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Bitmaps and icons overview

Classes: wxBitmap, wxBitmapHandler, wxIcon, wxCursor.

The wxBitmap class encapsulates the concept of a platform-dependent bitmap, either monochrome or colour. Platform-specific methods for creating a wxBitmap object from an existing file are catered for, and this is an occasion where conditional compilation will sometimes be required.

A bitmap created dynamically or loaded from a file can be selected into a memory device context (instance of wxMemoryDC). This enables the bitmap to be copied to a window or memory device context using wxDC::Blit, or to be used as a drawing surface. The wxToolBarSimple class is implemented using bitmaps, and the toolbar demo shows one of the toolbar bitmaps being used for drawing a miniature version of the graphic which appears on the main window.

See wxMemoryDC for an example of drawing onto a bitmap.

The following shows the conditional compilation required to load a bitmap under Unix and in Windows. The alternative is to use the string version of the bitmap constructor, which loads a file under Unix and a resource or file under Windows, but has the disadvantage of requiring the XPM icon file to be available at run-time.

#if defined(__WXGTK__) || defined(__WXMOTIF__)
#include "mondrian.xpm"
#endif
A macro, wxICON, is available which creates an icon using an XPM on the appropriate platform, or an icon resource on Windows.

wxIcon icon(wxICON(mondrian));

// Equivalent to:

#if defined(__WXGTK__) || defined(__WXMOTIF__)
wxIcon icon(mondrian_xpm);
#endif

#if defined(__WXMSW__)
wxIcon icon("mondrian");
#endif
There is also a corresponding wxBITMAP macro which allows to create the bitmaps in much the same way as wxICON creates icons. It assumes that bitmaps live in resources under Windows or OS2 and XPM files under all other platforms (for XPMs, the corresponding file must be included before this macro is used, of course, and the name of the bitmap should be the same as the resource name under Windows with _xpm suffix). For example:

// an easy and portable way to create a bitmap
wxBitmap bmp(wxBITMAP(bmpname));

// which is roughly equivalent to the following
#if defined(__WXMSW__) || defined(__WXPM__)
    wxBitmap bmp("bmpname", wxBITMAP_TYPE_RESOURCE);
#else // Unix
    wxBitmap bmp(bmpname_xpm, wxBITMAP_TYPE_XPM);
#endif
You should always use wxICON and wxBITMAP macros because they work for any platform (unlike the code above which doesn't deal with wxMac, wxBe, ...) and are more short and clear than versions with #ifdefs.

Supported bitmap file formats
Bitmap format handlers

Supported bitmap file formats

The following lists the formats handled on different platforms. Note that missing or partially-implemented formats are automatically supplemented by the wxImage to load the data, and then converting it to wxBitmap form. Note that using wxImage is the preferred way to load images in wxWidgets, with the exception of resources (XPM-files or native Windows resources). Writing an image format handler for wxImage is also far easier than writing one for wxBitmap, because wxImage has exactly one format on all platforms whereas wxBitmap can store pixel data very differently, depending on colour depths and platform.

wxBitmap

Under Windows, wxBitmap may load the following formats:

Under wxGTK, wxBitmap may load the following formats:

Under wxMotif, wxBitmap may load the following formats:

wxIcon

Under Windows, wxIcon may load the following formats:

Under wxGTK, wxIcon may load the following formats:

Under wxMotif, wxIcon may load the following formats:

wxCursor

Under Windows, wxCursor may load the following formats:

Under wxGTK, wxCursor may load the following formats (in additional to stock cursors):

Under wxMotif, wxCursor may load the following formats:

Bitmap format handlers

To provide extensibility, the functionality for loading and saving bitmap formats is not implemented in the wxBitmap class, but in a number of handler classes, derived from wxBitmapHandler. There is a static list of handlers which wxBitmap examines when a file load/save operation is requested. Some handlers are provided as standard, but if you have special requirements, you may wish to initialise the wxBitmap class with some extra handlers which you write yourself or receive from a third party.

To add a handler object to wxBitmap, your application needs to include the header which implements it, and then call the static function wxBitmap::AddHandler. For example:

  #include <wx/xpmhand.h>
  ...
  // Initialisation
  wxBitmap::AddHandler(new wxXPMFileHandler);
  wxBitmap::AddHandler(new wxXPMDataHandler);
  ...

Assuming the handlers have been written correctly, you should now be able to load and save XPM files using the usual wxBitmap API.

Note: bitmap handlers are not implemented on all platforms. Currently, the above is only necessary on Windows, to save the extra overhead of formats that may not be necessary (if you don't use them, they are not linked into the executable). Unix platforms have XPM capability built-in (where supported).

Also, just because a handler (such as a PNG handler) is not present does not mean that wxBitmap does not support that file format. If wxBitmap fails to find a suitable handler, the file-loading capabilities of wxImage are used instead.