I made a plugin to do this myself; it's fairly simple to script.

This is what is involved in optimizing for 18bit:

First, make a 64color palette; this will be a gray gradient from black to white, matching the intensities displayable on the TFT. The easiest way to do this is to reset FG/BG colors to default, and import the palette from the gradient 'FG to BG (RGB)' with 'number of colors' == 64.

For each image:
* Decompose the image (Colors->components->decompose, choose RGB) * Indexize the resulting image to your pre-made palette, with dithering (you may want to experiment with different dithering methods; I personally prefer 'positional'/'fixed' because it is predictable.)
* Recompose the resulting image (Colors->components->recompose) * Close the 'work' image. The original image is now optimized for 18bit. You should be aware that the dithering will make the image harder to compress.

If you just want a quick preview of the image using 18bit: * Colors->Posterize, select 64 levels of posterization. * If you don't want dithering, performing that step is all you need to throw away any data that is meaningless for 18bit display, so the image will compress better.

Note: the above workflows are for 2.4rc2. If you are using a significantly earlier version of the GIMP, the decompose and recompose filters are instead found somewhere in a submenu of the 'Filters' menu.

On 9/11/07, Francois du Toit wrote:

I want to optimize images for display on a cellphone with a 262K color (18bit) TFT screen.
What's the easiest way to do this in gimp?

Thanks!