Half cylinder/Closed cylinder in opengl

Question

I currently have a cylinder

glPushMatrix();
glTranslatef(cylinder->pos[0], cylinder->pos[1], cylinder->pos[2]);
glRotatef(cylinder->angle, -1.0, 0, 0);
gluDisk(quad, 0.0, cylinder->radius, 300, 90);
gluCylinder(quad, cylinder->radius, cylinder->radius, cylinder->height, 30, 1);
gluDeleteQuadric(quad);
glPopMatrix();

but it only renders half of a cylinder instead of the whole thing (the half coming towards the view). Second issue is I don't know how to close the cylinder. The gludisk only closes the bottom of the cylinder and I'm not sure how to close the top side.

Answer 1

If I understand what you're describing, it sounds like you might have back face culling enabled. Do you have a glEnable(GL_CULL_FACE) call in your code? This will eliminate all polygons that face away from you. In any case, once you draw a cylinder that is closed at both sides, it will be fine to keep it enabled.

You can close the cylinder at the top with another gluDisk() call, after applying a translation that moves it to (0, 0, height). For correctness, I believe you need to rotate the disk that closes the cylinder at the bottom. Otherwise it will face to the inside of the cylinder instead of to the outside, which affects the normals, and would get it eliminated by back face culling when it should be visible.

For the full thing, it should look something like this (untested):

glPushMatrix();
glTranslatef(cylinder->pos[0], cylinder->pos[1], cylinder->pos[2]);
glRotatef(cylinder->angle, -1.0f, 0.0f, 0.0f);
gluCylinder(quad, cylinder->radius, cylinder->radius, cylinder->height, 30, 1);
glPushMatrix();
glTranslatef(0.0f, 0.0f, cylinder->height);
gluDisk(quad, 0.0f, cylinder->radius, 30, 1);
glPopMatrix();
glRotatef(180.0f, 1.0f, 0.0f, 0.0f);
gluDisk(quad, 0.0f, cylinder->radius, 30, 1);
gluDeleteQuadric(quad);
glPopMatrix();