Ok, here's explain:
Source image is ZX Spectrum bitmap. ZXS has 15 color palette (black has only one level). Colors are very similar to CGA colors (I mean full 16 color palette). I need not to emulate CGA.
These sources I need to convert to Commodore 128 VDC color palette - again very similar colors like CGA. For best results I need to match exact color - it removes dithering.
You can now see that in 15 and 16 color palettes is dithering not required - I need convert one palette to second - I need not similar colors, I need exact match to avoid dithering.
I'm creating ZX Spectrum emulator for Commodore 128. So, when is anything emulated it needs to look like original - and dithering looks different to original.
If you will need I upload palettes.
Miro