Draw Sphere - order of vertices

问题

I would like draw sphere in pure OpenGL ES 2.0 without any engines. I write next code:

 int GenerateSphere (int Slices, float radius, GLfloat **vertices, GLfloat **colors) {
 srand(time(NULL));
 int i=0, j = 0;
 int Parallels = Slices ;
 float tempColor = 0.0f;    
 int VerticesCount = ( Parallels + 1 ) * ( Slices + 1 );
 float angleStep = (2.0f * M_PI) / ((float) Slices);

 // Allocate memory for buffers
 if ( vertices != NULL ) {
    *vertices = malloc ( sizeof(GLfloat) * 3 * VerticesCount );
 }
 if ( colors != NULL) {
    *colors = malloc( sizeof(GLfloat) * 4 * VerticesCount);
 }

 for ( i = 0; i < Parallels+1; i++ ) {
     for ( j = 0; j < Slices+1 ; j++ ) {

         int vertex = ( i * (Slices + 1) + j ) * 3;

         (*vertices)[vertex + 0] = radius * sinf ( angleStep * (float)i ) *
                    sinf ( angleStep * (float)j );
         (*vertices)[vertex + 1] = radius * cosf ( angleStep * (float)i );
         (*vertices)[vertex + 2] = radius * sinf ( angleStep * (float)i ) *
                        cosf ( angleStep * (float)j );
         if ( colors ) {
                int colorIndex = ( i * (Slices + 1) + j ) * 4;
                tempColor = (float)(rand()%100)/100.0f;

                (*colors)[colorIndex + 0] =  0.0f;
                (*colors)[colorIndex + 1] =  0.0f;
                (*colors)[colorIndex + 2] =  0.0f;
                (*colors)[colorIndex + (rand()%4)] = tempColor;
                (*colors)[colorIndex + 3] =  1.0f;
            }
        }
    }
    return VerticesCount;
}

I'm drawing it with using next code:

glDrawArrays(GL_TRIANGLE_STRIP, 0, userData->numVertices);

Where userData->numVertices - VerticesCount from function GenerateSphere. But on screen draws series triangles, these aren't sphere approximation! I think, I need to numerate vertices and use OpenGL ES 2.0 function glDrawElements() (with array, contained number vertices). But series of triangles drawn on the screen is not a sphere approximation. How can I draw sphere approximation? How specify order vertices (indices in OpenGL ES 2.0 terms)?

回答1:

Before you start with anything in OpenGL ES, here is some advice:

Avoid bloating CPU/GPU performance

Removing intense cycles of calculations by rendering the shapes offline using another program will surely help. These programs will provide additional details about the shapes/meshes apart from exporting the resultant collection of points [x,y,z] comprising the shapes etc.

I went through all this pain way back, because I kept trying to search for algorithms to render spheres etc and then trying to optimize them. I just wanted to save your time in the future. Just use Blender and then your favorite programming language to parse the obj files that are exported from Blender, I use Perl. Here are the steps to render sphere: (use glDrawElements because the obj file contains the array of indices)

1) Download and install Blender.

2) From the menu, add sphere and then reduce the number of rings and segments.

3) Select the entire shape and triangulate it.

4) Export an obj file and parse it for the meshes.

You should be able to grasp the logic to render sphere from this file: http://pastebin.com/4esQdVPP. It is for Android, but the concepts are same. Hope this helps.

回答2:

I struggled with spheres and other geometric shapes. I worked at it a while and created an Objective-C class to create coordinates, normals, and texture coordinates both using indexed and non-indexed mechanisms, the class is here:

http://www.whynotsometime.com/Why_Not_Sometime/Code_Snippets.html

What is interesting to see the resulting triangles representing the geometry is to reduce the resolution (set the resolution property before generating the coordinates). Also, you can use GL_LINE_STRIP instead of GL_TRIANGLES to see a bit more.

I agree with the comment from wimp that since calculating the coordinates generally happens once, not many CPU cycles are used. Also, sometimes one does want to draw only a ball or world or...

来源：https://stackoverflow.com/questions/14359425/draw-sphere-order-of-vertices

标签

geometry

opengl-es-2.0