Enhanced posterise function?

[Subclavius]

I'm interested in editing black and white images in particular with the posterise adjustment layer with low levels of posterisation (3-8 levels or thereabouts). The posterise produces the requisite number of evenly spaced levels e.g. 3 levels gives black/mid-grey/white and 5 levels gives black/25%/50%/75% and white, all as expected. When working at these lower numbers of levels I find it would be very helpful to be able to vary the banding/grouping that the posterise function uses to be user-selectable/non-linear.  Whilst some of this effect can be compensated with careful use of curves, it would be a whole lot simpler if the boundaries betwen levels were accessible and hence this request for enhancement.

It's possible that there's a workaround using multiple groups with levels/curves adjustments combined with blend modes but I haven't found it yet.  Any comments would be much appreciated.

[Fixx]

As operations are non-destructive it is possible to apply posterisation layer and place a curves adjustment layer below it. That way you can adjust posterisation result "quite" easily.

Having adjustment tools right in the posterisation layer would maybe be easier.

Anyway, I dislike the way posterisation works on each colour channel separately, it would be better if it would be more like applying indexed colour mode – you would just define how many colours you want. 

[Subclavius]

Hello Fixx,

Thank you. For me, it was slightly easier in that I am working with b+w, so only 1 channel. I had already applied curve and levels adjustments underneath the posterise layer but I found it didn't really achieve the level of control I was after.  In the end I found a way to achieve what I was after, ultimately using blend modes in groups (one for each level of posterisation).  This is accurate but very 'clunky' and laborious to change and I would still like to see a much greater level of control.

It would certainly be interesting to see how posterise worked against a colour LUT in indexed colour mode to produce a probably more predictable or coherent result.