Posterize help says it reduces to 2^LEVELS instead of less than LEVELS^3 total colours

Submitted by Carl

Description

Created attachment 329531 Posterize this with 3 levels to demonstrate a 27-colour result.

The Posterize documentation claims that this tool reduces the total number of colours to 2^LEVELS, where LEVELS is the number shown in the slider. This is not true. The actual number is 3^LEVELS and this is a maximum (ie. you may get less).

Where the documentation says the slider "allows[s] you to set the number of levels (2-256) in each RGB channel", I think the phrase "number of levels" should read "maximum number of colours".

Where it says "The total number of colors is the combination of these levels", I think "total number" should be replaced by "maximum total number".

Where it says "A level to 3 will give 2^3 = 8 colors", it should actually say "Setting the level to 3 will reduce the total colours to 3^3 = 27 colours or less". Better yet, reduce ambiguity with "Setting the level to 4 will reduce the total colours to 4^3 = 64 colours or less".

It was trivial to Posterize various images and then use the Colorcube Analysis tool to verify that colour counts achieved frequently exceed the obvious 2^LEVELS math error. Such experiments also find examples where the colour count is equal to or less than LEVELS^3.

My own feeling is that talking about "levels" in the documentation and using that same term in the tool GUI is needlessly confusing. Can we not just use "colours per channel"?

When posterized to 3 levels on GIMP v2.8.14 (Windows), the attached PNG image has 27 total colours.

Attachment 329531, "Posterize this with 3 levels to demonstrate a 27-colour result.":

Version: 2.8