我在WPF应用程序中借助WritableBitmapEx在WritableBitmap上绘制了很多形状。 不幸的是，没有准备好去绘制弧线的功能。
我该怎么做：
1。在WritableBitmap上画一个圆弧？
2。在WritableBitmap上绘制具有可变粗细的抗锯齿弧？
我只需要绘制circular arcs。
有可能在Canvas上绘制一个具有可变厚度的漂亮的抗锯齿弧（System.Windows.Media.ArcSegment） - 但是有数千种形状，Canvas的性能很差 - 这就是我使用WritableBitmap的原因。
如果某些算法需要它，我已经计算了弧参数，如： CenterPoint，Radius，StartPoint，EndPoint，StartAngle，EndAngle，ArcLength，IsLarge或Direction
我试图用类似的代码手动绘制它：
int number_of_points = 1000;
for(int i=0; i<=number_of_points; i++){
double progress=(double)i/number_of_points;
double theta = (StartAngle + ArcLength * progress) * Math.PI / 180.0;
draw_pixel(
Center.X + Radius * Math.Cos(theta),
Center.Y + Radius * Math.Sin(theta)
);
}
但是随着图像分辨率的变化，弧的大小变化（如何计算最佳的number_of_points？），不同的弧形厚度和抗锯齿，它开始变得有点棘手。
答案 0 :(得分：1)
在github上分析mono libgdiplus源后，我发现他们正在使用贝塞尔曲线绘制弧线。
我已将其部分功能移植到c＃。 可以使用DrawArc扩展函数（借助WritableBitmapEx的DrawBezier）绘制一个简单的弧。 WritableBitmapEx中没有反锯齿版本的DrawBezier，所以此解决方案回答（仅）我的第一个问题：
namespace System.Windows.Media.Imaging
{
public static partial class WriteableBitmapArcExtensions
{
//port of mono libgdiplus function
//append_arcs (GpPath *path, float x, float y, float width, float height, float startAngle, float sweepAngle)
//from: https://github.com/mono/libgdiplus/blob/master/src/graphics-path.c
public static void DrawArc(this WriteableBitmap bmp, float x, float y, float width, float height, float startAngle, float sweepAngle, Color color)
{
int i;
float drawn = 0;
int increment;
float endAngle;
bool enough = false;
if (Math.Abs(sweepAngle) >= 360)
{
bmp.DrawEllipse((int)x, (int)y, (int)width, (int)height, color);
return;
}
endAngle = startAngle + sweepAngle;
increment = (endAngle < startAngle) ? -90 : 90;
/* i is the number of sub-arcs drawn, each sub-arc can be at most 90 degrees.*/
/* there can be no more then 4 subarcs, ie. 90 + 90 + 90 + (something less than 90) */
for (i = 0; i < 4; i++)
{
float current = startAngle + drawn;
float additional;
if (enough)
return;
additional = endAngle - current; /* otherwise, add the remainder */
if (Math.Abs(additional) > 90)
{
additional = increment;
}
else
{
/* a near zero value will introduce bad artefact in the drawing */
if ((additional >= -0.0001f) && (additional <= 0.0001f))
return;
enough = true;
}
bmp._DrawArc(
x, y,
width, height, /* bounding rectangle */
current, current + additional, color);
drawn += additional;
}
}
//port of mono libgdiplus function
//append_arc (GpPath *path, BOOL start, float x, float y, float width, float height, float startAngle, float endAngle)
//from: https://github.com/mono/libgdiplus/blob/master/src/graphics-path.c
private static void _DrawArc(this WriteableBitmap bmp, float x, float y, float width, float height, float startAngle, float endAngle, Color color)
{
double sin_alpha, sin_beta, cos_alpha, cos_beta;
var rx = width / 2;
var ry = height / 2;
/* center */
var cx = x + rx;
var cy = y + ry;
/* angles in radians */
var alpha = startAngle * Math.PI / 180;
var beta = endAngle * Math.PI / 180;
/* adjust angles for ellipses */
alpha = Math.Atan2(rx * Math.Sin(alpha), ry * Math.Cos(alpha));
beta = Math.Atan2(rx * Math.Sin(beta), ry * Math.Cos(beta));
if (Math.Abs(beta - alpha) > Math.PI)
{
if (beta > alpha)
beta -= 2 * Math.PI;
else
alpha -= 2 * Math.PI;
}
var delta = beta - alpha;
// http://www.stillhq.com/ctpfaq/2001/comp.text.pdf-faq-2001-04.txt (section 2.13)
var bcp = 4.0 / 3 * (1 - Math.Cos(delta / 2)) / Math.Sin(delta / 2);
sin_alpha = Math.Sin(alpha);
sin_beta = Math.Sin(beta);
cos_alpha = Math.Cos(alpha);
cos_beta = Math.Cos(beta);
/* starting point */
double sx = cx + rx * cos_alpha;
double sy = cy + ry * sin_alpha;
//DrawBezier comes from WritableBitmapEx library
bmp.DrawBezier(
(int)(sx),
(int)(sy),
(int)(cx + rx * (cos_alpha - bcp * sin_alpha)),
(int)(cy + ry * (sin_alpha + bcp * cos_alpha)),
(int)(cx + rx * (cos_beta + bcp * sin_beta)),
(int)(cy + ry * (sin_beta - bcp * cos_beta)),
(int)(cx + rx * cos_beta),
(int)(cy + ry * sin_beta),
color
);
}
}
}
我在WritableBitmapEx网站上评论了一个问题：I would like to draw arcs - 所以也许这部分代码会包含在WritableBitmapEx库中。
在阅读了ForeverZer0的评论后，我使用System.Drawing.Graphics和WritableBitmap进行了一些实验。在getting a DrawingContext for a wpf WriteableBitmap的帮助下，我用这样的代码完成了它：
WritableBitmap ret = BitmapFactory.New(img_width, img_height);
ret.Lock();
var bmp = new System.Drawing.Bitmap(
ret.PixelWidth,
ret.PixelHeight,
ret.BackBufferStride,
System.Drawing.Imaging.PixelFormat.Format32bppPArgb,
ret.BackBuffer
);
System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bmp);
g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
g.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.HighQuality;
g.DrawArc(...); //<-- draws an antialiased arc with variable thickness
g.Dispose();
bmp.Dispose();
ret.AddDirtyRect(new Int32Rect(0, 0, ret.PixelWidth, ret.PixelHeight));
ret.Unlock();
return ret; //<-- WritableBitmap with beautifull arc on it;