VB.Net BufferedGraphics透明背景

我正在使用BufferedGraphics绘制一些东西,然后我将它绘制到用户控件上.这是代码：

Private context As BufferedGraphicsContext
Private grafx As BufferedGraphics

    Private Sub PaintDoc_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Me.SetStyle(ControlStyles.OptimizedDoubleBuffer, True)
        Me.UpdateStyles()
        context = BufferedGraphicsManager.Current
        context.MaximumBuffer = New Size(Me.Width + 1, Me.Height + 1)
        grafx = context.Allocate(Me.CreateGraphics, New Rectangle(0, 0, Me.Width, Me.Height))
        grafx.Graphics.InterpolationMode = Drawing2D.InterpolationMode.HighQualityBilinear
        grafx.Graphics.PixelOffsetMode = Drawing2D.PixelOffsetMode.HighQuality
        grafx.Graphics.SmoothingMode = Drawing2D.SmoothingMode.HighQuality
        grafx.Graphics.Clear(Color.Transparent)
    End Sub

Private Sub PaintDoc_MouseMove(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles Me.MouseMove
    If e.Button = Windows.Forms.MouseButtons.Left Then
        grafx.Graphics.FillEllipse(New SolidBrush(DrawColorFinal), CInt(e.x - (BrushWidth / 2)), CInt(e.y - (BrushWidth / 2)), BrushWidth, BrushWidth)
        Me.Invalidate()
    End If
End Sub

Private Sub PaintDoc_Paint(ByVal sender As Object, ByVal e As System.Windows.Forms.PaintEventArgs) Handles Me.Paint
    grafx.Render(e.Graphics)
End Sub

问题是,grafx不具有透明背景,但是背景为黑色.有什么想法吗？

更新：

感谢Hans Passant解决了问题,这里是支持快速多层图像绘制的最终代码：

Public Class PaintDoc
Public backBuffer(1) As Bitmap
Public bufferGraphics(1) As Graphics
Public layerIndex As Integer = 0

Private Sub PaintDoc_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    Me.SetStyle(ControlStyles.OptimizedDoubleBuffer Or ControlStyles.UserPaint Or ControlStyles.AllPaintingInWmPaint, True)
    Me.UpdateStyles()
    For i As Integer = 0 To backBuffer.Length - 1
        backBuffer(i) = New Bitmap(Me.ClientSize.Width, Me.ClientSize.Height, System.Drawing.Imaging.PixelFormat.Format32bppPArgb)
        bufferGraphics(i) = Graphics.FromImage(backBuffer(i))
        bufferGraphics(i).InterpolationMode = Drawing2D.InterpolationMode.HighQualityBilinear
        bufferGraphics(i).PixelOffsetMode = Drawing2D.PixelOffsetMode.HighQuality
        bufferGraphics(i).SmoothingMode = Drawing2D.SmoothingMode.HighQuality
    Next
End Sub

Private Sub PaintDoc_MouseMove(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles Me.MouseMove
    If e.Button = Windows.Forms.MouseButtons.Left Then
        bufferGraphics(layerIndex).FillEllipse(New SolidBrush(DrawColorFinal), CInt(e.X - (BrushWidth / 2)), CInt(e.Y - (BrushWidth / 2)), BrushWidth, BrushWidth)
        Me.Invalidate()
    End If
End Sub

Private Sub PaintDoc_Paint(ByVal sender As Object, ByVal e As System.Windows.Forms.PaintEventArgs) Handles Me.Paint
    e.Graphics.Clear(Color.Transparent)
    For i As Integer = 0 To backBuffer.Length - 1
        e.Graphics.DrawImage(backBuffer(i), Point.Empty)
    Next
End Sub

Private Sub PaintDoc_Resize(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Resize
    For i As Integer = 0 To backBuffer.Length - 1
        Dim newBuffer As Bitmap = New Bitmap(Me.ClientSize.Width, Me.ClientSize.Height, System.Drawing.Imaging.PixelFormat.Format32bppPArgb)
        If backBuffer(i) IsNot Nothing Then
            Using gr As Graphics = Graphics.FromImage(newBuffer)
                gr.DrawImage(backBuffer(i), Point.Empty)
            End Using
            backBuffer(i) = Nothing
        End If
        backBuffer(i) = newBuffer
    Next
End Sub
End Class

从BufferedGraphics获得的缓冲区没用,它的像素格式错误.您想要创建自己的缓冲区,可以支持透明度.这需要32bpp格式.使其格式化32bppPArgb以使其尽可能快.重写此代码时需要一些额外的支持来调整表单大小并消除所有闪烁：

Public Class Form1

    Private backBuffer As Bitmap

    Public Sub New()
        InitializeComponent()
        Me.DoubleBuffered = True
        Me.ResizeRedraw = True
    End Sub

    Protected Overrides Sub OnResize(ByVal e As System.EventArgs)
        MyBase.OnResize(e)
        Dim newBuffer = New Bitmap(Me.ClientSize.Width, Me.ClientSize.Height, System.Drawing.Imaging.PixelFormat.Format32bppPArgb)
        If backBuffer IsNot Nothing Then
            Using gr As Graphics = Graphics.FromImage(newBuffer)
                gr.DrawImage(backBuffer, Point.Empty)
            End Using
            backBuffer.Dispose()
        End If
        backBuffer = newBuffer
    End Sub

    Protected Overrides Sub OnMouseMove(ByVal e As System.Windows.Forms.MouseEventArgs)
        MyBase.OnMouseMove(e)
        If e.Button = Windows.Forms.MouseButtons.Left Then
            Using gr As Graphics = Graphics.FromImage(backBuffer)
                gr.FillEllipse(New SolidBrush(Color.Blue), CInt(e.X - (100 / 2)), CInt(e.Y - (100 / 2)), 100, 100)
            End Using
            Me.Invalidate()
        End If
    End Sub

    Protected Overrides Sub OnPaint(ByVal e As System.Windows.Forms.PaintEventArgs)
        e.Graphics.DrawImage(backBuffer, Point.Empty)
        MyBase.OnPaint(e)
    End Sub
End Class