A whirl effect in GLSL

whirl.frag

// simple fragment shader

// 'time' contains seconds since the program was linked.
uniform float time;

uniform sampler2D tex;

float radius = .5;

void main()
{
	vec2 coords = gl_TexCoord[0].st;
	float dist = distance(coords, vec2(.5));
	mat2 rotmat;
	if (dist < radius) {
		float percent = (radius - dist) / radius;
		float angle = percent * percent * sin(time) * 4.;
		float sina = sin(angle);
		float cosa = cos(angle);
		rotmat = mat2(cosa, sina, -sina, cosa);
	} else {
		rotmat = mat2(1, 0, 0, 1);
	}

	vec2 texCoord = rotmat * (coords - vec2(.5)) + vec2(.5);
	vec4 diffuse = texture2D(tex, texCoord);
	gl_FragColor = diffuse;
}

whirl.vert

uniform float time;

// simple vertex shader

void main()
{
	gl_Position    = gl_ModelViewProjectionMatrix * gl_Vertex;
	gl_FrontColor  = gl_Color;
	gl_TexCoord[0] = gl_MultiTexCoord0;
}