I have finished the color table generation code. My first try was using a lookup table that maps a density ( in the range [0-1] ) to a color, that interpolated colors based on 4 to 7 "color keys". Each color key was determined randomly.. as expected, it didn't look very good. I decided to use another approach: generate a start color for density 0; generate an end color for density 1; then use the mid-point displacement algorithm in 1D to generate the intermediate values.
Here's an example of a color table ( stretched in 2D to see the colors better ).
And Here's an example of a gas giant texture ( there are 6 per layer, and 6 layers total -> there's 36 of those ) with the applied color table.
I also rewrote the color table lookup code itself, to use a simple 256-values array, and pre-computing the colors and the alpha for each value. Then, to texture the gas giant, I only need 1 line: get the density, convert it to an integer in the 0-255 range, then lookup the RGBA values. It provided a nice performance boost ( 5 times faster than the previous method ). You wouldn't notice a difference though, because this part of the algorithm wasn't a bottleneck to start with.
I rewrote the atmospheric scattering shaders in GLSL. Here's a gas giant with its 6 layers ( each cloud layer is moving at different speeds, although it's subtle ). Notice how the scattering on the top-right makes the atmosphere go yellow-ish:
Last but not least, I've been trying to get good results when entering inside the atmosphere of the gas giant. It's still very experimental, at the moment my main problem is to blend the atmosphere look from space and from inside.. and I haven't succeeded yet. But that's how it looks:
In the upper atmosphere:
In the medium atmosphere:
In the deep atmosphere, you probably won't be able to go deeper in a spaceship anyway: