ex : u32
      ) -> VOut {
        let pos = array(
          vec2f( 0.0,  0.5),  // top center
          vec2f(-0.5, -0.5),  // bottom left
          vec2f( 0.5, -0.5)   // bottom right
        );
        let bary = array(
          vec3f(1, 0, 0),
          vec3f(0, 1, 0),
          vec3f(0, 0, 1),
        );
        var vout: VOut;
        vout.position = vec4f(pos[vertexIndex], 0.0, 1.0);
        vout.baryCoord = bary[vertexIndex];
        return vout;
      }

      @fragment fn fs(vin: VOut) -> @locat