unction checkPixels() {
        function checkPixel(x, y, color) {
            const screen = (s, t) => s * (t * 0.5 + 0.5);
            wtu.checkCanvasRect(gl, screen(w, x), screen(h, y), 1, 1, color,
                                `(${x.toFixed(3)}, ${y.toFixed(3)}): ${color} `);
        }
        for (const signX of [+1, -1]) {
            for (const signY of [+1, -1]) {
                // This function expects screen-space coordinates
                // normalized to [-1, +1]. The region from [0, 0]
  