public override void BeforeRender(WpfDrawingRenderer renderer)
        {
            base.BeforeRender(renderer);

            WpfDrawingContext context = renderer.Context;

            Geometry  clipGeom  = this.ClipGeometry;
            Transform transform = this.Transform;

            SvgUseElement useElement = (SvgUseElement)_svgElement;

            string elementId = this.GetElementName();

            float opacityValue = -1;

            string opacity = useElement.GetAttribute("opacity");

            if (string.IsNullOrWhiteSpace(opacity))
            {
                opacity = useElement.GetPropertyValue("opacity");
            }
            if (!string.IsNullOrWhiteSpace(opacity))
            {
                opacityValue = (float)SvgNumber.ParseNumber(opacity);
                opacityValue = Math.Min(opacityValue, 1);
                opacityValue = Math.Max(opacityValue, 0);
            }

            _drawGroup = new DrawingGroup();

            DrawingGroup currentGroup = context.Peek();

            if (currentGroup == null)
            {
                throw new InvalidOperationException("An existing group is expected.");
            }

            currentGroup.Children.Add(_drawGroup);
            context.Push(_drawGroup);

            if (clipGeom != null || transform != null || (opacityValue >= 0 && opacityValue < 1) ||
                (!string.IsNullOrWhiteSpace(elementId) && !context.IsRegisteredId(elementId)))
            {
                if (clipGeom != null)
                {
                    _drawGroup.ClipGeometry = clipGeom;
                }

                if (transform != null)
                {
                    _drawGroup.Transform = transform;
                }

                if (opacityValue >= 0 && opacityValue < 1)
                {
                    _drawGroup.Opacity = opacityValue;
                }

                if (!string.IsNullOrWhiteSpace(elementId) && !context.IsRegisteredId(elementId))
                {
                    SvgObject.SetName(_drawGroup, elementId);

                    context.RegisterId(elementId);

                    if (context.IncludeRuntime)
                    {
                        SvgObject.SetId(_drawGroup, elementId);
                    }
                }
            }
        }