Question for the planet artists here: has anyone seen this before, and more importantly, do you know how to fix it? (Effect slightly exaggerated.) Gaussian blur does nothing. I think it's caused by playing with the colour/brightness , but I can't work it away. Even when I down-scale the planet to 2048x2048, it's still visible.
Well there's two things that can cause it...
-Color banding of your display (this happens quite often with LCD's, for more information look
here. This causes even the smoothest of gradients to show
some amount of banding (or rather they seem to use slightly different RGB balance to achieve different brightness values, and thus the image doesn't appear as uniform gray). If you see banding on the images on that site, bad luck; get a CRT or better display. Mine shows some banding on the darker tones as well, so you need to just learn to see what is caused by the display and ignore it, and what is caused by image editing.
Wide gradients (large areas) of low contrast and brightness usually suffer from this the most of all.
-Image editing. Because the images we use currently have usually only 8 bits per channel and 24/32 bits per pixel, it means that when you edit something like smooth gradient with contrast/brightness/hue changes, rounding errors start to appear - you can check this out by taking a photograph that shows something like sky with a gradient, then start editing it (colour levels, back and forth, contrast/brightness with small changes)... With multiple edits, rounding errors start to accumulate and bands like that can start appearing in the sky. Same with calculating the final colour of a pixel with multiple layers with transparency.
If you can use 64-bit image mode while editing, you can decrease this problem as far as rounding errors go, but unfortunately your display will still only be able to use 32-bit colour mode, and in the end you need to convert the image to 32-bit (or if you don't need alpha, 24-bit) colour mode.
-...and yeah, image compression can do bad things to gradients as well. DXT compression uses less than 8 bits per pixel for some channels. It is not good for anything with significant amount of gradients in it. Use u888 instead (or u8888 if you need alpha).