When exporting the $808080 gray as a 2-color GIF the diffusion even at 100% doesnt come into effect because the color table is Gray+White, resulting in flat gray pixels, but deleting the Gray makes it revert to B&W, and then presto - dither checkerboard:
The only problem is i dont know what kernel or divisor Adobe uses, and actually i think they have a variable divisor! I compared a dither of a photo from Photoshop and it looks very similar to my Floyd if i switch the divisor from 16 to 35 (yes 35 on that particular photo not 32 so i think variable), giving a result more like Atkinson in that the light and dark areas are a bit cleaner.
But at least it's confirmation that a good dither of $808080 should result in the checkerboard (if it's good enough for Photoshop!?!) All the other kernels i tried result in that, except Atkinson has a 2x2 checkerboard instead! Anyway dithering has been fun but time to move on and learn something else lol. Thankyou wilbert for your feedback and thoughts!
