No i don't use HLFIX for anything i do. If i can't create a valid brush, then i don't.
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I have not tried making a dome with the arch tool. But i think i might know how to do so... in-fact. I'm gonna try right now.
Redka, the way i do it is..thusly: Since a 12 sided cylinder has 3 wedges per 90 degrees, as captain terror mentioned, one of those sides HAS to be a 45 degree angle.
We all know that Hammer doesn't like anything that's not a 45 deg, or 90 deg angle. So i only make the 45 deg angle brushes at different angles at either end. If that confuses you... here's a pic explaining what i mean.
You can see from this pic that the top, bottom, left, and right sides, when flipped, will all be facing in a good direction. And as for the 45 deg angle, as long as you make the sheer in each direction symmetrical, there will be no errors in your map.
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So. 8, 12, 24 sides.. are all good to use for curves and domes and spheres and such. Anything that's going to be hollow, anyway.