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 uPressure;
uniform sampler2D uVelocity;

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

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

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

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

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

    velocity.xy -= vec2(R - L, T - B);

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