DrawToBitmap 返回空白图像 [英] DrawToBitmap returning blank image

问题描述

我在从我的 winform 应用程序中创建位图图像时遇到问题.

I have a problem on creating bitmap image out of my winform application.

情况:

我有一个名为CanvasControl"的 UserControl，它接受 OnPaint 方法作为我的 Draw Pad 应用程序的画布.在此用户控件中，我有一个函数PrintCanvas()"，它将创建 UserControl 的屏幕截图图像到 PNG 文件中.下面是 PrintCanvas() 函数:

I have a UserControl named as "CanvasControl" that accepts OnPaint method acting as canvas for my Draw Pad application. Inside this user control I have a function "PrintCanvas()" that will create a screenshot image of the UserControl into PNG file. Below is the PrintCanvas() function:

public void PrintCanvas(string filename = "sample.png")
{
    Graphics g = this.CreateGraphics();
    //new bitmap object to save the image        
    Bitmap bmp = new Bitmap(this.Width, this.Height);
    //Drawing control to the bitmap        
    this.DrawToBitmap(bmp, new Rectangle(0, 0, this.Width, this.Height));
    bmp.Save(Application.StartupPath + 
        @"ExperimentFilesExperiment1" + filename, ImageFormat.Png);
    bmp.Dispose();
}

这个用户控件(CanvasControl)在我的主窗体中被调用，用户将在其中绘制一些东西，然后可以选择使用保存按钮进行保存.保存按钮会调出UserControl的PrintCanvas()"函数.

This user control (CanvasControl) is called out inside my main form where user will draw something and have an option to save afterwards using a save button. The save button will call out the "PrintCanvas()" function of the UserControl.

我得到了预期的输出图像文件，但问题是它是一个空白图像.

I get the output image file as expected, but the problem is it was a blank image.

到目前为止我所尝试的:

What I have tried so far:

为了测试这不是语法问题，我尝试将 PrintCanvas() 函数转移到我的主窗体中，令人惊讶的是，我在文件中获得了整个主窗体的图像，但 UserControl 在那里不可见.

To test that it is not a syntax issue, I tried to transfer the PrintCanvas() function into my main form and surprisingly I get an image of the whole main form on file but the UserControl is not visible there.

我是否错过了任何其他设置以使 winform UserControl 可打印?

Is there any other setup i missed out to make a winform UserControl printable?

更新:(绘图程序)

1. 充当画布的用户控件 - 此处代码

推荐答案

问题中的代码给出了第一个提示，但链接中的代码显示了问题的根源:您使用了 Graphics 绘图对象:

The code in the question gave a first hint but the code in the link showed the source of the problem: You use a 'wrong' instance of the Graphics object for drawing:

protected override void OnPaint(PaintEventArgs e)
{
  // If there is an image and it has a location,
  // paint it when the Form is repainted.
  Graphics graphics = this.CreateGraphics();
  ..

这是 winforms 图形最常见的错误之一！永远不要使用 CreateGraphics ！您始终应该在 Paint 或 DrawXXX 事件中使用 Graphics 对象在控制表面上绘图.这些事件有一个参数 e.Graphics，它是唯一可以绘制 persistent 的参数 图形.

This is one of the most common mistakes with winforms graphics! Never use CreateGraphics ! You always should draw onto the control surface with the Graphics object in a Paint or DrawXXX event. These events have a parameter e.Graphics which is the only one that can draw persistent graphics.

持久意味着它总是会在必要时刷新，而不仅仅是在你触发它的时候.这是一个令人讨厌的错误，因为在您遇到外部事件需要重绘之前，一切似乎都正常工作:

Persistent means that it will always be refreshed when necessary, not just when you trigger it. This is a nasty error because everything seems to work until you come upon a situation when an outside event makes redrawing necessary:

• 最小化然后最大化表单
• 将其移出屏幕并再次移回
• 调用DrawToBitmap
• ...

请注意，只有当您使用 PaintEventArgs e 参数中的 有效和当前 Graphics 对象时，所有功能才会真正起作用.

Note that all will only really work if you use the valid and current Graphics object from the PaintEventArgs e parameter.

所以，解决方法很简单:

So, the solution is simple:

protected override void OnPaint(PaintEventArgs e)
{
  // If there is an image and it has a location,
  // paint it when the Form is repainted.
  Graphics graphics = e.Graphics();  // << === !!
  ..

但是 CreateGraphics 有什么用?只对引诱新手犯这种错误有好处??

But what is the CreateGraphics good for? It is only good for luring newbies into that error??

不完全；以下是它的一些用途:

Not quite; here are some uses for it:

• 绘制非永久性图形，例如橡皮筋矩形或特殊鼠标光标
• 在没有实际绘制的情况下使用 TextRenderer 或 MeasureString 方法测量文本大小
• 使用Graphics.DpiX/Y
查询屏幕或Bitmap分辨率

• Drawing non-persistent graphics like a rubber-band rectangle or a special mouse cursor
• Measuring text sizes without actually drawing it with a TextRenderer or the MeasureString method
• Querying the screen or Bitmap resolution with Graphics.DpiX/Y

可能还有其他一些我现在想不起来的……

and probably some others I can't think of at the moment..

所以对于控件上的正常绘图总是使用 e.Grapahics 对象！您可以将其传递给子程序以使代码更有条理，但不要尝试缓存它；它必须是最新的！

So for normal drawing onto controls always use the e.Grapahics object! You can pass it on to subroutines to make the code more structured, but do not try to cache it; it needs to be current!

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