gl.ELEMENT_ARRAY_BUFFER, new Uint32Array([
            baseIndex + 0,
            baseIndex + 1,
            baseIndex + 2,
            baseIndex + 2,
            baseIndex + 3,
            baseIndex + 0]), drawType);

        var opt_positionLocation = 0;
        gl.enableVertexAttribArray(opt_positionLocation);
        gl.vertexAttribPointer(opt_positionLocation, 3, gl.FLOAT, false, 0, 0);
    };
    function readLocation(x, y) {
        var pixels = new Uint8Array(1 * 1 * 4);
        gl.readPixels(x, y, 