Here's the skybox l_swampn from Arcane Dimensions:
before and
now. These shots are from one of the lower corners of the skybox, and skyboxes in Retroquad are (still) rendered without any lighting of fog, just the raw preprocessed textures.
Now all six sides of the skybox are preprocessed as a single image, to smooth out the edges, and I've noticed that this helped to improve the image quality as a whole, getting rid of image artifacts like the pattern on the lower side of this skybox, therefore improving the overall color fidelity.
Right now, skyboxes are the only thing using this image format, but I'm going to port this image preprocessing algorithm into a texture compiler tool, to allow the engine to use 24-bit textures everywhere with fast load times. I'm really curious to see how the combination of this texture color quality with the 4-step dithered lighting will look.
All this has near zero impact on performance, by the way. I'm getting around 60 fps when rendering nothing but the skybox at full HD in my Core i5 laptop. Regular BSP geometry is slower to render due to lighting and their 3D mesh complexity.