ying vec2 vT;
    varying vec2 vB;
    uniform vec2 u_texel;

    void main () {
        vUv = .5 * (a_position + 1.);
        vL = vUv - vec2(u_texel.x, 0.);
        vR = vUv + vec2(u_texel.x, 0.);
        vT = vUv + vec2(0., u_texel.y);
        vB = vUv - vec2(0., u_texel.y);
        gl_Position = vec4(a_position, 0., 1.);
    }
</script>

<script type="x-shader/x-fragment" id="fragShaderAdvection">
    precision highp float;
    precision highp sampler2D;

    varying vec2 vUv;
    uni