Implementation Details
When using FMO, the image can be divided in different scan patterns of the macroblocks, with several built-in patterns defined in the spec, signaled as 0-5 in the unit slice_group_map_type, and one option to include an entire explicitly assigned MBAmap, signaled as 6. The map type and a new MBAmap can be sent at any time.
- Interleaved slice groups, type 0: Every row is a different slice, alternating as many times as slice groups. Only horizontal prediction vectors are allowed.
- Scattered or dispersed slice groups, type 1: Every macroblock is a different slice. With two slice groups, it creates a checkerboard pattern; four or more groups also interleave rows, and with six slice groups, no macroblock will ever touch another from the same slice group in any direction, maximizing error concealment opportunities. No vector prediction is possible.
- Foreground groups, type 2: Specifying only the top-left and bottom-right of static rectangles to create regions of interest. All areas not covered are assigned to a final group. Vector prediction is possible within each rectable and within the background. The behavior of overlapping rectangles is undefined, but in the reference software the last slice group to define it is used.
- Changing groups, types 3-5: Similar to type 2, but dynamic types that grow and shrink in a cyclic way. Only the growth rate, the direction and the position in the cycle have to be known.
- Explicit groups, type 6: An entire MBAmap is transmitted with groups arranged in any way the encoder wishes. Vector prediction is possible within any contiguous regions of the same group.
(In the above image, "Type 0" shows standard H.264 slices, not interleaved slice groups.)
Read more about this topic: Flexible Macroblock Ordering
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