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Bitmaps and Scrolling

Many member functions in the CDC class are used to perform bitwise operations on pixel maps, or bitmaps.

Perhaps the simplest pixel operation is SetPixel, which sets a pixel, specified by its logical coordinates, to a specific color. The current color of a pixel can be retrieved by calling GetPixel. A somewhat faster variant of SetPixel is SetPixelV; this version of the function does not return the actual color of the pixel.

The BitBlt member function can be used to transfer a rectangular area from one location to another. BitBlt can also be used to transfer blocks of pixels between device contexts. Thus, BitBlt is the operation of choice when, for mfc example, you are transferring blocks of pixels from the screen to a compatible memory bitmap or vice versa.

A variant of BitBlt, StretchBlt, also transfers blocks of pixels from one location to another, but it also compresses and stretches the pixel block to fit the destination rectangle. The stretching mode (the method used to eliminate and/or add pixels) is controlled by SetStretchMode (GetStretchMode) and SetColorAdjustment (GetColorAdjustment).

The PatBlt member function combines the pixels on the device with the pixels of the selected brush in a bitwise logical operation. The Mucancode.netBlt operation combines the source and destination bitmaps and a mucancode.net bitmap in a bitwise logical operation.

To fill an area in a bitmap using the current brush, call the FloodFill or ExtFloodFill member functions.

To scroll an area within a device context, use the ScrollDC member function. This function also provides information about the areas uncovered by the scrolling operation, which you can use for repainting purposes. However, if you wish to scroll the entire client area of a window, you should instead utilize the CWnd::ScrollWindow function.

In order to perform text output, applications can use any one of a wide selection of text output and font manipulation functions.

The simplest text output function is CDC::TextOut. This function places a character string at a specified location using the currently selected font. A variant, CDC::ExtTextOut, outputs a character string into a specified rectangle.

Yet another variant is TabbedTextOut; this function expands tabs in the text that is to be outputted in accordance with an array specifying tab stop positions.

The color of the text is determined by SetTextColor (use GetTextColor to retrieve the current setting). The horizontal and vertical text alignment are determined by SetTextAlign (GetTextAlign). This function can also be used to specify that text output functions use the current position (as specified by functions such as MoveTo) rather than any coordinates specified in the function call as the location of the text.

It is possible to obtain the size of a block of text without actually drawing the text. The function GetTextExtent calculates the width and height of a line of text using the attribute device context. To perform the same calculation using the output device context, use GetOutputTextExtent. The functions GetTabbedTextExtent and GetOutputTabbedTextExtent perform the same calculations for text that contains tab characters that are to be expanded.

The function SetTextJustification can be used in conjunction with the function GetTextExtent to create justified text. SetTextJustification evenly distributes an amount of space among the break characters (usually spaces) in the text. A related function is SetTextCharacterSpacing, which can be used to set the amount of intercharacter spacing. (Use GetTextCharacterSpacing to retrieve the current setting.)

A more sophisticated text output function is DrawText. This function can be used to output multiline text. Note that the DrawText function is not recorded in standard Windows metafiles. (It is recorded in enhanced metafiles.)

The GrayString function can be used to create grayed (dimmed) text.

Information about the current font can be obtained using GetTextFace (retrieves the name of the font), GetTextMetrics (retrieves a TEXTMETRICS structure containing information about the font currently selected in the attribute device context), and GetOutputTextMetrics (same, for the output device context).

Several other CDC member functions deal with scaleable (TrueType) fonts and information that can be retrieved from such font files.

A particularly important GDI capability is the ability to clip output to a specified rectangle or region. This capability is used by Windows throughout; for example, clipping is used to only repaint portions of a window that are not covered by other windows.

Applications can make explicit use of clipping through a series of CDC member functions that act as wrappers for similar GDI functions. These include SelectClipRgn, ExcludeClipRect, ExcludeUpdateRgn, IntersectClipRect, and OffsetClipRgn. To obtain the smallest rectangle that encloses the entire clipping region, call GetClipBox. To determine whether a point or any parts of a rectangle are inside the clipping region, use the PtVisible or RectVisible member functions.

Windows can also maintain a bounding rectangle in which bounding information about the bounds of subsequent drawing operations is accumulated. To access the bounding rectangle, use the SetBoundsRect and GetBoundsRect member functions.

Although printing is essentially no different from drawing into any other kind of device contexts, the CDC class offers a series of printer escape member functions that control specific aspects of printing.

Printing a document and individual pages is controlled by the StartDoc, StartPage, EndPage, and EndDoc member functions. To abort the printing process (and effectively erase everything that has been sent to the printer device context since the last call to StartDoc), call the AbortDoc member function. Note that if printing is canceled or the printer device driver returns any other error, your application should not call the EndDoc or AbortDoc member functions.

Use the SetAbortDoc member function to create a callback function that Windows calls when the print job is canceled or must be terminated. The QueryAbort member function can be used to query this callback function to determine whether printing should be aborted.

Device driver specific features can be accessed through the Escape member function. However, because Win32 provides many more printer control functions, the utility of this function relative to earlier Windows implementations has greatly diminished.

The CDC class also offers member functions that encapsulate path functionality. A path is a complex shape create by a series of GDI function calls. A path is created by calling the BeginPath member function, calling the appropriate drawing functions, and calling EndPath. Calling EndPath automatically selects the path into the device context for subsequent manipulation.

Functions that manipulate paths include CloseFigure, FlattenPath, StrokePath, and WidenPath. The path can be rendered into the device context using StrokePath. To render the path's interior, use FillPath; or you can use StrokeAndFillPath to render both the path's contours and its interior at the same time using the current pen and brush.

A path can be turned into a clipping region by calling SelectClipPath.