http://sugi.cc/post/57405474012/get-angle-in-shader UnityのShaderで、角度を求める(0~1)

getAngle.shader

	float PI = 3.14159265359;
	float x = (atan2(v.vertex.z,v.vertex.x) - PI/2.0);
	float y = 2.0*PI;
	float angle = (x - y * floor(x/y))/(2.0*PI);

	//いちばん、Unityで、ただしそうな、かきかた
	float2 p = IN.uv - float2(0.5,0.5);
	angle = fmod(atan2(p.y,p.x)-PI/2+2*PI, 2*PI)/(2*PI);