What does "No DCT Decimation" do?

[Chainmax]

I searched around on the forum and in berrinam's signature wiki link but found no answers. Would enabling it increase quality?

[Prettz]

I asked this on their IRC channel and Manano (I think) said that it lets the encoder skip further consideration of a block under certain circumstances, sort of like the Fast P-Skip option.

He said to enable it (i.e. disable DCT Decimation) if you are using Trellis (and you probably are).

[Chainmax]

So DCT Decimation is somewhat comparable to Fast P-Skip and is advisable to use then, thanks for the quick answer :). Time to go pester Sharktooth about profile updates then ;).

[bkman]

I've done some testing with No-DCT-Decimation in CQ mode, and it only makes the output bigger with little-to-no appreciable quality gain. But YMMV.

[DeathTheSheep]

--same results as bkman, odd--

[Manao]

Not odd at all : when you decimate a DCT block ( i.e, you consider it's small enough not to matter ), you don't write it into the bitstream. So you will write more DCT blocks with --no-dct-decimate than without. And the more thing you write, the bigger the size.

Now, visually, you don't see the difference. Well, that's why by default the block decimation is enabled. Yet, it appears some are seeing real differences on some special cases, where the DCT decimation was zeroing blocks that shouldn't have been zeroed.

Finally, beware of such quality comparison when the quality difference is too small to notice. I doubt you can see the quality difference between two files encoded with the same settings, but with a slightly different bitrate. Yet that doesn't mean the lower bitrate is a better compromise.

[DeathTheSheep]

Nice, nice. Of course I don't expect a noticable quality change at a constant quantizer of 24, and no one switch (save for the super-important B-frame, cabac, etc) is expected to produce any easily discernable quality difference at a nice-looking constant quantizer anyways... So ya. Ya!

PS: In my previous post, I was just "adding" my result to the others, but I misspelled "add" as "odd" on my Pocket PC... ;)

[elguaxo]

*bump* :p

I'm reviewing my presets. We have now AQ and PSY RDO or FGO depending on the patched version we use. Before all these options I was using CQMs and/or low deblocking values. Now I don't need them, but --no-dct-decimate is still there on all my 'quality' presets.

Does it make any sense to keep this option if I'm already using AQ and PSY RDO (or FGO)?

[Ranguvar]

If you're using PsyRDO, it's advisable to use full trellis. And if you're using full trellis, I've heard it said before that --no-dct-decimate is a good idea. I believe it also helps a little with grain retention, but the difference is small.

I definitely wouldn't use it on anime/cartoons, though.