10.4 Wm_contextmenu message processing

The WM_CONTEXTMENU message notifies a window that the user clicked the right mouse button (right clicked) in the window.

hwnd = (HWND) wParam;

xPos = LOWORD(lParam);

yPos = HIWORD(lParam);

hwnd - Handle to the window in which the user right clicked the mouse. This can be a child window of the window receiving the message. xPos - Horizontal position of the cursor, in screen coordinates, at the time of the mouse click.

yPos - Vertical position of the cursor, in screen coordinates, at the time of the mouse click.

A window can process this message by displaying a shortcut menu using the TrackPopupMenu or TrackPopupMenuEx function.

If a window does not display a shortcut menu it should pass this message to the DefWindowProc function. If a window is a child window, DefWindowProc sends the message to the parent. Otherwise, DefWindowProc displays a default shortcut menu if the specified position is in the window’s caption.

DefWindowProc generates the WM_CONTEXTMENU message when it processes the WM_RBUTTONUP or WM_NCRBUTTONUP message.

11. Manipulating Device-Independent Bitmaps

11.1 The types of bitmap

The term bitmap is frequently misunderstood by new Windows programmers. For most people, a bitmap is a picture that can be displayed on the screen. While this definition is generally true, there are actually two main types of bitmaps used with Windows applications. The first, called device-independent bitmaps (DIBs), are found in picture files with a .BMP file extension and are the type of bitmap with which most people are familiar. You'll learn about DIBs in the next chapter.

The second type of bitmap, called a device-dependent bitmap (DDB), resides only in the computer's memory and is usually not a picture per se but rather some sort of image that a Windows application needs to create its display. In this chapter, you learn about DDBs and how they're used to build an application's display.

DIBs are the picture files that most people think of as bitmaps. DIBs are device independent because their file includes the color information needed to reproduce the picture on other devices. DDBs, on the other hand, don't include color tables, so you'll never (well, maybe I shouldn't say never) find DDBs saved to files. Instead, these types of images usually are created directly in the computer's memory and disappear when the application that created them terminates.

Because of their nature, DDBs are more utilitarian than DIBs. That is, the Windows programmer uses DDBs as tools for creating an application's display rather than displaying DDBs simply as pictures. Think of a Windows paint application. Often, a paint application enables the user to copy and paste a portion of the display. The user might, for example, copy the image of a tree and paste it down in various places on the display to create a forest. The tree image is a bitmap in the computer's memory. It's unlikely that the tree image will ever be saved to disk. On the other hand, the entire forest picture probably will be saved to disk as a DIB.

Another popular way to use DDBs (hereafter called bitmaps) is to use them to represent an application's entire display area in memory. The application makes changes to its display by drawing on the bitmap in memory and then copying the bitmap to the application's window. Using a bitmap in this way, an application can quickly update its display whenever it needs to without having to redraw the data from scratch. In the following section, you'll create a Windows application that handles its display in exactly this way.

Device-dependent bitmaps are graphical images that can be displayed on only one type of physical device. For example, when you use Windows functions such as CreateBitmap() and LoadBitmap(), you're creating in memory a bitmap image that is compatible with a certain device, usually the screen. These types of bitmaps are also sometimes called GDI bitmaps because Windows' GDI (graphics device interface) can handle them directly. DDBs are stored without color tables because they use the colors of their associated device. Moreover, DDBs usually reside only in memory, rather than as files on a disk.

Device-independent bitmaps are graphical images that can be displayed on many different devices. These types of bitmaps carry with them a color table that the current device must use to display the bitmap so that the bitmap looks similar from one device to another. For example, a DIB should look almost the same under Windows as it does under DOS or OS/2. Because DIBs are generally portable between systems, you often find them as disk files. If you look in your Windows directory, for example, you'll see many files that have the BMP extension. These are DIBs. You can create your own DIBs using various types of paint programs, including Windows Paintbrush, which comes with every copy of Windows. You also can use Visual C++'s bitmap editor.

Whether a DIB is stored on disk or in memory, it has almost exactly the same structure. Actually, a DIB is made up of several different types of structures, one following the other. These structures include the BITMAPFILEHEADER, BITMAPINFO, BITMAPINFOHEADER, and RGBQUAD types. The following sections describe these structure types and how they're used in Windows programs.