- High r_speeds/polygon count
- Extraneous brushes
- Invalid brushes
- Rounding errors
- Splits in illogical and cumbersome directions
- Brushes not in the carving area also split
- The dreaded LEAK
- Portal errors
And many more...
Let's say you want to make a carve with an 8-sided cylinder. You will most certainly get the following result: Notice the angles that the carve split the brush at. They are haphazard and very difficult to modify (imagine having to add another 8-sided cavity). Also, examine the boxed area - this vertex doesn't even align with the smallest grid size. This can create, among higher r_speeds, the dreaded LEAK.
Instead, build 90 degree brushes around the hole. Then add 45-45-90 degree triangles for the remaining 4 sides. The result is much more efficient, will save your map from such earlier mentioned problems, and looks just as good: Alternately, you could do something like create an arch and set it's thickness so that the inside circle aligns with your cavity, then clip the edges to fit your cavity:
Here is another common situation. You are given a brush and you want to create a simple hall. In this case, carving is appropriate: However, you now decide to add another hallway 90 degrees to your original hall, spanning the entire width of the original brush. If you carve here, you will loose efficiency. Here, 3 brushes created by the carve are highlighted when there should only be 1 brush. in their place: Though carving has proved somewhat inefficient in this case, a little knowledge can easily fix this problem. Simply remove the extraneous brushes:
It all comes down to common sense. Carve only if you know what the results will be and if you know how to manually fine-tune the mess that carving will create. Don't carve with things at funny angles, angles that will result in vertices being placed off the grid. They will cause rounding errors in your vertices. Ultimately this will cause LEAKs and/or inefficiencies, not to mention portal errors and invalid brushes, and will ruin your map!
So please, if you're going to carve at all, DO IT RESPONSIBLY!
(And don't drink and drive...)
It is a bad habit of many new mappers to rely on Hammer's "carve" function. The reason is that carving has a plethroa of potential negative effects. Levels built upon carving more often than not experience many of the following: