Some line unexpectedly hidden at some angles


I draw a cube (with 6 polygons) and a cylinder (using gluCylinder()). The cube and cylinder are drawn in the GL_LINE polygon mode. Now the cube works just fine. But at some angles, some lines of the cylinder partly disappear. They look like the unseen part is outside the clipping volume. But it shouldn’t be the projection voume problem because rendering becomes normal when changing from GL_LINE to GL_FILL.

I increase the line width but it doesn’t help. Any suggestion is most appreciated.