Bitmap from Drawing not render outside of Load_Form

Question

I'm trying to render bitmap created from drawing to screen but only render after minimize and maximize again.

I follow these steps: Using Bitmaps for Persistent Graphics in C#

But only can render bitmap in screen outside of Load_Form.

If I put the code:

using System.Drawing;
...

Graphics graphicsObj;
myBitmap = new Bitmap(this.ClientRectangle.Width, 
        this.ClientRectangle.Height, 
        Imaging.PixelFormat.Format24bppRgb);
graphicsObj = Graphics.FromImage(myBitmap);

Pen myPen = new Pen(Color.Plum, 3);
Rectangle rectangleObj = new Rectangle(10, 10, 200, 200);
graphicsObj.DrawEllipse(myPen, rectangleObj);
graphicsObj.Dispose();

In other place, for example a button, I need to minimize and maximize to see the image.

Edit:

bmp is a Bitmap global variable I create an instance in form event Load_Form1

bmp = new Bitmap(this.ClientRectangle.Width,
                 this.ClientRectangle.Height,
                 System.Drawing.Imaging.PixelFormat.Format24bppRgb);

Paint event of Form for redraw:

private void Form1_Paint(object sender, PaintEventArgs e)
{
    Graphics graphicsObj = e.Graphics;
    graphicsObj.DrawImage(myBitmap, 0, 0, myBitmap.Width, myBitmap.Height);
    graphicsObj.Dispose();
}

But I need draw inmediately after create the drawing.

The code you show is creating a Bitmap (and leaking a Pen). Where do you think it should show?? Do you assign it to something, like an Image or a BackgroundImage of something?? The mini/max 'trick' sounds as if you are missing an Invalidate to display things drawn in a Paint event, which has nothing to do with the above code.. — TaW
I want to show in the open Form. I only can draw when Paint_Form1 is fired like show in "Edit" but I need draw after the Bitmap is generate. I try to show with graphicsObj.DrawImage(myBitmap, 0, 0, myBitmap.Width, myBitmap.Height); but only work in Form1_Load. I cannot show from the button click that create the bitmap. — gvd
OK, If that's what you want (weird as it sounds), all you need to do is add an this.Invalidate(); after creating the bitmap. And please do not dispose anything you didn't create, like e.Grahphics!! - Of course, simply looking at the code you show us, you do not need any of it at all, except: using( Pen myPen = new Pen(Color.Plum, 3) e.Graphics.DrawEllipse(myPen, rectangleObj);' and the Rectangle, of course. The rest seems to be useless, unless you actually need the Bitmap for some other purpose as well.. - Finally: 24bpp is not recommended unless you really need it, use 32bpp — TaW

TaW TaW · Accepted Answer · 2015-04-26T15:37:41

Not telling us about the bigger picture makes it hard to recommend the best course of action but let me simply assume two possible aims:

either you simply want something drawn onto the Form
or you want to display a bitmap into which you sucessively draw more and more things.

For the first all you need is to code the Paint event like this:

private void Form1_Paint(object sender, PaintEventArgs e)
{
    Rectangle rectangleObj = new Rectangle(10, 10, 200, 200);
    using (Pen myPen = new Pen(Color.Plum, 3))
      e.Graphics.DrawEllipse(myPen, rectangleObj);
}

If the data that control the drawing are dynamic you should store them at class level variables or lists of them and change them as needed so you can use them in the Paint event.

For the latter aim there will be various times when you add to the Bitmap.

So you start by creating a, probably, class level Bitmap:

    public Form1()
    {
        InitializeComponent();
        bmp = new Bitmap(this.ClientRectangle.Width,
                         this.ClientRectangle.Height);         
    }

    Bitmap bmp = null;

And have one or more places where you draw into it like this:

void drawALittle()
{
    Rectangle rectangleObj = new Rectangle(10, 10, 200, 200);
    using (Pen myPen = new Pen(Color.Plum, 3))
    using (Graphics G = Graphics.FromImage(bmp))
    {
        G.DrawEllipse(myPen, rectangleObj);
        //..
    }
    this.Invalidate();
}

Note how I Invalidate the Form after changing the Bitmap, so the Paint event is triggered.

Also note that if these update happen really often it will be a good idea to keep the Graphics object alive between calls; either by making it a class variable like the Bitmap or by keeping it locally in the method that does all the updates and passing it out as a parameter to the drawing method..

In the form's Paint event all you need is

private void Form1_Paint(object sender, PaintEventArgs e)
{
      e.Graphics.DrawImage(bmp, 0, 0);
}

Also note that 32bppARGB is the recommended default format. Is is used for anything you display in any case, so it is most efficient to use it to begin with..

Bitmap from Drawing not render outside of Load_Form

1 Answers