Video Compression

VideoNerd

The HEVC/H.265 standards requires that picture luma sizes (both width and height) are multiple of MinCbSizeY (this parameter is configurable, but it typical value is 8).

The standard says:

pic_height_in_luma_samples and pic_width_in_luma_samples “shall be an integer multiple of MinCbSizeY”.

Where ‘MinCbSizeY’ is derived from SPS parameter ‘log2_min_luma_coding_block_size_minus3‘, typical magnitude of ‘MinCbSizeY’ is 8.

What to do if original picture size (width and/or height) is not multiple of 8?

1) One of solutions is to extend original pictures to 8-pixels boundaries (CbY alignment) in bottom-right extension or by left-top extension and encode the extended pictures. To instruct a player about actual output sizes, it’s necessary to signal (in SPS) corresponding cropping parameters: conf_win_left_offset, conf_win_right_offset, conf_win_top_offset and conf_win_bottom_offset. Otherwise the player displays the extended pictures with padded margins.

2) Alternative solution is to crop original pictures to 8-pixels boundaries. In such case the cropping parameters are not signaled since encoded picture size is equal to displayed one.

3) Most sophisticated solution is upscale original pictures to 8-pixels boundaries.

As per the first solution, it’s better for coding efficiency to extend original frames in same way by padding with same values. In such case then the energy of inter predicted residual blocks is smaller than the case where pictures are extended by mirroring or edge duplication. Adaptive pixel extension can be useful for I-frames only, if the encoder extends pictures in such way that spatial residuals of a chosen intra mode are minimal.

 

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