Examining the OnDraw() Function

Another function that you need to look at in more detail is the view object's OnDraw() function, which is responsible for updating the main window's display. In the ShowDib application, OnDraw() must display the currently selected DIB.

Because the CDib object is part of the document class, OnDraw() first gets a pointer to the CDib object, accessing the document object through the pDoc pointer supplied by OnDraw(). (The code that supplies this pointer was generated by AppWizard.) As you can see, pDoc points to the application's document object:

CDib* pDib = pDoc->m_pDib;

If the application has loaded a DIB (pDib is not 0), OnDraw() gets pointers to the DIB's image and BITMAPINFO structure:

BYTE* pBmBits = pDib->GetDibBitsPtr();

LPBITMAPINFO pBmInfo = pDib->GetDibInfoPtr();

The function also gets the DIB's width and height:

UINT bmWidth = pDib->GetDibWidth();

UINT bmHeight = pDib->GetDibHeight();

The program then displays the DIB by calling the Windows function StretchDIBits():

StretchDIBits(pDC->m_hDC, 10, 10, bmWidth, bmHeight,

0, 0, bmWidth, bmHeight, pBmBits, pBmInfo,

DIB_RGB_COLORS, SRCCOPY);

This function requires a whopping 13 arguments. The first argument is a handle to the destination device context (that is, the device context on which the DIB should be displayed). Because OnDraw() supplies a pointer to the destination device context (DC), you must use this pointer to access the DC's handle, which is stored in the DC class's m_hDC data member.

To display data, a Windows application must obtain a device context. In review, a device context is little more than a collection of attributes that describe how data should be displayed in a window. These attributes include pen colors and sizes, brush colors and sizes, fill colors, and other important graphical information.

In a normal Windows program, you'd be responsible for creating and managing a device context whenever you wanted to display something on the screen. However, Microsoft Foundation Classes takes care of much of the dirty work for you. In functions like OnDraw(), MFC supplies the device context as one of the function's parameters.

The next four arguments are the X coordinate, Y coordinate, width, and height of the destination rectangle. This rectangle is the area of the destination DC into which you want to copy the DIB. The next four arguments define the source rectangle. In this case, because you want to display the entire DIB, this rectangle has X,Y coordinates of 0,0 and is the same width and height as the DIB.

The ninth and tenth arguments are pointers to the DIB's image data and BITMAPINFO structure. The eleventh argument is a flag that describes the colors used with the DIB. The flag DIB_RGB_COLORS means that the DIB's color table contains actual RGB values, whereas the DIB_PAL_COLORS flag indicates that the color table contains 16-bit indexes into the logical palette. (You learn about logical palettes later in this chapter, in the section "Creating ShowDib, Version 2.") Finally, the last argument is a flag that tells the function how to combine the source and destination rectangles. In this case, SRCCOPY means that the source rectangle should overwrite the destination rectangle.

The StretchDIBits() function can automatically stretch or shrink the size of an image when the size of the destination surface is larger or smaller than the bitmap.

Now that you know how the program works, it's time to try it out.

The ShowDib application can now load and display a DIB. To use the program, select the File menu's Open command and select a DIB file. ShowDib loads the selected DIB and displays it in its main window.

When using this first version of ShowDib, you'll probably notice that, although the program can load and display a DIB, the resulting display sometimes looks pretty weird. This is because, currently, ShowDib ignores the DIB's color table. To display a DIB correctly, a program must create a logical palette from the DIB's color table and then select that palette before actually updating the display window. In the next section, then, you'll add palette-handling features to ShowDib so that it can display DIBs using the correct colors.