Flexible Macroblock Ordering - Tradeoffs

Tradeoffs

FMO is only allowed within the Baseline and Extended profiles. The much more common Constrained Baseline, Main, and all High profiles do not support it, and software that can create or decode it is rare. Some videoconferencing units use it; otherwise, the JM reference software is the primary support.

Using multiple slices per picture always lowers coding efficiency, and FMO can further impact it. The more spread out the slices are, the worse it becomes, with checkerboard patterns (see Scattered Slices below) being the worst. The goals of spreading out errors and coding efficiency are directly in conflict. FMO allows inter prediction for immediate neighboring slices in the same group, effectively making a contiguous region nearly act like a single slice; in some situations, where slice groups are shaped into a Region of Interest, it can actually slightly improve efficiency over simple standard slices, but the benefit is rare and small. Due to this, FMO should only be used where packet losses are common and expected.

Aside from increased complexity in encoding and decoding, and lower efficiency, inloop deblocking also creates a problem: Slices can be sent in any order, but the deblocker requires all . Either the deblocker has to run in multiple passes whenever another slice is received, or an entire picture needs to be buffered before beginning the deblocking, possibly creating additional latency if slices are delayed long enough that the next picture's slices start coming in first.