kernel vec4 curl(sampler image, float r, float a)
{
	vec2 p = destCoord();
	float ra = r * a;
	float z = ra - p.y;
	float angle = asin(z / r);
	
	// with perspective:
	//float newx = p.x + (0.5 + p.x / samplerExtent(image).z) * r * (cos(angle) - 1.0);
	
	// less:
	float newx = p.x + (0.8 + p.x / samplerExtent(image).z / 3.0) * r * (cos(angle) - 1.0);
	
	// none:
	//float newx = p.x + r * (cos(angle) - 1.0);
	
	float newy = r * (ra / r - angle);

	//p.x = compare(z, p.x, newx);
	//p.y = compare(z, p.y, newy);

	vec2 s = compare(vec2(z, z), p, vec2(newx, newy));
	
	return sample(image, samplerTransform(image, s));
}