GIMP Version: 2.10.12
Operating System: Windows
OS Version: Windows 7 Professional
GIMP Experience: Basic Level

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Hi all,

I've been wondering, if there is a filter for GIMP that corresponds to the "Posterize" filter from Adobe Photoshop?

I've found this filter very useful for reducing the number of colours needed by a picture. I suppose I don't really need it at the time of writing, except perhaps for toying around.

Cheers,
Albert.

Posterize is towards the bottom of the Colors menu

...but.. a tip for Gimp 2.10 Make sure there is a canvas open. The forward slash key brings up a search facility. Type in a search term, click on a result.

Playing around with reducing pictures to 16 or less colours reminds me of DOS graphics, especially the 16-colour modes.

Is there an easy way to constrain the choice of colours to VGA default palettes? I mean 0=black, 1=blue, 2=green, 7=gray, etc.

-Albert.

1. Create an image with a black background
2. Image > Mode > Indexed and set to 16 colours
3.Windows > Dockable Dialogues > Colormap
4. Set foreground colour to blue
5. In the colormap click on "Add current foreground color"
Repeat 5. until all 16 colours have been added
6. Select colours from the colormap

Custom palettes. You can find retro/oldschool palettes for Gimp using google search.
I attached few, the ones that are highlighted.
If you convert image to C64 or Spectrum palette, it won't be correct though because of the graphic modes those systems had.

Drop the palettes in the user palettes folder = C:/Users/Yourname/AppData/Roaming/GIMP/2.10/palettes
Refresh palettes.
Menu: Image/Mode/Indexed...use custom palette, pick the one you want to use.

Gimp is an RGB editor and I really advise working in RGB. For example if you start with a blank white canvas, convert to Indexed and whatever palette is chosen there are no colours in the colormap and you will have to add as described by Blighty. So stick with RGB mode. If you need indexed, convert as a final operation.

You need to make sure that the colors you use are in the chosen palette.
If the palette dialogue is not open then Windows -> Dockable Dialogues -> Palettes will open it. That allows selection of a palette and there are default ones that will do. Default 23 colours and EGA (240) actually has 16.

Using them:
You can click in the FG/BG icon which brings up the Change FG(BG) dialogue. There is a Palette Tab that shows the active palette. Choose the colour there.

Alternative if you do a lot of drawing. Open the Colors Dialogue Windows -> Dockable Dialogues -> Colours Same as before choose the palette tab. Pick your colours there.

Bit of a jumbled up screenshot, but it is all there

The problem is that converting to indexed mode, in and of itself, reduces the number of colours, so there is not much left for posterize to work with.

I did an experiment, testing two operations; a) posterize with 10 levels, and b) change to indexed mode with a 186 colour optimised palette. 186 was chosen because that was the result of operation (a), as reported by color cube analysis. The result is that operation (b) yields a somewhat better picture.

I don't understand what's going on here. According to the help for posterize, 10 levels allows 10*10*10=1000 colours, so why does it use only 186?

The original picture has 68117 unique colours according to the color cube analysis.

Setting to indexed mode with CGA palette (16 colours) results in a very poor image, using only 7 colours, mostly light gray and dark gray. It does not appear to use blue at all, even though the picture has a blue shirt and the sky is largely blue.

With CGA palette and dithering, the picture looks great given only 16 colours. Colorcube analysis reports that it uses 15 colours; why not 16?

-Albert.

I found that the CGA palette has the colours in the wrong order. See: Wikipedia CGA article



What would be wrong about it?



I did so, and I added a correct CGA palette I found.

-Albert.

I agree. I'm just playing.



I don't. Mostly I edit photographs.

-Albert.

Too many colors in a cell. I lived my childhood with C64, but Spectrum also had similar limitations.
The image on those systems was divided into cells, and you could use only certain amount of colors in a single cell.
To get "correct" C64 or Spectrum image I would use emulator, or application you can draw it correctly like Timanthes.
There is also gimp-cbmplugs that I used earlier in Linux.
If you just play around it doesn't matter if you don't follow the graphic mode rules,
palette conversion make them look retro and nice in my opinion.

And yes, if you convert to indexed mode do it last.
When I make little retro images I export them to png without saving to XCF like bigger projects.

This is Gimp forum, but GrafX2 is also cool for oldschool.

Edit. Just found this will test later.


Amiga Palette in python, with bit planes and 4 dithering possibilities.