like this: http://loh.fr/gallery/?Qwd=./Panorama/Schlossplatz&Qif=Erlanger-Schlossplatz_-_7_-_Tunnel_-_final.jpg

fishEye.fsh

uniform sampler2D tex;

float M_2PI = 6.283185307179586;
float SQRT2 = 1.4142135623730951;

void main()
{
	float x = (gl_TexCoord[0].x - 0.5);
	float y = (gl_TexCoord[0].y - 0.5);
	float r = sqrt(x*x+y*y);
	float s = atan(y, x)/M_2PI;
	if (y<0.0) {
		s = s + 1.0;
	}
	s = (1.75-s);
	if (s>1.0) { s = s - 1.0; }
	gl_FragColor = texture2D(tex, vec2(s, r*SQRT2));
	// gl_FragColor = texture2D(tex, vec2(s, 1.0 - r*SQRT2));
}