DUE FOR REWRITE

This page is due for a rewrite to include the very useful debanders from vs-debandshit. You might find more success with dumb3kdb and f3kpf than what's listed here.

Debanding

This is the most common issue one will encounter. Banding usually happens when bitstarving and poor settings lead to smoother gradients becoming abrupt color changes, which obviously ends up looking bad. These can be fixed by performing blur-like operations and limiting their outputs.

Note that, as blurring is a very destructive process, it's advised to only apply this to necessary parts of your video and use masks to further limit the changes.

There are three great tools for VapourSynth that are used to fix banding: neo_f3kdb, fvsfunc's gradfun3, which has a built-in mask, and vs-placebo's placebo.Deband.

Banding example fixed with f3kdb default settings.

neo_f3kdb

deband = core.neo_f3kdb.deband(src=clip, range=15, y=64, cb=64, cr=64, grainy=64, grainc=64, dynamic_grain=False, sample_mode=2)

These settings may come off as self-explanatory for some, but here's what they do:

• src This is obviously your source clip.

• range This specifies the range of pixels that are used to calculate whether something is banded. A higher range means more pixels are used for calculation, meaning it requires more processing power. The default of 15 should usually be fine. Raising this may help make larger gradients with less steps look smoother, while lower values will help catch smaller instances.

• y The most important setting, since most (noticeable) banding takes place on the luma plane. It specifies how big the difference has to be for something on the luma plane to be considered as banded. You should start low and slowly but surely build this up until the banding is gone. If it's set too high, lots of details will be seen as banding and hence be blurred. Depending on your sample mode, y values will either only have an effect in steps of 16 (mode 2) or 32 (modes 1, 3, 4). This means that y=20 is equivalent to y=30.

• cb and cr The same as y but for chroma. However, banding on the chroma planes is comparatively uncommon, so you can often leave this off.

• grainy and grainc In order to keep banding from re-occurring and to counteract smoothing, grain is usually added after the debanding process. However, as this fake grain is quite noticeable, it's recommended to be conservative. Alternatively, you can use a custom grainer, which will get you a far nicer output (see the graining section for more on this).

• dynamic_grain By default, grain added by f3kdb is static. This compresses better, since there's obviously less variation, but it usually looks off with live action content, so it's normally recommended to set this to True unless you're working with animated content.

• sample_mode Is explained in the README. Consider switching to 4, since it might have less detail loss.

GradFun3

The most popular alternative to f3kdb is gradfun3. This function is more resource intensive and less straightforward parameters, but can also prove useful in cases where f3kdb struggles. As there are a ton of parameters, this guide will only cover the most important ones:

import fvsfunc as fvf
deband = fvf.GradFun3(src, thr=0.35, radius=12, elast=3.0, mask=2, mode=3, smode=2, debug=False, planes=list(range(src.format.num_planes)), ref=src)

• thr is equivalent to y, cb, and cr in what it does. You'll likely want to raise or lower it.

• radius has the same effect as f3kdb's range.

• smode sets the smooth mode. It's usually best left at its default, or set to 5 if you'd like to use a CUDA-enabled GPU instead of your CPU. Uses ref (defaults to input clip) as a reference clip.

• mask sets the mask strength. 0 to disable. The default is a sane value.

• planes sets which planes should be processed.

• debug allows you to view the mask.

• elast controls blending between debanded and source clip. Default is sane. Higher values prioritize debanded clip more.

placebo.Deband

This debander is quite new to the VapourSynth scene, but it's very good at fixing strong banding. However, as such, it is also prone to needless detail loss and hence should only be used when necessary and ideally combined with a detail/edge mask. It's (current) parameters:

placebo.Deband(clip clip[, int planes = 1, int iterations = 1, float threshold = 4.0, float radius = 16.0, float grain = 6.0, int dither = True, int dither_algo = 0])

It's not unlikely that this function will see significant change in the future, hence the README is also very much worth reading.

Parameters you'll want to look at:

• planes obviously the to-be-processed planes. The syntax is different here, check the README. In short, default for luma-only, 1 | 2 | 4 for luma and chroma.

• iterations sets how often the debander is looped. It's not recommended to change this from the default, although this can be useful in extreme cases.

• threshold sets the debander's strength or rather the threshold when a pixel is changed. You probably don't want to go much higher than 12. Go up in steps of 1 and fine-tune if possible.

• radius does the same as for the previous functions.

• grain is again the same as f3kdb, although the grain is a lot nicer.

Banding detection

If you want to automate your banding detection, you can use banddtct from awsmfunc. Make sure to adjust the values properly and check the full output. Check this link for an explanation on how to use it. You can also just run adptvgrnMod or adaptive_grain with a high luma_scaling value in hopes that the grain covers it up fully. More on this in the graining section. Note that both of these methods won't be able to pick up/fix every kind of banding. banddtct can't find banding covered by grain, and graining to fix banding only works for smaller instances.

Deblocking

Deblocking is mostly equivalent to smoothing the source, usually with another mask on top. The most popular function here is Deblock_QED from havsfunc. The main parameters are

• quant1: Strength of block edge deblocking. Default is 24. You may want to raise this value significantly.

• quant2: Strength of block internal deblocking. Default is 26. Again, raising this value may prove to be beneficial.

Other popular options are deblock.Deblock, which is quite strong, but almost always works,

dfttest.DFTTest, which is weaker, but still quite aggressive, and fvf.AutoDeblock, which is quite useful for deblocking MPEG-2 sources and can be applied on the entire video. Another popular method is to simply deband, as deblocking and debanding are very similar. This is a decent option for AVC Blu-ray sources.