How do I quantizise alpha channel in photoshop?

[Taavi98A4]

Is it even possible in PS?
Normally I have alpha values from 0-255, but I need them 0-8 for example. I need these to save space and have better compression. 
Other colour information should remain in original depth.

Its possible to make my own tool for this or search for external solution, but thinking of asking here first before wasting my time.

[jefbr]

In short:

- ¨Possible in ps?: yes but. (read below)

- Make your own tool: yes but. If you make a tool, you are the only one with that tool until everyone that needs your work has the tool as well. Depending on what you want to achieve, this is good/bad.

In photoshop, you can apply a posterize filter to your alpha channel with x values (x=8 for 0-8 as requested) to reduce the alpha values to 8 values used on a 0-255 scale.

With a posterize filter of 8 applied to your alpha channel, the alpha values per pixel will be redistributed to one of 8 values.

I don't see how it will save a huge amount of space, as your alpha histogram will still be 0-255 and not the 0-8 as I think you desire, but perhaps for your case this optimization is enough.

You might get more results faster by using a different file format than the one you are using now. (gif, png, webp, tiff, ...)

[c.pfaffenbichler]

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I don't see how it will save a huge amount of space, as your alpha histogram will still be 0-255 and not the 0-8 as I think you desire, but perhaps for your case this optimization is enough.

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In a psd Channels appear to get compressed by default, so the savings might be noticable depending on the Channel’s original content.