][k][6] * P6

where mode refers to one of the five supported filter intra modes listed in the table below:

Table 1. Filter intra modes.
filter_intra_mode index Filter intra mode
0 FILTER_DC_PRED
1 FILTER_V_PRED
2 FILTER_H_PRED
3 FILTER_D157_PRED
4 FILTER_PAETH_PRED

The filter coefficients are listed in Table 2 as a function of the intra prediction mode and predicted sample.

Table 2. Filter coefficients as a function of the intra prediction mode and the predicted sample.

filter_intra_table-2

The best filtering mode (i.e. set of weights) is selected through, for example, a rate distortion performance measure.

The 4x2 blocks can be processed in a wavefront manner. For the 8x8 block example shown in Figure 2, the order of processing the 4x2 blocks could be: Block_0, then Block_1 and Block_2, then Block_3 and Block_4, then Block_5 and Block_6.

The Filter Intra feature is applicable only to luma intra prediction and to blocks that have width and height less than or equal to 32.

2. Implementation of the algorithm

Control macros/flags

the control flags associated with the filter intra flag are listed in Table 3 below.

Flag Level (sequence/picture) Description
enable_filter_intra sequence Enable/disable the feature at the sequence header.
pic_filter_intra_mode picture Enable/disable the feature at the picture level.

Main function calls

A diagram of the main function calls associated with the filter intra algorithm is shown in Figure 5 below. The functions are shown according to the depth of the function call.

filter_intra_fig4

Figure 5. Main function calls associated with the filter intra algorithm.

Candidate Injection

The function inject_filter_intra_candidates is responsible of injecting all Filter intra candidates in MD. Candidates are injected for a given block if the flag pic_filter_intra_mode is not zero, enable_filter_intra is set to 1, and both the block width and block height are smaller than or equal to 32.

A total of 5 intra based candidates are injected where the candidate field filter_intra_mode is assigned a value from the following list: FILTER_DC_PRED / FILTER_V_PRED / FILTER_H_PRED / FILTER_D157_PRED / FILTER_PAETH_PRED. For other regular intra candidates filter_intra_mode is assigned a special value (FILTER_INTRA_MODES) to make sure it is not a filter intra candidate. A special class is assigned to all Filter Intra candidates.

Note that when inter-intra compound is used, no filter intra modes are allowed in the intra part of the prediction.

Generation of the filter intra prediction

The generation of the filter predictions is performed in the function eb_av1_filter_intra_predictor. The generation of the prediction is performed as described above. The block is split into 4x2 blocks and the predictions for the 4x2 blocks are generated in a raster scan order.

3. Optimization of the algorithm

To reduce the complexity associated with the filter intra feature, the latter is enabled only for base layer pictures when the feature is enabled at the sequence level (i.e. enable_filter_intra is set to 1) and sc_content_detected is zero (i.e. normal video content).

4. Signaling

At the sequence header the flag enable_filter_intra will enable filter intra in the bit-stream.

When filter intra mode is selected:

  • The intra mode that is sent to the decoder is DC_PRED.

  • There is no need to send intra angular information in the block info part of the bit-stream.

  • Palette prediction should be disabled.