precision highp float;

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

uniform sampler2D uVelocity;

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

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

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

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

    float vorticity = R - L - T + B;

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