Twirl glsl

Twirl-glsl.frag

uniform sampler2D textureMap; //image
uniform float amount; //amount of twirl
void main() {
   vec2 uv = gl_TexCoord[0].st-0.5;
   float angle = atan2(uv.y,uv.x);
   float radius = length(uv);
   angle+= radius*amount;
   vec2 shifted = radius*vec2(cos(angle), sin(angle));
   gl_FragColor = texture(textureMap, (shifted+0.5));
}