uniform float s;

#vertex

void main() {
    gl_TexCoord[0] = gl_MultiTexCoord0;
    gl_Position = ftransform();
}

#fragment

uniform sampler2D tex;

void main() {
    vec2 tc = gl_TexCoord[0].xy - 0.5;
    float r = sqrt(tc.x*tc.x + tc.y*tc.y);
    tc /= r;
    r = sin(r * s) / s;
    gl_FragColor = texture2D(tex, (tc * r) + 0.5);
}