Pro OpenGL ES for iOS

80 CHAPTER 3: Building a 3D World^

//Get x-y-z for the first vertex of stack.

vPtr [0] = m_Scale*cosPhi0 * cosTheta; //10

vPtr [1] = m_Scale*sinPhi0*m_Squash;

vPtr [2] = m_Scale*cosPhi0 * sinTheta;

//The same but for the vertex immediately above the

previous one.

vPtr [3] = m_Scale*cosPhi1 * cosTheta;

vPtr [4] = m_Scale*sinPhi1*m_Squash;

vPtr [5] = m_Scale* cosPhi1 * sinTheta;

cPtr [0] = red; //11

cPtr [1] = 0;

cPtr [2] = blue;

cPtr [4] = red;

cPtr [5] = 0;

cPtr [6] = blue;

cPtr [3] = cPtr[7] = 255;

cPtr += 2*4; //12

vPtr += 2*3;

}

blue+=colorIncrment; //13

red-=colorIncrment;

}

}

return self;

}

Okay, so it takes a lot of code to create something as basic as a sphere. Using the

triangle lists is more involved than using the quads in standard OpenGL, but that’s what

we have to work with.

The basic algorithm divides the sphere into stacks and slices. Stacks are the lateral

pieces while slices are vertical. The boundaries of the stacks are generated two at a time

as partners. These form the boundaries of the triangle strip. So, stack A and B are

calculated and subdivided into triangles based on the number of slices around the

circle. The next time through, take stacks B and C and rinse and repeat. Two boundary

conditions apply: