GDI+ Drawing Tips
FAQ Home
1. Where can I
see a basic tutorial on GDI+?
2. What are some things to remember when drawing in Window Forms?
3. When can I not use AutoScrolling? In other words why would I ever need
any other form of autoscrolling?
4. Are AutoScrolling bounds setup in World or Device coordinates?
5. Why am I not being able to set a Color.Transparent color as a background
to my control?
6. Why does a Control not draw transparent even after setting it's BackColor
to Transparent?
7. How can I get the screen resolution of my display?
8. How do I draw rotated text?
9. Why does my transparent Panel whose parent has a gradient background
flicker a lot when resized?
10. How do I get a count of the gdi handles currently in use in my
application?
11. How can I minimize flickering when drawing a control?
12. How do I draw circles, rectangles, lines and text?
13. How do I get a snapshot of my desktop?
14. How can I draw without handling a paint message?
1 Where can I see a basic tutorial on GDI+?
Take a look at Mahash Chand's GDI+ Tutorial for Beginners found on
C# Corner.
2 What are some things to remember when drawing in Window Forms?
Check out the Painting techniques using Windows Forms by Fred Balsigerat
gotnetdot.com. It is a good basic discussion of how to get the best
performance from Windows Forms drawing. His hints include leveraging the
power of the .Net Framework by using the proper controls and control styles
as well as consolidating painting code in the OnPaint and OnPaintBackground
methods.
3 When can I not use AutoScrolling? In other words why would I ever need
any other form of autoscrolling?
AutoScrolling does not allow you to dynamically control the scrolling
interval. If you are drawing a complex control such as grid then you want to
be able to scroll based on the current row height, column width etc. With
AutoScrolling you cannot do this. You have to implement Scrolling yourself
in such cases.
Autoscrolling is also not very useful if you want multiple views to share a
scrollbar. The most common place where you see this is with a workbook.
There is no direct way in Winforms to hook up your own scrollbars with the
AutoScrolling implementation.
4 Are AutoScrolling bounds setup in World or Device coordinates?
AutoScrollingMinSize is setup in device coordinates. If you are using a
world coordinate system other than pixels this means that you have to
translate between world and device coordinates before you set these value.
You can use the Graphics.TransformPoints API to do this as shown below.
g.TransformPoints(CoordinateSpace.Device, CoordinateSpace.World,
ptScrollSize);
All this does is go from the GraphicsUnit that you are using to pixels. For
example if your GraphicsUnit is Inch, then with a Graphics object that
represents a monitor screen you will have translated values that come to
about 100 pixels for every logical inch. Using this API ensures that you are
insulated from the underlying device. The values will be much higher for
printers. Another important point to remember is that your Transform in the
painting code (please refer earlier FAQ in this section on AutoScrolling
implementation) will have to be in logical coordinates and will hence have
to translate between logical and device coordinates.
protected void OnHandlePaint(object
sender, PaintEventArgs args)
{
Graphics g = args.Graphics;
g.PageUnit = GraphicsUnit.Inch;
// this will always be in pixels
Point[] ptAutoScrollPos = new Point[]{this.AutoScrollPosition};
// We have to convert the pixel value (device) to inches (logical..world)
g.TransformPoints(CoordinateSpace.World, CoordinateSpace.Device,
ptAutoScrollPos);
// set up a simple translation so that we draw with respect to the doc
bounds
// and not the physical bounds
g.TranslateTransform(ptAutoScrollPos[0].X, ptAutoScrollPos[0].Y);
g.DrawEllipse(_pen, _rectEllipse);
}
Play around with the sample and all should be clear. When you run the sample
nothing will be seen on the screen. Scroll around. Remember the logical
units are in inches. You will be looking at a huge ellipse! Source:
Syncfusion Staff
5 Why am I not being able to set a Color.Transparent color as a
background to my control?
Sometimes the framework will throw an exception if you try to set the bg
color to be transparent. This is because the Control doesn't support
transparent colors. To work around this you should call this method from
within the Control:
[C#]
this.SetStyle(ControlStyles.SupportsTransparentBackColor, true);
[VB.Net]
Me.SetStyle(ControlStyles.SupportsTransparentBackColor, True)
Depending on the Control, you might also have to perform some custom
drawing, then.
6 Why does a Control not draw transparent even after setting it's
BackColor to Transparent?
This is possible if the Control is drawn by the system, rather than by the
framework. This is the case for example with the Label control when it's set
to FlatStyle.System. You should then set to to something else
other than System for your transparent drawing to work.
7 How can I
get the screen resolution of my display?
Use this property: System.Windows.Forms.Screen.PrimaryScreen.Bounds
8 How do I draw rotated text?
private void
pictureBox1_Paint(object sender, System.Windows.Forms.PaintEventArgs e)
{
Graphics g = e.Graphics;
g.TranslateTransform(100.0f, 100.0f);
g.RotateTransform(-90.0f);
g.DrawString("Vertical Text", Font, Brushes.Blue, 0.0f, 0.0f);
g.ResetTransform();
g.TranslateTransform(100.0f, 100.0f);
g.RotateTransform(-45.0f);
g.DrawString("Slanted Text", new Font(Font, FontStyle.Bold),
Brushes.Red, 0.0f, 0.0f);
g.ResetTransform();
}
9 Why does my transparent Panel whose parent has a gradient background
flicker a lot when resized?
Make sure to set these flags for your panel, in it's constructor (for
example). You will have to derive a class from Panel otherwise if you were
initially using the Panel directly.
base.SetStyle(ControlStyles.DoubleBuffer|ControlStyles.DoubleBuffer|ControlStyles.UserPaint,
true);
10 How do I get a count of the gdi handles currently in use in my
application?
You can do so using the native GetGuiResources api. Here is a
sample:
///
/// uiFlags: 0 - Count of GDI objects
/// uiFlags: 1 - Count of USER objects
/// - Win32 GDI objects (pens, brushes, fonts, palettes, regions,
device contexts, bitmap headers)
/// - Win32 USER objects:
/// - WIN32 resources (accelerator tables, bitmap resources,
dialog box templates, font resources, menu resources, raw data resources,
string table entries, message table entries, cursors/icons)
/// - Other USER objects (windows, menus)
///
[DllImport("User32")]
extern public static int GetGuiResources(IntPtr hProcess, int
uiFlags);
public static int GetGuiResourcesGDICount()
{
return GetGuiResources(Process.GetCurrentProcess().Handle,
0);
}
public static int GetGuiResourcesUserCount()
{
return GetGuiResources(Process.GetCurrentProcess().Handle,
1);
}
'
' uiFlags: 0 - Count of GDI objects
' uiFlags: 1 - Count of USER objects
' - Win32 GDI objects (pens, brushes, fonts, palettes, regions,
device contexts, bitmap headers)
' - Win32 USER objects:
' - WIN32 resources (accelerator tables, bitmap resources, dialog
box templates, font resources, menu resources, raw data resources, string
table entries, message table entries, cursors/icons)
' - Other USER objects (windows, menus)
'
extern Public static Integer GetGuiResources(IntPtr hProcess,
Integer uiFlags)
Public Shared Function GetGuiResourcesGDICount() As Integer
Return GetGuiResources(Process.GetCurrentProcess().Handle,0)
End Function
Public Shared Function GetGuiResourcesUserCount() As Integer
Return GetGuiResources(Process.GetCurrentProcess().Handle,1)
End Function
11 How can I minimize flickering when drawing a control?
The Window.Forms framework offers support for double buffering to avoid
flickers through ControlStyles. Double buffering is a technique that
attempts to reduce flicker by doing all the drawing operations on an
off-screen canvas, and then exposing this canvas all at once. To turn on a
control's double buffering, you need to set three styles.
public UserPictureBox()
//derived from System.Windows.Forms.Control
{
// This call is required by the Windows.Forms Form Designer.
InitializeComponent();
// Activates double buffering
SetStyle(ControlStyles.UserPaint, true);
SetStyle(ControlStyles.AllPaintingInWmPaint, true);
SetStyle(ControlStyles.DoubleBuffer, true);
// TODO: Add any initialization after the InitForm call
}
12 How do I draw circles, rectangles, lines and text?
Handle the Paint event for your control or form.
private void
Form1_Paint(object sender, System.Windows.Forms.PaintEventArgs e)
{
Graphics g = e.Graphics;
Pen pen = new Pen(Color.White, 2);
SolidBrush redBrush = new SolidBrush(Color.Red);
g.DrawEllipse(pen, 100,150,100,100);
g.DrawString("Circle", this.Font, redBrush, 80, 150);
g.FillRectangle(redBrush, 140, 35, 20, 40);
g.DrawString("Rectangle", this.Font, redBrush, 80, 50);
g.DrawLine(pen, 114, 110, 150, 110);
g.DrawString("Line", this.Font, redBrush, 80, 104);
}
13 How do I get a snapshot of my desktop?
Here is some code that will do it.
[C#]
internal class NativeMethods
{
[DllImport("user32.dll")]
public extern static IntPtr GetDesktopWindow();
[System.Runtime.InteropServices.DllImport("user32.dll")]
public static extern IntPtr GetWindowDC(IntPtr hwnd);
[System.Runtime.InteropServices.DllImport("gdi32.dll")]
public static extern UInt64 BitBlt
(IntPtr hDestDC,
int x,
int y,
int nWidth,
int nHeight,
IntPtr hSrcDC,
int xSrc,
int ySrc,
System.Int32 dwRop);
}
// Save the screen capture into a jpg
public void SaveScreen()
{
Image myImage = new Bitmap(Screen.PrimaryScreen.Bounds.Width,
Screen.PrimaryScreen.Bounds.Height);
Graphics gr1 = Graphics.FromImage(myImage);
IntPtr dc1 = gr1.GetHdc();
IntPtr dc2 =
NativeMethods.GetWindowDC(NativeMethods.GetDesktopWindow());
NativeMethods.BitBlt(dc1, 0, 0,
Screen.PrimaryScreen.Bounds.Width,
Screen.PrimaryScreen.Bounds.Height, dc2, 0, 0,
13369376);
gr1.ReleaseHdc(dc1);
myImage.Save("screenshot.jpg", ImageFormat.Jpeg);
}
[VB.Net]
Friend Class NativeMethods
Public Shared Function GetDesktopWindow() As IntPtr
End Function
Public Shared Function GetWindowDC(ByVal hwnd As IntPtr) As IntPtr
End Function
Public Shared Function BitBlt(ByVal hDestDC As IntPtr, ByVal x As Integer,
ByVal y As Integer, ByVal nWidth As Integer, ByVal nHeight As Integer, ByVal
hSrcDC As IntPtr, ByVal xSrc As Integer, ByVal ySrc As Integer, ByVal dwRop
As System.Int32) As UInt64
End Function
End Class 'NativeMethods
'Save the screen capture into a jpg
Private Sub SaveScreen()
Dim myImage = New Bitmap(Screen.PrimaryScreen.Bounds.Width,
Screen.PrimaryScreen.Bounds.Height)
Dim gr1 As Graphics = Graphics.FromImage(myImage)
Dim dc1 As IntPtr = gr1.GetHdc()
Dim dc2 As IntPtr = NativeMethods.GetWindowDC(NativeMethods.GetDesktopWindow())
NativeMethods.BitBlt(dc1, 0, 0, Screen.PrimaryScreen.Bounds.Width,
Screen.PrimaryScreen.Bounds.Height, dc2, 0, 0, 13369376)
gr1.ReleaseHdc(dc1)
myImage.Save("screenshot.jpg", ImageFormat.Jpeg)
End Sub 'SaveScreen
14 How can I draw without handling a paint message?
To draw on any hwnd, you need to get a Graphics object from that hwnd. Once
you have the Graphics object, you can then use its methods to draw. Of
course, since this drawing is not done in the Paint handler, it will not be
automatically refreshed when the control is redrawn for any reason.
Graphics g =
Graphics.FromHwnd(this.Handle);
SolidBrush brush = new SolidBrush(Color.Red);
Rectangle rectangle = new Rectangle(25, 25, 100, 100);
g.FillRectangle(brush, rectangle);
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