2 files changed, 7 insertions, 0 deletions
diff --git a/object.h b/object.h
index 26548c5..c679d83 100644
--- a/object.h
+++ b/object.h
@@ -28,6 +28,9 @@ public:
         return hit(r, t, t1, std::numeric_limits<double>::infinity());
     }
 
+    // If given position is on the object's surface.
+    virtual bool is_on(const vec3d &p) const = 0;
+
     // Given a point on the surface, returns the normalized outer normal vector on that point.
     virtual vec3d normal_vector(const vec3d &where) const = 0;
 
diff --git a/sphere.h b/sphere.h
index fa181e7..a5a935e 100644
--- a/sphere.h
+++ b/sphere.h
@@ -81,6 +81,10 @@ public:
         return hit;
     }
 
+    bool is_on(const vec3d &p) const override {
+        return ((p - center).mod2() - radius * radius) < 1e-10;
+    }
+
     pixel8b color() const override {
         return pixel8b::from_normalized(1.0, 0.0, 0.0);
     }