在Windows窗体中使用alpha通道保存一种颜色位图会保存另一种(错误的)颜色

在C#、. NET 2.0,Windows窗体,Visual Studio Express 2010中,我保存的是相同颜色的图像：

  Bitmap bitmap = new Bitmap(width, height, PixelFormat.Format32bppArgb);
  using (Graphics graphics = Graphics.FromImage(bitmap))
  {
      Brush brush = new SolidBrush(color);
      graphics.FillRectangle(brush, 0, 0, width, height);
      brush.Dispose();
  }

  bitmap.Save("test.png");
  bitmap.Save("test.bmp");

例如,如果我正在使用

颜色[A = 153,R = 193,G = 204,B = 17]或#C1CC11

保存图像并在外部查看器(例如Paint.NET,IrfanView,XNView等)中打开它后,会被告知图像的颜色实际上是：

颜色[A = 153,R = 193,G = 203,B = 16]或#C1CB10

所以它是相似的颜色,但是不一样！

我尝试了保存PNG和BMP.

当涉及透明度(alpha)时,.NET将保存不同的颜色！
当Alpha为255(无透明度)时,它会保存相应的颜色.

解决方法:

谢谢Joe和Hans Passant的评论.

是的,正如乔所说,问题就在网上：

graphics.FillRectangle(brush, 0, 0, width, height);

在这里,GDI会用类似的颜色修改颜色,但不是精确的颜色.

看来解决方案是使用Bitmap.LockBits和Marshal.Copy直接在像素中写入颜色值：

        Bitmap bitmap = new Bitmap(this.currentSampleWidth, this.currentSampleHeight, PixelFormat.Format32bppArgb);

        // Lock the bitmap's bits.  
        Rectangle rect = new Rectangle(0, 0, bitmap.Width, bitmap.Height);
        BitmapData bmpData = bitmap.LockBits(rect, System.Drawing.Imaging.ImageLockMode.ReadWrite, bitmap.PixelFormat);

        // Get the address of the first line.
        IntPtr ptr = bmpData.Scan0;

        // Declare an array to hold the bytes of the bitmap (32 bits per pixel)
        int pixelsCount = bitmap.Width * bitmap.Height;
        int[] argbValues = new int[pixelsCount];

        // Copy the RGB values into the array.
        System.Runtime.InteropServices.Marshal.Copy(ptr, argbValues, 0, pixelsCount);

        // Set the color value for each pixel.
        for (int counter = 0; counter < argbValues.Length; counter++)
            argbValues[counter] = color.ToArgb();

        // Copy the RGB values back to the bitmap
        System.Runtime.InteropServices.Marshal.Copy(argbValues, 0, ptr, pixelsCount);

        // Unlock the bits.
        bitmap.UnlockBits(bmpData);

        return bitmap;

标签：alpha-transparency,system-drawing,bitmap,c,winforms
来源： https://codeday.me/bug/20191101/1987215.html