precision highp float;

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

uniform sampler2D uVelocity;
uniform sampler2D uCurl;
uniform float uCurlValue;
uniform float dt;

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

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

    float T = texture2D(uCurl, vT).x;

    float B = texture2D(uCurl, vB).x;

    float C = texture2D(uCurl, vUv).x;

    vec2 force = vec2(abs(T) - abs(B), abs(R) - abs(L)) * 0.5;

    force /= length(force) + 1.;

    force *= uCurlValue * C;

    force.y *= -1.;

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

    gl_FragColor = vec4(vel + force * dt, 0.0, 1.0);
}