Here's another (enhanced) Spectrum 512 slideshow for the Atari STE
but this time the maximum number of colours has been raised to 32768 (of which the Atari STE is only able to display up to 29791). The Archive
includes one folder of the slideshow to copy the files directly to your Atari HD and an .ST floppy image with the same content from which you can boot.
The method of switching two images and/or palettes is not really new since other graphic converter tools like Doug's Photochrome
offer such a feature in a more advanced way. Also the Spectrum 512 format does not support overscan resolutions and can be considered being a rather outdated format. OTOH it's quite a well known format and supported by STOS and other programs.
So what's the real reason to make another Spectrum slideshow?
Well I noticed that the colours stored within the Spectrum 512 image didn't use the whole 16 bit colour word. In fact, it uses only 4 + 4 + 4 = 12 bits for the Atari STE. So I thought it could be cool to add another bit to increase the colour channels to 5 bits each. The new least significant bit for each channel are now stored in the upper nibble of the colour word and so the image .SPU format itself is kept completely "backwards" compatible. Fortunately, since the added bits are being stored in an unused space the (uncompressed) .SPU image size doesn't even change.
With this new additional colour bit information the display program generates two palettes but not two images(!) to switch alternatively between them in the VBL routine.
Here are some of the slideshow pictures and how they look on the Atari STE: