How to create stones like karren?

[kalwalt]

Another picture, cracks are more distant:
[attach=1]
I'm wondering how I can enlarge the cracks

[Dune]

That's a strange way of connecting nodes, but if it works...
You can change the size of the fractals or use a transform shader for bigger size.

[Tangled-Universe]

As far as I can see your 2 negative displacement shaders do nothing, since they don't have any input on the right port to displace (negatively).

The port on the left is just to connect it into the network, it allows the shader to do something, but it needs some kind of input in that right triangle to actually do something.

You can indeed resize the cracks using a transform node, but be aware that this does not scale the displacement amplitudes along.

So best is to resize your noise function itself, along with the respective displacement settings.
When displacement is involved I do it like this, because then the ratio of scale to displacement stays clear to me and using a transform shader adds an extra variable to take into account.

[kalwalt]

I'm sorry if it seems strange some my nodes, actually some of them don't do so much. i disabled some of them. I get how to enlarge the cracks: in the DandelO clip there is a color adjust shader, reducing the White point value, and increasing the scale of the noise, did the job. Of course this is only  one of the step to complete the work. I also considering to change the heightmap to a procedural one, or a simpler heightmap created with WM without erosion.
[attach=2]

[kalwalt]

I would to stretch in the X or Z axis the voronoi 3D diff scalar but if i connect after that a transform input shader and increasing the scaling in one of the axes does nothing. Is there some specific function for this?

[Tangled-Universe]

I would to stretch in the X or Z axis the voronoi 3D diff scalar but if i connect after that a transform input shader and increasing the scaling in one of the axes does nothing. Is there some specific function for this?

If you replace the "get position 01" with a "get position in texture" node then you should be able to adjust the scale etc. with a transform node.

[kalwalt]

I would to stretch in the X or Z axis the voronoi 3D diff scalar but if i connect after that a transform input shader and increasing the scaling in one of the axes does nothing. Is there some specific function for this?

If you replace the "get position 01" with a "get position in texture" node then you should be able to adjust the scale etc. with a transform node.

It's perfect! It does exactly what i want! 
another question: does exist a blur function in the TG Function nodes? or there is a way to perform a blur operation combining different nodes?
Many thank's!

[Tangled-Universe]

You're welcome

No, procedural blur is a kind of holy grail and does not yet exist or at least not in a way suitable for TG's procedurals.

What do you want to try? Perhaps there's another way?

[kalwalt]

You're welcome

No, procedural blur is a kind of holy grail and does not yet exist or at least not in a way suitable for TG's procedurals.

What do you want to try? Perhaps there's another way?

Yes, I saw different topics in this forum that argued the difficulties of this approach. My idea was to blur the cracks in order to obtain a smoother transitions from the cracks and the inner part of the stones. I encrased a bit more the size of the cracks and i used a soothstep function, but i think it's not the same as blurring. Probably i need to use a different approach to the problem.

[Tangled-Universe]

I often try to use a "colour adjust" node to tweak a crack function.
So connect the output of your crack function to a colour adjust shader.

If you decrease the white point of the colour adjust shaders then the cracks will become sharper/more narrow.
Increasing whitepoint does the opposite and thus will make the cracks "softer" and wider.

Just click on the little blue "window" button to open the preview of the shader and play with the sliders to see what they do.

The colour adjust shader basically remaps values like Photoshop Levels function.

[kalwalt]

I often try to use a "colour adjust" node to tweak a crack function.
So connect the output of your crack function to a colour adjust shader.

If you decrease the white point of the colour adjust shaders then the cracks will become sharper/more narrow.
Increasing whitepoint does the opposite and thus will make the cracks "softer" and wider.

Just click on the little blue "window" button to open the preview of the shader and play with the sliders to see what they do.

The colour adjust shader basically remaps values like Photoshop Levels function.

I was doing the same thing, i have already a color adjust node as you can see in the picture http://www.planetside.co.uk/forums/index.php?action=dlattach;topic=18577.0;attach=51383;image of the nodes cracks, but i was wondering if there were other methods, but as you had confirmed, they aren't.
Thank you again for your answer.