.vert * (pixel[0] + pixel[1] * 2 + pixel[2] * 3);

            dzdx &#x3D; (z1 - z0) / dp;

            // determine elevation for (pixelX, y0)
            offset &#x3D; (y0 * width + pixelX) * 4;
            pixel[0] &#x3D; elevationData[offset];
            pixel[1] &#x3D; elevationData[offset + 1];
            pixel[2] &#x3D; elevationData[offset + 2];
            pixel[3] &#x3D; elevationData[offset + 3];
            z0 &#x3D; data.vert * (pixel[0] + pixel[1] * 2 + pixel[2] * 3);

            // determi