X264 TODO

This page contains an incomplete list of things available in x264 for you to do. It's organized into sections covering various parts of x264.

Sequential elimination (SEA), used for exhaustive search, might be more generally applicable to algorithms like UMH, by letting us skip a lot of SADs. The downside is we won't be able to use SAD_X4 anymore.
(T)ESA is currently wrong for motion searches done on weightp duplicates. This effect is miniscule, but it still should be fixed.
Hierarchical motion estimation might be a useful way to catch very long motion vectors without the cost of UMH or ESA. It might also help regularize motion.
Somehow take into account the effect of motion vector decision on future blocks.
- Hierarchical motion estimation
- Approximations from lookahead MVs
- Iterative ME (as per Snow)
- Trellis motion estimation

Make the early terminations smarter. Currently they're just hacks -- some statistical analysis might be useful.
SAD (subme 1) i8x8 vs i4x4 decision is a bit bad. Can it be improved without significant speed loss?

Can we find more ways to skip more motion searches in multiref?
On extremely fast encoding settings, fast skip is actually kind of slow. But anything dumber (e.g. SAD) is completely useless. Is there some better balance that can be achieved here?
Chroma-aware mode decision for B-frames?

Psy-RD is a hack. It works, but it's a hack. If you apply QNS with Psy-RD as the metric, it goes way overboard and gives terrible results. This means that Psy-RD only works because normal mode decision is limited in the way it can modify the image to better suit the metric. Is there a way to make it better?
Should RD be linear at all? Perhaps we should weight more heavily against low quality blocks and also try to ignore minuscule distortion that viewers can't see.
Psy-trellis (and maybe psy-RD?) are too strong at very high QPs.
Psy-trellis should be merged with Psy-RD. There are patches for this, but they probably won't be committed until psy-RD itself is fixed.
RD should take into account local variance.
Lambda should be varied on a per-DCT-block basis instead of a per-macroblock basis.
Lambda should be picked independent of quantizer (i.e. with greater precision).

Lookahead should be multithreaded, either by splitting the frame (sliced threads) or running multiple frame analysis calls at once.
Temporal MV predictors in lookahead? There's a patch for these somewhere, but they biased heavily in favor of B-frames, likely by improving the motion search.

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