Question

I have been trying to make a cylinder in renderscript. This is the code I’ve tried:

public Mesh cylinder(){
    float radius=1.25f, halfLength=5;
    int slices=16;

    Mesh.TriangleMeshBuilder mbo= new TriangleMeshBuilder(mRSGL,3, Mesh.TriangleMeshBuilder.TEXTURE_0);
    for(int i=0; i<slices; i++) {
    float theta = (float) (((float)i)*2.0*Math.PI);
    float nextTheta = (float) (((float)i+1)*2.0*Math.PI);

        /*vertex at middle of end*/
    mbo.addVertex(0.0f, halfLength, 0.0f);

        /*vertices at edges of circle*/
    mbo.addVertex((float)(radius*Math.cos(theta)), halfLength, (float)(radius*Math.sin(theta)));
        mbo.addVertex((float)(radius*Math.cos(nextTheta)), halfLength, (float)(radius*Math.sin(nextTheta)));

        /* the same vertices at the bottom of the cylinder*/

    mbo.addVertex((float)(radius*Math.cos(nextTheta)), -halfLength, (float)(radius*Math.sin(nextTheta)));
    mbo.addVertex((float)(radius*Math.cos(theta)), halfLength, (float)(radius*Math.sin(theta)));
    mbo.addVertex(0.0f, -halfLength, 0.0f);

    mbo.addTriangle(0, 1, 2);
    mbo.addTriangle(3, 4, 5);


}
    return mbo.create(true);

}

But this code gives me a rectangle of length 5. Any ideas where I’m going wrong?

Answer 1

You actually have a few problems here. First, your angles are always equal to multiples of 2pi. You need to divide by the number of sectors when you calculate your angles. Additionally in this step you have an unnecessary explicit type conversion, java will handle the conversion of integer to double for you.

Second, you are constantly adding the same two triangles to the mesh and not adding any triangles for the side of the cylinder, just the two end faces. In your loop when calling addTriangle() you should use indices, for example addTriangle(n, n+1, n+2).

Finally, you were missing a negative sign when you created your 4th vertex, so it was actually at halfLength, not -halfLength.

Try this:

public Mesh cylinder(){
    float radius=1.25f, halfLength=5;
    int slices=16;

    Mesh.TriangleMeshBuilder mbo= new TriangleMeshBuilder(mRSGL,3, Mesh.TriangleMeshBuilder.TEXTURE_0);

    /*vertex at middle of end*/
    mbo.addVertex(0.0f, halfLength, 0.0f);
    mbo.addVertex(0.0f, -halfLength, 0.0f);

    for(int i=0; i<slices; i++) {
         float theta = (float) (i*2.0*Math.PI / slices);
         float nextTheta = (float) ((i+1)*2.0*Math.PI / slices);

         /*vertices at edges of circle*/
         mbo.addVertex((float)(radius*Math.cos(theta)), halfLength, (float)(radius*Math.sin(theta)));
         mbo.addVertex((float)(radius*Math.cos(nextTheta)), halfLength, (float)(radius*Math.sin(nextTheta)));

         /* the same vertices at the bottom of the cylinder*/
         mbo.addVertex((float)(radius*Math.cos(nextTheta)), -halfLength, (float)(radius*Math.sin(nextTheta)));
         mbo.addVertex((float)(radius*Math.cos(theta)), -halfLength, (float)(radius*Math.sin(theta)));

         /*Add the faces for the ends, ordered for back face culling*/
         mbo.addTriangle(4*i+3, 4*i+2, 0); 
         //The offsets here are to adjust for the first two indices being the center points. The sector number (i) is multiplied by 4 because the way you are building this mesh, there are 4 vertices added with each sector
         mbo.addTriangle(4*i+5, 4*i+4, 1);
         /*Add the faces for the side*/
         mbo.addTriangle(4*i+2, 4*i+4, 4*i+5); 
         mbo.addTriangle(4*i+4, 4*i+2, 4*i+3);
    }
return mbo.create(true);

}

I have also added a slight optimization where the vertices for the centers of the circles are created only once, thus saving memory. The order of indices here is for back face culling. Reverse it if you want front face. Should your needs require a more efficient method eventually, allocation builders allow for using trifans and tristrips, but for a mesh of this complexity the ease of triangle meshes is merited. I have run this code on my own system to verify that it works.