CSG objects can be extremely complex. They can be deeply nested. In other words there can be unions of differences or intersections of merges or differences of intersections or even unions of intersections of differences of merges... ad infinitum. CSG objects are (almost always) finite objects and so respond to auto-bounding and can be transformed like any other POV primitive shape.
Now let's add two spheres each translated 0.5 units along the x-axis in each direction. Color one blue and the other red.
Try tracing this file now at 200x150 -A . Now place a union block around the two spheres. This will create a single CSG union out of the two objects.
Trace the file again. The union will appear no different from what each sphere looked like on its own, but now we can give the entire union a single texture and transform it as a whole. Let's do that now.
Trace the file again. As you can see, the object has changed dramatically. Experiment with different values of scale and rotate and try some different textures.
There are some advantages of assigning only one texture to a CSG object instead of assigning the texture to each individual component. First, it is much easier to use one texture if your CSG object has a lot of components because changing the objects appereance involves changing only one single texture. Second, the file parses faster because the texture has to be parsed only once. This may be a great factor when doing large scenes or animatons. Third, using only one texture saves memory because the texture is only stored once and referenced by all components of the CSG object. Assigning the texture to all n components means that it is stored n times.
Trace the file and you will see a lens-shaped object instead of the two spheres. This is because an intersection consists of the area shared by both shapes, in this case the lens-shaped area where the two spheres overlap. We like this lens-shaped object so we will use it to demostrate differences.
Now let's create a cylinder and stick it right in the middle of the lens.
Render the scene now to see the position of the cylinder. We will place a difference block around both the lens-shaped intersection and the cylinder like this:
Now render the file. You should see the lens-shaped intersection with a neat hole in the middle of it where the cylinder was. The cylinder has been subtracted from the intersection. Note that the pigment of the cylinder causes the surface of the hole to be colored blue. If you eliminate this pigment the surface of the hole will be red.
OK, let's get a little wilder now. Let's declare our perforated lens object to give it a name. Let's also eliminate all textures in the declared object because we will want them to be in the final union instead.
Now, let's use union to build a complex shape composed of copies of this object.
Render it. An interesting object to be sure. But let's try something more. Let's make it a partially-transparent object by adding some filter to the pigment block.
Now render the file again. This looks pretty good... only... you can see parts of each of the lens objects inside the union! This is no good.
Sure enough, it does!
Look at the following example where a cylinder is used to cut a hole in a larger box.
If you trace this object you'll see red speckles where the hole is supposed to be. This is caused by the coincident surfaces of the cylinder and the box. One time the cylinder's surface is hit first by a viewing ray, resulting in the correct rendering of the hole, and another time the box is hit first, leading to a wrong result where the hole vanishes and red speckles appear.
This problem can be avoided by increasing the size of the cylinder to get rid of the coincidence surface. This is done by:
In general you have to make the subtracted object a little bit larger in a CSG difference. Just look for coincident surfaces and increase the subtracted object appropreatly to get rid of those surfaces.
The same problem occurs in CSG intersections and is also avoided by scaling some of the involved objects.