The first thing we can do is change the scale and threshold applied to the intensity in the pixel shader. The texture is storing the full [0, 1] range but we can choose which part of this to show to provide more or less cloud cover:
Here the left most image is showing 50% of the cloud data, the middle one just 10% and the right hand one 90%. Note that on this final image the contrast scalar has also been modified to produce a more 'overcast' type result. Because these values can be changed on the fly it leaves the door open to having different cloud conditions on different planets or even animating the cloud effect over time as the weather changes.
Next I thought it would be interesting to add peturbations to the cloud function itself to break up some of it's unrealistic uniformity. First a simple simulation of the global Coriolis Effect that essentially means that the clouds in the atmosphere are subject to varying rotational forces as they move closer to or further away from the equator due to the varying tangental speeds at differing lattitudes.
The real effect is of course highly complex but with a simple bit of rotation around the Y axis based upon the radius of the planet at the point of evaluation I can give the clouds a bit of a twist to at least create the right impression:
Here the image on the left is without the Coriolis effect and the image on the right is with the Coriolis effect. The amount of rotation can be played with based on the planet to make it more or less dramatic but even at low levels the distortion in the cloud layer that it creates really helps counter-act the usual grid style regularity you usually get with noise based 3D effects.
With the mechanism in place to support the global peturbation from the Coriolis effect, I then thought it would be interesting to have a go at creating some more localised distortions to try to break up the regularity further and hopefully make the cloud layer a bit more realistic looking or at least more interesting.
The kind of distortions I was after were the swirls, eddies and flows caused by cyclical weather fronts, mainly typhoons and hurricanes of greater or lesser strength. To try to achieve this I created a number of axes in random directions from the centre of the planet each of which had a random area of influence defining how close a point had to be to it to be affected by it's peturbation and a random strength defining how strongly affected points would be influenced.
Each point being evaluated is then tested against this set of axes (40 currently) and for each one if it's close enough it is rotated around the axis by an amount relative to it's distance from the axis. So points at the edge of the axis' area of influence hardly move while points very close to the axis are rotated around it more. This I reckoned would create some interesting swirls and eddies which in fact it does:
It can produce some fairly solid looking clumps which is not great but on average I think it adds positively to the effect. (Looking at the type of distortion produced I suspect it may also be useful for creating neat swirly gas planets or stars with an appropriate colour ramp - something for the future there I hope).
So far I've got swirly white clouds but to make them seem a bit more varied the next thing I tried was to modulate not just the alpha of the cloud at each point but the shade as well. At first I was going to go with a second greyscale fBm channel in the cube map using different lacunarity and scales to produce a suitable result, but then I thought why not use the same channel but just take a second sample from a different point on the texture. This worked out to be a pretty decent substitute, by taking a sample from the opposite side of the cube map from the one being rendered and using this as a shade value rather than an alpha it introduces some nice billowy peturbations in the clouds that I think helps to give them an illusion of depth and generally look better:
The main benefit of re-using the single alpha channel is it leaves me all three remaining channels in my 32bit texture for storing the normal of the clouds so I can do some lighting. Now for your usual normal mapping approach you only need to store two components of the surface normal in the normal map texture as you can work out the third in the pixel shader, but doing this means you lose the sign of the re-constituted component. This isn't a problem for usual normal mapping where the map is flat and you use the polygon normal, bi-normal and tangent to orient it appropriately, but in my case the single map is applied to a sphere so the sign of the third component is important. Fortunately having three channels free means I can simply store the normal raw and not worry about re-constituting any components - as it's a sphere I also don't need to worry about transforming the normal from image space to object space.
Calculating the normal to store is an interesting problem also, I tried first using standard techniques to generate it from the alphas already stored in the cube map but this understandibly produced seams along the edges of each cube face. Turns out a better way is to treat the fBm function as the 3D function it is, and use the gradient of the function at each point to calculate the normal - this is essentially the same way as the normals are calculated for the landscape geometry during the marching cubes algorithm.
With the normal calculated and stored, I then put some fairly basic lighting calculations into the pixel shader to do cosine based lighting. The first version produced very harsh results as there isn't really any ambient light to fill in the dark side of the clouds so I changed it to use a fair degree of double sided lighting to smooth out the effect and make it a bit more subtle/believable.
Shown below is the planet without lighting, the generated normal map, the lighting component on it's own and finally the lighting component combined with the planet rendering:
As you can see from the two right hand images there are still some problems with the system as it stands - the finite resolution of the cube map means there are obvious texel artifacts in the lighting where the normals are interpolated for a start - but I think it's still a worthwhile addition.
Although I'm not 100% happy with the final base cloud layer effect I think that's probably enough for now, I'm currently deciding what to do next - possibly try positioning my planet and sun at realistic astronomical distances and sizes to check that the master co-ordinate system is working. This is not visually fun but necessary for the overall project although it will probably also require some work on the camera control system to make moving between and around bodies at such distances workable.
More interestingly I might try adding some glare and lens flare to the sun rendering to make it a bit more visually pleasing - the existing disc with fade-off is pretty dull really.