Does the order of indices matter?

Question

this._vertices = new Float32Array([
            -0.5, 0, 0, // left
            0, 0.5, 0, // top
            0.5, 0, 0 // right
        ]);

this._indicies = new Uint16Array([0, 1, 2]);

As you can see I have 3 points for a triangle. The problem with this is that my triangle doesn't end up getting rendered unless I change the indices to

this._indicies = new Uint16Array([0, 2, 1]);

do you know why that is? Why does the order of the indices matter? And how do I know the correct order to put the indices in?

Ps. It works when setting the draw type to LINE_LOOP but it doesn't work on triangles.

Answer 1

If culling is on gl.enable(gl.CULL_FACE) then triangles are culled if their vertex are counter clockwise in clip space (ie, after the vertex shader). You can choose which triangles, clockwise or counter-clockwise get culled with gl.cullFace(...)

     0             0
    / \           / \
   /   \         /   \
  2-----1       1-----2

clockwise   counter-clockwise