如您所述,ArcTo無法繪製完整的橢圓。事實上,當你嘗試減少「平坦」區域時,它變得數字不穩定。另一個考慮是弧線繪製比現代硬件上的Bezier繪圖要慢。這種最現代的系統使用四條貝塞爾曲線來近似一個橢圓而不是繪製一個真正的橢圓。
你可以看到,WPF的EllipseGeometry通過執行以下代碼,上破DrawBezierFigure方法調用,並在調試檢查的PathFigure做到這一點:
using(var ctx = geometry.Open())
{
var ellipse = new EllipseGeometry(new Point(100,100), 10, 10);
var figure = PathGeometry.CreateFromGeometry(ellipse).Figures[0];
DrawBezierFigure(ctx, figure);
}
void DrawBezierFigure(StreamGeometryContext ctx, PathFigure figure)
{
ctx.BeginFigure(figure.StartPoint, figure.IsFilled, figure.IsClosed);
foreach(var segment in figure.Segments.OfType<BezierSegment>())
ctx.BezierTo(segment.Point1, segment.Point2, segment.Point3, segment.IsStroked, segment.IsSmoothJoin);
}
上面的代碼繪製一個簡單方法高效的橢圓到StreamGeometry中,但是非常特殊的代碼。在實際操作中我用繪製任意幾何成StreamGeometryContext定義了幾種通用的擴展方法,所以我可以簡單地寫:
using(var ctx = geometry.Open())
{
ctx.DrawGeometry(new EllipseGeometry(new Point(100,100), 10, 10));
}
這裏是DrawGeometry擴展方法的實現:
public static class GeometryExtensions
{
public static void DrawGeometry(this StreamGeometryContext ctx, Geometry geo)
{
var pathGeometry = geo as PathGeometry ?? PathGeometry.CreateFromGeometry(geo);
foreach(var figure in pathGeometry.Figures)
ctx.DrawFigure(figure);
}
public static void DrawFigure(this StreamGeometryContext ctx, PathFigure figure)
{
ctx.BeginFigure(figure.StartPoint, figure.IsFilled, figure.IsClosed);
foreach(var segment in figure.Segments)
{
var lineSegment = segment as LineSegment;
if(lineSegment!=null) { ctx.LineTo(lineSegment.Point, lineSegment.IsStroked, lineSegment.IsSmoothJoin); continue; }
var bezierSegment = segment as BezierSegment;
if(bezierSegment!=null) { ctx.BezierTo(bezierSegment.Point1, bezierSegment.Point2, bezierSegment.Point3, bezierSegment.IsStroked, bezierSegment.IsSmoothJoin); continue; }
var quadraticSegment = segment as QuadraticBezierSegment;
if(quadraticSegment!=null) { ctx.QuadraticBezierTo(quadraticSegment.Point1, quadraticSegment.Point2, quadraticSegment.IsStroked, quadraticSegment.IsSmoothJoin); continue; }
var polyLineSegment = segment as PolyLineSegment;
if(polyLineSegment!=null) { ctx.PolyLineTo(polyLineSegment.Points, polyLineSegment.IsStroked, polyLineSegment.IsSmoothJoin); continue; }
var polyBezierSegment = segment as PolyBezierSegment;
if(polyBezierSegment!=null) { ctx.PolyBezierTo(polyBezierSegment.Points, polyBezierSegment.IsStroked, polyBezierSegment.IsSmoothJoin); continue; }
var polyQuadraticSegment = segment as PolyQuadraticBezierSegment;
if(polyQuadraticSegment!=null) { ctx.PolyQuadraticBezierTo(polyQuadraticSegment.Points, polyQuadraticSegment.IsStroked, polyQuadraticSegment.IsSmoothJoin); continue; }
var arcSegment = segment as ArcSegment;
if(arcSegment!=null) { ctx.ArcTo(arcSegment.Point, arcSegment.Size, arcSegment.RotationAngle, arcSegment.IsLargeArc, arcSegment.SweepDirection, arcSegment.IsStroked, arcSegment.IsSmoothJoin); continue; }
}
}
}
另一種選擇是自己計算點。通過將控制點設置爲半徑的(Math.Sqrt(2)-1)* 4/3來找到橢圓的最佳近似值。所以,你可以明確地計算積分並繪製貝塞爾如下:
const double ControlPointRatio = (Math.Sqrt(2)-1)*4/3;
var x0 = centerX - radiusX;
var x1 = centerX - radiusX * ControlPointRatio;
var x2 = centerX;
var x3 = centerX + radiusX * ControlPointRatio;
var x4 = centerX + radiusX;
var y0 = centerY - radiusY;
var y1 = centerY - radiusY * ControlPointRatio;
var y2 = centerY;
var y3 = centerY + radiusY * ControlPointRatio;
var y4 = centerY + radiusY;
ctx.BeginFigure(new Point(x2,y0), true, true);
ctx.BezierTo(new Point(x3, y0), new Point(x4, y1), new Point(x4,y2), true, true);
ctx.BezierTo(new Point(x4, y3), new Point(x3, y4), new Point(x2,y4), true, true);
ctx.BezierTo(new Point(x1, y4), new Point(x0, y3), new Point(x0,y2), true, true);
ctx.BezierTo(new Point(x0, y1), new Point(x1, y0), new Point(x2,y0), true, true);
另一種選擇是使用兩個包含arcTo要求,但正如我之前提到的,這是低效率的。如果你想這樣做,我相信你可以弄清楚兩個ArcTo調用的細節。
感謝您的詳細解答! – 2010-06-06 16:06:50