diff --git a/README.md b/README.md
index 95f58b0..1797efe 100644
--- a/README.md
+++ b/README.md
@@ -9,6 +9,7 @@
 A rewrite of the original CAD package [`sdfx`](https://github.com/deadsy/sdfx) for generating 2D and 3D geometry using [Go](https://go.dev/).
 
 ## Highlights
+* GUI with real-time rendering using [sdf3ui](https://github.com/soypat/sdf3ui).
 * 3d and 2d objects modelled with signed distance functions (SDFs).
 * Minimal and idiomatic API.
 * Render objects as triangles or save to STL, 3MF(experimental) file format.
diff --git a/internal/d3/box.go b/internal/d3/box.go
index a0b5344..fa99274 100644
--- a/internal/d3/box.go
+++ b/internal/d3/box.go
@@ -16,6 +16,14 @@ func NewBox(center, size r3.Vec) Box {
 	return Box{Min: r3.Sub(center, half), Max: r3.Add(center, half)}
 }
 
+// CenteredBox creates a Box with a given center and size.
+// Negative components of size will be interpreted as zero.
+func CenteredBox(center, size r3.Vec) Box {
+	size = MaxElem(size, r3.Vec{}) // set negative values to zero.
+	half := r3.Scale(0.5, size)
+	return Box{Min: r3.Sub(center, half), Max: r3.Add(center, half)}
+}
+
 // Equals test the equality of 3d boxes.
 func (a Box) Equals(b Box, tol float64) bool {
 	return EqualWithin(a.Min, b.Min, tol) && EqualWithin(a.Max, b.Max, tol)