A new feature has been added to the Retro Image Tool: creating Spectrum 512/4k images. Just set "Colours" to "Spectrum 512/4k" and choose either "512 (ST)" or "4096 (STE)" for the palette option, also please set the dither option to "Floyd-Steinberg 75%".
Now the original images will be resized to fit the 320x200 pixels resolution when loaded. Recommended source pictures should have a 16:10 aspect ratio like 1280x800 or 1920x1200 pixels. It's still not perfect but probably a neat tool to play around with.
Cyprian wrote:looks really cool. what is the difference between "16 PCHG" and "512/4k"?
16 PCHG (PaletteCHanGe) means that a new 16 colour palette is available for each line while the classic Spectrum 512/4k approach alters the palette two times in a line so up to 48 colours can be shown. Unfortunately the 48 colours cannot be chosen freely as only 16 different colours are available at each X position within a line. So in general the resulting quality is better using the Spectrum 512/4k format. Note that that format, however, is only compatible with the 8 MHz machines since it is based on an exact pixel timing.
Hint: you can use RECOIL to display the SPU images.