Create circles/ellipses in Unity UI system in one of three modes: FillInside, FillOutside and Edge.

CircleGraphic.cs

using UnityEngine;
using UnityEngine.UI;

#if UNITY_EDITOR
using UnityEditor;

// Custom Editor to order the variables in the Inspector similar to Image component
[CustomEditor( typeof( CircleGraphic ) ), CanEditMultipleObjects]
public class CircleGraphicEditor : Editor
{
	private SerializedProperty colorProp;

	private void OnEnable()
	{
		colorProp = serializedObject.FindProperty( "m_Color" );
	}

	public override void OnInspectorGUI()
	{
		serializedObject.Update();

		EditorGUILayout.PropertyField( colorProp );
		DrawPropertiesExcluding( serializedObject, "m_Script", "m_Color", "m_OnCullStateChanged" );

		serializedObject.ApplyModifiedProperties();
	}
}
#endif

[RequireComponent( typeof( CanvasRenderer ) )]
[AddComponentMenu( "UI/Circle Graphic", 12 )]
public class CircleGraphic : MaskableGraphic
{
	public enum Mode { FillInside = 0, FillOutside = 1, Edge = 2 };

#pragma warning disable 0649
	[SerializeField]
	private int detail = 64;

	[SerializeField]
	private Mode mode;

	[SerializeField]
	[Tooltip( "Edge mode only" )]
	private float edgeThickness = 1;
#pragma warning restore 0649

	private Vector2 uv = Vector2.zero;
	private Color32 color32;

	private float width = 1f, height = 1f;
	private float deltaWidth, deltaHeight;
	private float deltaRadians;

	protected override void OnPopulateMesh( VertexHelper vh )
	{
		Rect r = GetPixelAdjustedRect();

		color32 = color;
		width = r.width * 0.5f;
		height = r.height * 0.5f;

		vh.Clear();

		Vector2 pivot = rectTransform.pivot;
		deltaWidth = r.width * ( 0.5f - pivot.x );
		deltaHeight = r.height * ( 0.5f - pivot.y );

		if( mode == Mode.FillInside )
		{
			deltaRadians = 360f / detail * Mathf.Deg2Rad;
			FillInside( vh );
		}
		else if( mode == Mode.FillOutside )
		{
			int quarterDetail = ( detail + 3 ) / 4;
			deltaRadians = 360f / ( quarterDetail * 4 ) * Mathf.Deg2Rad;

			vh.AddVert( new Vector3( width + deltaWidth, height + deltaHeight, 0f ), color32, uv );
			vh.AddVert( new Vector3( -width + deltaWidth, height + deltaHeight, 0f ), color32, uv );
			vh.AddVert( new Vector3( -width + deltaWidth, -height + deltaHeight, 0f ), color32, uv );
			vh.AddVert( new Vector3( width + deltaWidth, -height + deltaHeight, 0f ), color32, uv );

			int triangleIndex = 4;
			FillOutside( vh, new Vector3( width + deltaWidth, deltaHeight, 0f ), 0, quarterDetail, ref triangleIndex );
			FillOutside( vh, new Vector3( deltaWidth, height + deltaHeight, 0f ), 1, quarterDetail, ref triangleIndex );
			FillOutside( vh, new Vector3( -width + deltaWidth, deltaHeight, 0f ), 2, quarterDetail, ref triangleIndex );
			FillOutside( vh, new Vector3( deltaWidth, -height + deltaHeight, 0f ), 3, quarterDetail, ref triangleIndex );
		}
		else
		{
			deltaRadians = 360f / detail * Mathf.Deg2Rad;
			GenerateEdges( vh );
		}
	}

	// Uncomment for precise raycasts/clicks (might be processor-heavy)
	//public override bool Raycast( Vector2 sp, Camera eventCamera )
	//{
	//	if( base.Raycast( sp, eventCamera ) )
	//	{
	//		Vector2 localPoint;
	//		if( RectTransformUtility.ScreenPointToLocalPointInRectangle( rectTransform, sp, eventCamera, out localPoint ) )
	//		{
	//			Vector2 deltaPoint = localPoint - rectTransform.rect.center;
	//			float distance = deltaPoint.sqrMagnitude;

	//			float angle = Vector2.Angle( Vector2.right, new Vector2( deltaPoint.x / width, deltaPoint.y / height ) ) * Mathf.Deg2Rad;
	//			Vector2 edge = new Vector2( width * Mathf.Cos( angle ), height * Mathf.Sin( angle ) );

	//			if( mode == Mode.FillInside )
	//				return edge.sqrMagnitude >= distance;
	//			if( mode == Mode.FillOutside )
	//				return edge.sqrMagnitude <= distance;

	//			if( edge.sqrMagnitude < distance )
	//				return false;

	//			angle = Vector2.Angle( Vector2.right, new Vector2( deltaPoint.x / ( width - edgeThickness ), deltaPoint.y / ( height - edgeThickness ) ) ) * Mathf.Deg2Rad;
	//			Vector2 edgeInner = new Vector2( ( width - edgeThickness ) * Mathf.Cos( angle ), ( height - edgeThickness ) * Mathf.Sin( angle ) );
	//			Debug.DrawLine( transform.position, transform.position + (Vector3) edgeInner );
	//			Debug.DrawLine( transform.position, transform.position + (Vector3) deltaPoint );
	//			return edgeInner.sqrMagnitude <= distance;
	//		}
	//	}

	//	return false;
	//}

	private void FillInside( VertexHelper vh )
	{
		vh.AddVert( new Vector3( deltaWidth, deltaHeight, 0f ), color32, uv );
		vh.AddVert( new Vector3( width + deltaWidth, deltaHeight, 0f ), color32, uv );

		int triangleIndex = 2;
		for( int i = 1; i < detail; i++, triangleIndex++ )
		{
			float radians = i * deltaRadians;

			vh.AddVert( new Vector3( Mathf.Cos( radians ) * width + deltaWidth, Mathf.Sin( radians ) * height + deltaHeight, 0f ), color32, uv );
			vh.AddTriangle( triangleIndex, triangleIndex - 1, 0 );
		}

		vh.AddTriangle( 1, triangleIndex - 1, 0 );
	}

	private void FillOutside( VertexHelper vh, Vector3 initialPoint, int quarterIndex, int detail, ref int triangleIndex )
	{
		int startIndex = quarterIndex * detail;
		int endIndex = ( quarterIndex + 1 ) * detail;

		vh.AddVert( initialPoint, color32, uv );
		triangleIndex++;

		for( int i = startIndex + 1; i <= endIndex; i++, triangleIndex++ )
		{
			float radians = i * deltaRadians;

			vh.AddVert( new Vector3( Mathf.Cos( radians ) * width + deltaWidth, Mathf.Sin( radians ) * height + deltaHeight, 0f ), color32, uv );
			vh.AddTriangle( quarterIndex, triangleIndex - 1, triangleIndex );
		}
	}

	private void GenerateEdges( VertexHelper vh )
	{
		float innerWidth = width - edgeThickness;
		float innerHeight = height - edgeThickness;

		vh.AddVert( new Vector3( width + deltaWidth, deltaHeight, 0f ), color32, uv );
		vh.AddVert( new Vector3( innerWidth + deltaWidth, deltaHeight, 0f ), color32, uv );

		int triangleIndex = 2;
		for( int i = 1; i < detail; i++, triangleIndex += 2 )
		{
			float radians = i * deltaRadians;
			float cos = Mathf.Cos( radians );
			float sin = Mathf.Sin( radians );

			vh.AddVert( new Vector3( cos * width + deltaWidth, sin * height + deltaHeight, 0f ), color32, uv );
			vh.AddVert( new Vector3( cos * innerWidth + deltaWidth, sin * innerHeight + deltaHeight, 0f ), color32, uv );

			vh.AddTriangle( triangleIndex, triangleIndex - 2, triangleIndex - 1 );
			vh.AddTriangle( triangleIndex, triangleIndex - 1, triangleIndex + 1 );
		}

		vh.AddTriangle( 0, triangleIndex - 2, triangleIndex - 1 );
		vh.AddTriangle( 0, triangleIndex - 1, 1 );
	}
}