Win32 GDI Programming with Pens
Windows Drawing with CreatePen and CreatePenIndirect Tutorial
Oct 24, 2008
In order to draw on a device context, the application needs to select a pen into the device context. This pen can be one of the stock pens, selected using the GetStockObject function, but this does not allow the programmer to choose the style, width, or color of the pen.
To do this, the CreatePen and CreatePenIndirect functions must be used. First, however, it is necessary to introduce the data types that will be used in pen programming with the Windows GDI.
Windows Drawing Programming with Pens
A pen is defined by three key parameters:
- Style - various PS_ identifiers (i.e. PS_SOLID, PS_DASH, etc.);
- Width - in pixels;
- Color - a standard Windows API COLORREF.
These can also be included in a LOGPEN structure, which has three members:
- UINT lopnStyle
- POINT lopnWidth
- COLORREF lopnColor
In the pWidth is defined as a POINT, where only the x parameter (i.e. LOGPEN.lopnWidth.x) is actually used.
The Windows CreatePen Function
The CreatePen function simply takes the three parameters that define the pen that the programmer wants to create, and returns a handle to a pen. This handle can then be selected into the device context (with the SelectObject function), and used to draw with. The function is defined as follows:
- HPEN CreatePen ( UINT, POINT, COLORREF )
To create a solid blue pen, 2 pixels wide, the programmer would use code such as:
The PS_SOLID identifier is one of many that can be used to change the style of the line.
The PS_ Identifiers in the Win32 GDI API
The following PS_ (Pen Style) identifiers can be used in the CreatePen function:
- PS_SOLID - a solid line;PS_DASH - line consisting of dashes;PS_DOT - line consisting of dots;PS_DASHDOT - line of dash and dots;PS_DASHDOTDOT - line of dashes followed by two dots;PS_NULL - invisible pen.
The last of these produces a pen which cannot be seen, but leaves no border. Each of them can also be used in the LOGPEN structure when using the CreatePenIndirect function.
The Windows CreatePenIndirect Function
There are two key uses for the LOGPEN structure. One is to fill in data that can be passed to the CreatePenIndirect function:
- HPEN CreatePenIndirect ( LOGPEN * )
As in the CreatePen function, it returns a handle to a pen that can be used to draw with. However, it takes a pointer to a LOGPEN structure, so the programmer must use the & operator to pass it by reference. The code might look like the following:
This code creates the same blue pen as the previous example. While this may seem longwinded, it's utility is illustrated by the second use for the LOGPEN structure and CreatePenIndirect function - to use the definition of an existing GDI object to populate the structure, before then using it to create another object. This uses the GetObject function:
sizeof(oLogPen), &oLogPen );
Once the oLogPen structure has been filled out, it can be modified, and then used to create a new pen. This advanced use of pens can be practical when creating Windows drawing program code.
