Question

I have been doing some work with open gl on the android platform recently and I have been following this tutorial http://blog.jayway.com/2010/02/15/opengl-es-tutorial-for-android-%E2%80%93-part-v/ .
However there is one thing that isn’t really explained in the tutorial and that is what is an index or Indices. So my question is what is meant by indices? what do they do and how do they fit into drawing 3D shapes.

Answer 1

Usually you’ll have your vertices in a float[] in the order x, y, z, x, y, z.... Imagine for a second that each set of x, y, z is a single element in an array. An index is the location of a vertex in that new array.

So let’s say you have the vertices for something simple, a square:

float[] vertices = new float[]
{
    -x, y, 0,  //top left (index 0)
    -x, -y, 0, //bottom left (index 1)
    x, -y, 0,  //bottom right (index 2)
    x, y, 0    //top right (index 3)
};

The proper way to draw the square as GL_TRIANGLES using indices would be by using the indices 0, 1, 2, 0, 2, 3, where 0, 1, 2 create the first triangle and 0, 2, 3 create the second triangle.