原文地址（英）： https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/CSG_Modelling#difference
创建一个合并的对象，使用union操作符，如下图所示。
Usage example:
union() {
cylinder (h = 4, r=1, center = true, $fn=100);
rotate ([90,0,0]) cylinder (h = 4, r=0.9, center = true, $fn=100);
}
注意: union是隐含的，当未使用时。但当两个对象相交时，就成为必然的操作。
从主对象中把所有字对象挖出，如下所示：
Usage example:
difference() {
cylinder (h = 4, r=1, center = true, $fn=100);
rotate ([90,0,0]) cylinder (h = 4, r=0.9, center = true, $fn=100);
}
创建所有子对象的交集，保留重叠的部分。如下所示：
Usage example:
intersection() {
cylinder (h = 4, r=1, center = true, $fn=100);
rotate ([90,0,0]) cylinder (h = 4, r=0.9, center = true, $fn=100);
}
总是计算CSG model(even in OpenCSG preview mode).
Usage example:
render(convexity = 1) { ... }
| convexity | Integer. The convexity parameter specifies the maximum number of front sides (back sides) a ray intersecting the object might penetrate. This parameter is only needed for correctly displaying the object in OpenCSG preview mode and has no effect on the polyhedron rendering. |
This image shows a 2D shape with a convexity of 4, as the ray indicated in red crosses the 2D shape a maximum of 4 times. The convexity of a 3D shape would be determined in a similar way. Setting it to 10 should work fine for most cases.
原文：http://my.oschina.net/u/2306127/blog/376741