impl calcTextcoord() method for Sphere class

pyrt/geometry/sphere.py

@@ -5,10 +5,10 @@
 """
 
 from ..geometry import Shape
-from ..math import Ray, HitRecord, Vec3, dot3, G_EPSILON
+from ..math import Ray, HitRecord, Vec2, Vec3, dot3, G_EPSILON
 from ..material import Material, PhongMaterial
 from .bbox import BBox
-from math import sqrt, pi
+from math import sqrt, pi, asin, atan2
 
 
 class Sphere(Shape):
@@ -23,6 +23,24 @@ def __init__(self, center: Vec3, radius: float, material: Material = PhongMateri
         self.material = material
 
 
+    def calcTexcoord(self, p: Vec3) -> Vec2:
+        """
+        Returns texture-coordinate at cartesian position p
+        :param p:
+        :return: returns
+        """
+
+        # change coordinate system's origin to sphere's center
+        p_central = p - self.center
+
+        # sphere radius
+        r = p_central.length()
+
+        u = 1.0 - (atan2(p_central.z, p_central.x) + pi) / (2.0 * pi)
+        v = (asin(p_central.y / r) + pi / 2) / pi
+        return Vec2(u, v)
+
+
     def hit(self, ray: Ray, hitrecord: HitRecord) -> bool:
         """
         Hit ray with sphere.