Ok, I’m gonna run through my process for creating interesting rock formations.
Before you can make nice rocks, you first need to look at real rock formations and get an understanding of what makes them interesting. Google images is your friend, and you will find hundreds of reference photos with ease, if you search.
Ok, After you have your reference image its time to begin.
Create a single brush, sized accordingly (I usually start with the largest part of my rock formation, so this brush might be 128/128 or 256/256). Turn this cube into a displacement surface, with 2 iterations (Its way easier to bang out the form with a low resolution displacement).
Tweak this displacement surface, so that you get a nice organic rock shape. This is why you want the lowest resolution displacement – much easier to tweak the points and get a nice shape.
Now, once you have one good looking rock… duplicate it! Take this copy and scale it down to create the staggered sibling rocks for your formation.
Continue this process until you have created the nice solid form you were shooting for with your rocks. The point is to capture the general shape.
Don’t be afraid to push and pull these shapes a good bit. I usually scale them non-uniformally and rotate them on the Z axis, so that each shape feels somewhat different and they aren’t obvious clones of each other. Also, don’t be afraid to push displacement surfaces into each other. I do this all the time, and it works great. This is something that other games do frequently – creating basic models and pushing them into each other to create shapes. Since our world geometry has to be very precise, HL mappers aren’t used to this concept, but it can be done with displacements with no ill effects (as far as I know). Also, the beauty of displacement surfaces is that they actually render very cheap (but are heavy on the bsp filesize). I have found that you can usually afford to go a little further with displacements than you may think, without it effecting framerate.
Once you have created the basic rock shape, you can then take your time refining and cleaning it up. Bump up the displacement resolution and sculpt the rocks a little more. Delete any back or ground face displacements that the player wont ever see. Also, for the sake of efficiency, I usually clip my rocks at the ground plane, and clip away any large chunks that cant be seen, like the portion of the small rocks push into the center of the large central rock.
And that’s about it. You’d be surprised how quickly you can make decent rocks with this method.