diff --git a/src/object/quad.rs b/src/object/quad.rs
index e0bbd78..aca63ef 100644
--- a/src/object/quad.rs
+++ b/src/object/quad.rs
@@ -50,6 +50,10 @@ impl Renderable for Quad {
     fn as_any(&self) -> &dyn Any {
         self
     }
+
+    fn center(&self) -> crate::ds::Vector3 {
+        self.q + self.u/2.0 + self.v/2.0
+    }
 }
 
 impl ToGpu<GpuQuad> for Quad {
diff --git a/src/object/renderable.rs b/src/object/renderable.rs
index 520009f..33001dd 100644
--- a/src/object/renderable.rs
+++ b/src/object/renderable.rs
@@ -1,5 +1,6 @@
 pub trait Renderable: Send + Sync {
     fn as_any(&self) -> &dyn std::any::Any;
+    fn center(&self) -> crate::ds::Vector3;
 }
 
 pub trait ToGpu<T> {
diff --git a/src/object/sphere.rs b/src/object/sphere.rs
index e4cfc1f..bf13123 100644
--- a/src/object/sphere.rs
+++ b/src/object/sphere.rs
@@ -38,6 +38,10 @@ impl Renderable for Sphere {
     fn as_any(&self) -> &dyn Any {
         self
     }
+
+    fn center(&self) -> crate::ds::Vector3 {
+        self.center
+    }
 }
 
 impl ToGpu<GpuSphere> for Sphere {