im trying to create a realistic cloud system for a space render but im not getting very far just using a cloud layer and fiddling with the density scale and things, so im wonder is their a tutorial or something on how to create one or, how i can use my cloud texture map that i have to control density and position of the clouds based on the grayscale of the map
The main thing to remember with creating a planetary cloud system is that the default scales really need enlarging considerably for each cloud layer.
Try this out in the density fractal; Feature scale=250,000, Largest scale=1e+007, smallest scale=2500.
That should give you nice big patches of cloud all round a default planet. If there's not enough detail in the clouds, use this fractal only as the 'blend shader' to a smaller fractal. This will make the smaller scaled shapes only appear where the large fractal exists.
This really quick image shows the settings above, without any secondary fractals. You could play with 'coverage adjust(colour offset if you use power fractals)', noise variation(which sharpens cloud transition the lower the setting is, and softens their transitions the higher it is set) warp amount, etc. to get a nice looking coverage. It's a start, at least.
* Don't forget to hit 'random seed' until you get lucky with the shape/distribution without needing to sculpt and plan the noise shapes manually or to use maps.
can a grayscale map of a cloud image be used as like a density shader to control the position and density of the cloud system?
Yes. Probably best to use the image as the blend shader of the density fractal, rather than just using an image map.
If you only use a 2D image instead of a density fractal, the cloud layer will not have variation across its depth, other than the greyscale colour of the image.
An image map used as a blending shader to a fractal, which has infinite noise-pattern variation across all 3 axes, would be a better idea.
You can, although, get some really good results by using just an image map and using the 'depth modulation' function of the cloud layer, but it still won't have variation across every axis without a 3D fractal.
i rendered a patch of the planet with a image map shader pluged into the blending shader of the density shader of the cloud layer, and the clouds dont look right, seem to be all pixelated
Is any form of interpolation enabled in the image map shader?
What are your rendersettings?
Enable 'ray trace atmosphere' on the render settings, as this will give a smoother less pixelated look. Also on the cloud layer setting 'acceleration cache' to 'None' might help avoid pixelation in the self shadows of the clouds. If the fractal shapes themselves are too noisy, you might like to increase the 'coverage adjust' on the fractal (not on the cloud layer node), and/or increase the feature scale and lead-in scale.
I wrote up my experiments on clouds from space here:
http://terragen2.wordpress.com/2011/05/21/clouds-from-space/
In the end I purchased the Nvseal Global Clouds Pack from NWDA
http://www.nwdanet.com/buy/7-preset-packs/51-nvseal-global-clouds-pack