Posted by millerni456 on Sep 28, 2011; 3:31am
URL: https://forum.jogamp.org/glDrawElements-with-TRIANGLE-STRIP-tp3374872.html

Problem Solved: Skip to last post for answer.


Hello, I was working with glDrawElements and seem to have run into a problem and I was looking for assistance.
It would also help to clear things up on glDrawElements, because I haven't found a solid definition out there yet.

glDrawElements takes these parameters as far as I know.....

the mode (GL_TRAIGLE_STRIP),

the number of indices

the size in bytes of the indices

and the index buffer.

So, when I fill in the paramters I don't quite get want I expect.
Im looking to create a sort of truncated cone that looks like this. (looking down z axis).



Here's how I set up my vertices:

Note that the height and radius are specified by the program.
RadialX and Z is talking about coordinate locations. (Distance along those axes).
Also the loop iterates 20 times incrementing the angle by 18 degrees per iteration. (Thus a whole circle).
Since I'm making a triangle strip, the vertices alternate from the bottom of the cone to the top.

float[] data = new float[120]; //20 vertices on top and bottom, * 3 per xyz component.
               
                double h1 = .333 * height;
                                         
                int k = 0;
                for(int i = 0; i<data.length; i+=6)//iterates 20 times.
                {
                        double cos = Math.cos(Math.toRadians(k*18));
                        double sin = Math.sin(Math.toRadians(k*18));
                        System.out.println(k*18);
                        double radialX = radius * cos;//-1 to 1
                        double radialZ = radius * sin;//-1 to 1
                        System.out.println(k*36);
                       
                        /**BOTTOM or cylinder**/
                        //gl.glTexCoord2d(1, 1);
                        data[i] = (float) (radialX * .5);
                        data[i+1] = (float) (h1);
                        data[i+2] = (float) (radialZ * .5);
                       
                        /**TOP of ylinder**/
                        //gl.glTexCoord2d(1, 0);
                        data[i+3] = (float) (radialX);
                        data[i+4] = (float) (height);
                        data[i+5] = (float) (radialZ);
                        k++;
                }

So this should set up the vertices correctly (which it does, I've tested with GL_POINTS).