Hemispheres

OpenGL OpenGL: Basic Coding
1

How can i create a hollow hemisphere in OpenGL? (ie. half of a glutSolidSphere object)

It’s driving me up the wall trying to work it out.

Thanks

2

Get the source code for Glut, and adapt the glutSolidSphere code, it should be pretty easy.
http://www.opengl.org/resources/libraries/glut.html

3

You could try gluSphere() with an additional clipping plane.

4

1 Sorry, but I’m a bit of a noob, where exactly on that site can i get the code for glutSolidSphere?

  1. I thought about clipping planes, but after looking into it, i was under the impression that you can only have 6 clipping planes. I need to create 16 hemishere though.
5

You can get glut source (follow links) at http://www.opengl.org/resources/libraries/glut.html.

6

yeah, ive got the .tar.gz file, but theres a lot of files inside it :smiley:

any clue which one will have the code for gluSphere that i need in it?

7

You can use gluSphere with just 1 clipping plane … even for 16 hemispheres. Just specify the appropriate clipping plane equation before you draw each sphere.

double clipEq[16][4];
glEnable(GL_CLIP_PLANE0);
for(i=0; i<16; i++)
{
   glClipPlane(GL_CLIP_PLANE0, clipEq[i]);
   glPushMatrix();
   // Transform
   gluSphere(q, size, numSegs, numSegs);
   glPopMatrix();
}
8

Originally posted by <LordSigmund>:
which one will have the code for gluSphere that i need in it?
I don’t think GLUT has the gluSphere source … only the glut**Sphere source. See glut_shapes.c.

You can do a google search (e.g. with spherical coordinates) to get an algorithm to generate the mesh yourself. This might be more what you need.

Hope this helps,
Ben

9

Thanks for the help, i think thats the right track, but im having trouble understanding glClipPlane

how does the plane equation define the plane?

what im trying to do is create a transparent hollow cylinders ending in 2 hemispheres. im going to be looking out of them (theyre carriages in a ferris whell to be exact), so i cant just use spheres and hide the other half.

so, when i setup my carriage object, i need to define two vertical and parallel clipping planes at either end of my cylinder

how do i define the clipping planes to do this, i dont understand the manual explanation for the equation?

10

The equation for glClipPlane is the equation [A,B,C,D] that satisfies the relationship Ax+By+Cz+D=0. [x,y,z] is a point in 3-space. [A,B,C] defines the plane normal vector. D defines the distance from the origin to the plane along the plane normal.

In other words, for a given [x,y,z] you are on the plane if the equation yields zero. Anything behind the plane’s half-space (away from normal) is clipped. Anything in front is kept.

Playing around with the book example, clip.c, should help you understand. Try varying the clip equation.

11

thanks a lot mate, just got them to work, sorry for the hassle

12

Originally posted by <LordSigmund>:
… trying to do is create a transparent hollow cylinders ending in 2 hemispheres.
Try this …

   static GLUquadricObj * q = 0;
   if(!q)
   {
      q = gluNewQuadric();
      gluQuadricDrawStyle(q, GLU_FILL);
   }

   double radius = 10.0;
   double clipEq[2][4] =
      { { radius, 0.0, 0.0, 1.0 }, { -radius, 0.0, 0.0, 1.0 } };

   // Red hemisphere
   glPushMatrix();
   glColor3f(1.0, 0.0, 0.0);
   glTranslatef(radius, 0.0, 0.0);
   glEnable(GL_CLIP_PLANE0);
   glClipPlane(GL_CLIP_PLANE0, clipEq[0]);
   gluSphere(q, radius, 16, 16);
   glPopMatrix();

   // Green hemisphere
   glPushMatrix();
   glColor3f(0.0, 1.0, 0.0);
   glTranslatef(-radius, 0.0, 0.0);
   glEnable(GL_CLIP_PLANE0);
   glClipPlane(GL_CLIP_PLANE0, clipEq[1]);
   gluSphere(q, radius, 16, 16);
   glPopMatrix();

   // Blue cylinder
   glPushMatrix();
   glColor3f(0.0, 0.0, 1.0);
   glDisable(GL_CLIP_PLANE0);
   glRotatef(90.0, 0.0, 1.0, 0.0);
   glTranslatef(0.0, 0.0, -radius);
   gluCylinder(q, radius, radius, 2.0*radius, 16, 16);
   glPopMatrix();