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 <[EMAIL PROTECTED]> 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!
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