Advanced MFC Programming

Chapter 4. Button
using bit-wise AND operation, the foreground area of the target device will become black. This means on
the target device, every pixel in the foreground area is zero now. Since we use bit-wise XOR mode to
output normal image in the last step, and XORing anything with zero will not change the source, the
foreground area on the target device will finally contain the pattern of the source image. For mask area, the
second ANDing operation doesn’t make any change to it because ANDing a pixel with white color (all 1s)
doesn’t change that pixel. So the overall operations on the mask region is equivalent to two consecutive
XOR operations, which will resume all the pixels in this region to their original colors.
However there is still a slight problem here: if we draw the source and mask bitmaps directly to the
target device using the method mentioned above, we will see a quick flicker on the mask area of the target
device. Although it lasts only for a very short period, it is an undesirable effect. To overcome this, we can
prepare a bitmap in the memory, copy the target image pattern to this bitmap, do the transparent painting on
the memory bitmap, and copy the final result back to the target. Since the background area of the memory
bitmap has the same pattern with the background area of the target device, this final copy will not cause
any flicker.
To customize the drawing behavior of bitmap button, we need to override function
CBitmapButton::OnDrawItem(…). For an owner-draw button, this function will be called when a button
needs to be updated. Actually, menu and combo box also use similar functions. We can create an owner
draw menu or combo box by overriding the member functions with the same name. For these functions,
their parameters are all pointers to DRAWITEMSTRUCT type object. This structure stores information such as
button’s current state (i.e. focused, disabled), the device context that can be used to implement drawing,
and the rectangle indicating where we can output the image pattern.
New Class
Sample 4.5\Btn is based on sample 4.4\Btn, it demonstrates how to create bitmap buttons with
transparent background. In this sample, a new class MCBitmapButton is derived from CBitmapButton.
Besides the default properties inherited from base class, the following new features are added to this class:
1) The new class handles transparent background drawing automatically. Programmer can prepare a
black-and-white mask bitmap and associate it with the bitmap button together with other required
bitmaps. The drawing will be taken care in the member function of the class. Programmer doesn’t need
to add extra code.
2) The mask bitmap can be loaded along with other images by calling either AutoLoad(…) or
LoadBitmaps(…).
2) Function AutoLoad(…) is overridden to load the mask image automatically. In this case, the mask
bitmap must have a string ID that is created by suffixing an ‘M’ character to button’s caption text. For
example, if we want to create mask bitmap for a button whose caption text is “PLAY”, the ID of the
mask bitmap must be “PLAYM”. If we load the mask bitmap using automatic method, there is no
difference between using the new class and CBitmapButton.
2) Mask bitmap could also be loaded by calling function LoadBitmaps(…). The overridden function has
five parameters, the last of which is the ID of the mask bitmap.
2) If the mask bitmap is not present, the bitmap will be output directly to the target device using the
default implementation.
In the sample, a mask bitmap “PLAYM” is added to the application. It will be used to draw button
IDC_PLAY with transparent background.
The new class derived from CBitmapButton is MCBitmapButton. In this class, a new CBitmap type
variable m_bitmapMask is added to load the mask bitmap, also, functions AutoLoad(…) and LoadBitmaps(…)
are overridden. The following code fragment shows this new class:
class MCBitmapButton : public CBitmapButton
{
public:
MCBitmapButton();
BOOL LoadBitmaps
(
LPCTSTR lpszBitmapResource,
LPCTSTR lpszBitmapResourceSel=NULL,
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