[FRAGMENT SHADER]
precision mediump float;

varying vec2 texCoords;

uniform sampler2D render_texture;

uniform vec2 screen_size;

uniform vec2 swirl_center;

uniform float swirl_radius;
uniform float swirl_amount;
uniform float swirl_strength;

vec2 swirl(vec2 coords)
{
    //Convert to poolar coordinates
    //Get the relative position to the swirl_center
    coords = coords - swirl_center;
    float r = length(coords);
    if(r <= swirl_radius)
    {
        float phi = atan(coords.y, coords.x);
        float distortion = pow(swirl_amount * ((swirl_radius - r) / swirl_radius), 2);

        if(swirl_amount >= 0)
            phi = phi + distortion;
        else
            phi = phi - distortion;

        coords.x = r * cos(phi);
        coords.y = r * sin(phi);
    }
    //Convert it back to global space
    return coords + swirl_center;
}

void main()
{
    vec2 coords = texCoords * screen_size;
    coords = swirl(coords);
    coords /= screen_size;

    gl_FragColor = texture2D(render_texture, coords);
}

[VERTEX SHADER]
layout(location = 0) in vec3 vertexPos;
layout(location = 1) in vec2 vertexUV;

varying vec2 texCoords;

void main()
{
    gl_Position = vec4(vertexPos, 1.f);

    texCoords = vertexUV;
}