Advanced MFC Programming

Chapter 8. DC, Pen, Brush and Palette
1) Select the GDI object into the DC, use a pointer to store the old GDI object.
1) Perform drawing operations.
1) Select the old GDI object into the DC, this will select the new GDI object out of the DC.
1) Destroy the GDI object if necessary (If the GDI object was created in step 2 and will not be used by
other DCs from now on).
The following sections will discuss how to use specific GDI objects to draw various kind of graphic
objects.
8.1 Line
Creating Pen
Sample 8.1\GDI demonstrates how to create a pen and use it to draw lines. The sample is a standard
SDI application generated from Application Wizard.
To draw a line, we need the following information: starting and ending points, width of the line,
pattern of the line, and color. There are several types of pens that can be created: solid pen, dotted pen,
dashed pen. Besides drawing patterns, each pen can have a different color and different width. So if we
want to draw two types of lines, we need to create two different pens.
In MFC, pen is supported by class CPen, it has a member function CPen::CreatePen(…), which can be
used to create a pen. This function has several versions, the following is one of them:
BOOL CPen::CreatePen(int nPenStyle, int nWidth, COLORREF crColor);
Parameter nPenStyle specifies the pen style, it can be any of the following: PS_SOLID, PS_DASH,
PS_DOT, PS_DASHDOT, PS_DASHDOTDOT, PS_NULL, etc. The meanings of these styles are self-explanatory. The
second parameter nWidth specifies width of the pen. Please note that if we create pen with a style other than
PS_SOLID, this width can not exceed 1 device unit. Parameter crColor specifies color of the pen, which can
be specified using RGB macro.
In MFC’s document/view structure, the data should be stored in CDocument derived class and the
drawing should be carried out in CView derived class (for SDI or MDI applications). Since CView is derived
from CWnd, we can obtain its DC by either calling function CWnd::GetDC() or declare a CClientDC type
variable using the window’s pointer. To select a GDI object into the DC, we need to call function
CDC::SelectObject(…). This function returns a pointer to a GDI object of the same type that is being
selected by the DC. Before we delete the GDI object, we need to use this pointer to select the old GDI
object into the DC so that the new DC will be selected out of the DC.
We can not delete a GDI object when it is being selected by a DC. If we do so, the application will
become abnormal, and may cause GPF (General protection fault) error.
Drawing Mode
Besides drawing lines, sample 8.1\GDI also demonstrates how to implement an interactive
environment that lets the user use mouse to draw lines anywhere within the client window. In the sample,
the user can start drawing by clicking mouse’s left button, and dragging the mouse with left button held
down, releasing the button to finish the drawing. When the user is dragging the mouse, dotted outlines will
be drawn temporarily on the window. After the ending point is decided, the line will be actually drawn
(with red color and a width of 1). In order to implement this, the following messages are handled in the
application: WM_LBUTTONDOWN, WM_MOUSEMOVE and WM_LBUTTONUP.
Before the left mouse button is released, we have to erase the old outline before drawing a new one in
order to give the user an impression that the outline “moves” with the mouse cursor. The best way to
implement this type of operations is using XOR drawing mode. With this method, we can simply draw an
object twice in order to remove it from the device.
XOR bit-wise operation is very powerful, we can use it to generate many special drawing effects.
Remember when we perform a drawing operation, what actually happens in the hardware level is that data
202