precision mediump float;

varying vec2 vTextureCoord;

uniform sampler2D uSampler;
uniform float radius;
uniform float angle;
uniform vec2 offset;
uniform vec4 filterArea;

vec2 mapCoord( vec2 coord )
{
    coord *= filterArea.xy;
    coord += filterArea.zw;

    return coord;
}

vec2 unmapCoord( vec2 coord )
{
    coord -= filterArea.zw;
    coord /= filterArea.xy;

    return coord;
}

vec2 twist(vec2 coord)
{
    coord -= offset;

    float dist = length(coord);

    if (dist < radius)
    {
        float ratioDist = (radius - dist) / radius;
        float angleMod = ratioDist * ratioDist * angle;
        float s = sin(angleMod);
        float c = cos(angleMod);
        coord = vec2(coord.x * c - coord.y * s, coord.x * s + coord.y * c);
    }

    coord += offset;

    return coord;
}

void main(void)
{

    vec2 coord = mapCoord(vTextureCoord);

    coord = twist(coord);

    coord = unmapCoord(coord);

    gl_FragColor = texture2D(uSampler, coord );

}