improve circle geometry API

geometry/circle.go

@@ -10,33 +10,41 @@ import (
 	"math"
 )
 
-// Circle represents a circle geometry
+// Circle represents the geometry of a filled circle (i.e. a disk)
+// The center of the circle is at the origin, and theta runs counter-clockwise
+// on the XY plane, starting at (x,y,z)=(1,0,0).
 type Circle struct {
 	Geometry
 	Radius      float64
-	Segments    int
+	Segments    int // >= 3
 	ThetaStart  float64
 	ThetaLength float64
 }
 
-// NewCircle creates and returns a pointer to a new Circle geometry object.
-// The geometry is defined by its radius, the number of segments (triangles), minimum = 3,
-// the start angle in radians for the first segment (thetaStart) and
-// the central angle in radians (thetaLength) of the circular sector.
-func NewCircle(radius float64, segments int, thetaStart, thetaLength float64) *Circle {
+// NewCircle creates a new circle geometry with the specified radius
+// and number of radial segments/triangles (minimum 3).
+func NewCircle(radius float64, segments int) *Circle {
+	return NewCircleSector(radius, segments, 0, 2*math.Pi)
+}
+
+// NewCircleSector creates a new circle or circular sector geometry with the specified radius,
+// number of radial segments/triangles (minimum 3), sector start angle in radians (thetaStart),
+// and sector size angle in radians (thetaLength). This is the Circle constructor with most tunable parameters.
+func NewCircleSector(radius float64, segments int, thetaStart, thetaLength float64) *Circle {
 
 	circ := new(Circle)
 	circ.Geometry.Init()
 
+	// Validate arguments
+	if segments < 3 {
+		panic("Invalid argument: segments. The number of segments needs to be greater or equal to 3.")
+	}
+
 	circ.Radius = radius
 	circ.Segments = segments
 	circ.ThetaStart = thetaStart
 	circ.ThetaLength = thetaLength
 
-	if segments < 3 {
-		segments = 3
-	}
-
 	// Create buffers
 	positions := math32.NewArrayF32(0, 16)
 	normals := math32.NewArrayF32(0, 16)
@@ -52,10 +60,11 @@ func NewCircle(radius float64, segments int, thetaStart, thetaLength float64) *C
 	normal.Z = 1
 	normals.AppendVector3(&normal)
 
-	// Append circle center uv coord
+	// Append circle center uv coordinate
 	centerUV := math32.NewVector2(0.5, 0.5)
 	uvs.AppendVector2(centerUV)
 
+	// Generate the segments
 	for i := 0; i <= segments; i++ {
 		segment := thetaStart + float64(i)/float64(segments)*thetaLength