.NET Graphics - 创建具有透明背景的椭圆

Question

以下代码将在图像上绘制一个椭圆并用番茄颜色填充该椭圆

string imageWithTransEllipsePathToSaveTo = "~/Images/imageTest.png";
Graphics g = Graphics.FromImage(sourceImage);

g.FillEllipse(Brushes.Tomato, 50, 50, 200, 200);

sourceImage.Save(Server.MapPath(imageWithTransEllipsePathToSaveTo), ImageFormat.Png);

如果我将画笔更改为透明，它显然不会显示，因为椭圆将是透明的，下面的图像会显示。

如何将椭圆的“背景”设置为透明，以便图像包含透明点？

编辑：

很抱歉造成混乱，但像这样......

Answer 1

这是我的第二个答案，它使用图像而不是颜色画笔。不幸的是，没有 RadialImageBrush（我知道）。我已包含将图像保存到磁盘的代码，并包含usings 以确保您导入正确的组件。这确实使用 WPF，但它应该作为库或控制台应用程序的一部分工作。

using System.Windows.Media;
using System.Windows.Media.Imaging;
using System;
using System.Windows.Controls;
using System.Windows;
using System.Windows.Shapes;
namespace WpfApplication30
{
    class ImageEditor
    {
        public static void processImage(string loc)
        {
            ImageSource ic = new BitmapImage(new Uri(loc, UriKind.Relative));
            ImageBrush brush = new ImageBrush(ic);
            Path p = new Path();
            p.Fill = brush;
            CombinedGeometry cb = new CombinedGeometry();
            cb.GeometryCombineMode = GeometryCombineMode.Exclude;
            EllipseGeometry ellipse = new EllipseGeometry(new Point(50, 50), 5, 5);
            RectangleGeometry rect = new RectangleGeometry(new Rect(new Size(100, 100)));
            cb.Geometry1 = rect;
            cb.Geometry2 = ellipse;
            p.Data = cb;

            Canvas inkCanvas1 = new Canvas();
            inkCanvas1.Children.Add(p);
            inkCanvas1.Height = 96;
            inkCanvas1.Width = 96;

            inkCanvas1.Measure(new Size(96, 96));
            inkCanvas1.Arrange(new Rect(new Size(96, 96)));
            RenderTargetBitmap targetBitmap =
    new RenderTargetBitmap((int)inkCanvas1.ActualWidth,
                           (int)inkCanvas1.ActualHeight,
                           96d, 96d,
                           PixelFormats.Default);
            targetBitmap.Render(inkCanvas1);

            using (System.IO.FileStream outStream = new System.IO.FileStream( loc.Replace(".png","Copy.png"), System.IO.FileMode.Create))
            {
                PngBitmapEncoder encoder = new PngBitmapEncoder();
                encoder.Frames.Add(BitmapFrame.Create(targetBitmap));
                encoder.Save(outStream);
            }
        }
    }
}

结果如下：

Answer 2

您需要使用半透明颜色创建画笔。

您可以使用 Color.FromArgb(alpha, r, g, b) 执行此操作，其中 alpha 设置不透明度。

Example copied from MSDN:

public void FromArgb1(PaintEventArgs e)
{
    Graphics     g = e.Graphics;

    // Transparent red, green, and blue brushes.
    SolidBrush trnsRedBrush = new SolidBrush(Color.FromArgb(120, 255, 0, 0));
    SolidBrush trnsGreenBrush = new SolidBrush(Color.FromArgb(120, 0, 255, 0));
    SolidBrush trnsBlueBrush = new SolidBrush(Color.FromArgb(120, 0, 0, 255));

    // Base and height of the triangle that is used to position the
    // circles. Each vertex of the triangle is at the center of one of the
    // 3 circles. The base is equal to the diameter of the circles.
    float   triBase = 100;
    float   triHeight = (float)Math.Sqrt(3*(triBase*triBase)/4);

    // Coordinates of first circle's bounding rectangle.
    float   x1 = 40;
    float   y1 = 40;

    // Fill 3 over-lapping circles. Each circle is a different color.
    g.FillEllipse(trnsRedBrush, x1, y1, 2*triHeight, 2*triHeight);
    g.FillEllipse(trnsGreenBrush, x1 + triBase/2, y1 + triHeight,
        2*triHeight, 2*triHeight);
    g.FillEllipse(trnsBlueBrush, x1 + triBase, y1, 2*triHeight, 2*triHeight);
}

Answer 3

您需要使用 RadialGradientBrush：

RadialGradientBrush b = new RadialGradientBrush();
b.GradientOrigin = new Point(0.5, 0.5);
b.Center = new Point(0.5, 0.5);
b.RadiusX = 0.5;
b.RadiusY = 0.5;
b.GradientStops.Add(new GradientStop(Colors.Transparent,0));
b.GradientStops.Add(new GradientStop(Colors.Transparent,0.25));
b.GradientStops.Add(new GradientStop(Colors.Tomato, 0.25));
g.FillEllipse(b, 50, 50, 200, 200);