precision highp float;

varying highp vec2 vUv;
varying highp vec2 vL;
varying highp vec2 vR;
varying highp vec2 vT;
varying highp vec2 vB;

uniform sampler2D uVelocity;

void main() {
    float L = texture2D(uVelocity, vL).x;

    float R = texture2D(uVelocity, vR).x;

    float T = texture2D(uVelocity, vT).y;

    float B = texture2D(uVelocity, vB).y;

    vec2 C = texture2D(uVelocity, vUv).xy;

    if(vL.x < 0.0) {
        L = -C.x;
    }

    if(vR.x > 1.0) {
        R = -C.x;
    }

    if(vT.y > 1.0) {
        T = -C.y;
    }

    if(vB.y < 0.0) {
        B = -C.y;
    }

    float div = 0.5 * (R - L + T - B);

    gl_FragColor = vec4(div, 0.0, 0.0, 1.0);
}