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Bitmaps, Device Contexts and BitBlt

Example: bmp_one



The really great thing about MS Windows is that unlike DOS, you don't need to know anything about what video hardware you are using to display graphics. Instead, windows provides an API called the Graphics Device Interface, or GDI. The GDI uses a set of generic graphics objects that can be used to draw to the screen, to memory, or even to printers.

Device Contexts

The GDI revolves around an object called the Device Context (DC), represented by the data type HDC (Handle to Device Context). An HDC is basically a handle to something you can draw on; it can represent the entire screen, an entire window, the client area of a window, a bitmap stored in memory, or a printer. The nice part is that you don't even need to know which one it refers to, you can use it basically the same way, which is especially handy for writing custom drawing functions which you can then use on any of these devices without changing it for each one.

An HDC like most GDI objects is opaque, meaning that you can't access it's data directly... but you can pass it to various GDI functions that will operate on it, either to draw something, get information about it, or change the object in some way.

For example, if you wanted to draw on a window, first you would retreive an HDC representing the window with GetDC(), then you could use any of the GDI functions that take an HDC like BitBlt() for drawing images, TextOut() for drawing text, LineTo() for lines and so on.


Bitmaps can be loaded much like icons in earlier examples, there is LoadBitmap() for the most basic functionality of simply loading a bitmap resource, and LoadImage() can be used to load bitmaps from a *.bmp file just as it can for icons.

One of the quirks of GDI is that you can't draw to bitmap objects (HBITMAP type) directly. Remember that drawing operations are abstracted by Device Contexts, so in order to use these drawing functions on a bitmap, you need to create a Memory DC, and then select the HBITMAP into it with SelectObject(). The effect is that the "device" that the HDC refers to is the bitmap in memory, and when you operate on the HDC, the resulting graphic operations are applied to the bitmap. As I mentioned, this is actually a very conveiniant way of doing things, as you can write code that draws to an HDC and you can use it on a Window DC or a Memory DC without any checks or changes.

You do have the option of manipulating the bitmap data in memory yourself. You can do this with Device Independant Bitmaps (DIB), and you can even combine GDI and manual operations on the DIB. However for the time being, this is beyond the scope of the basic tutorial and for now we're just cover the simpler GDI operations on their own.

WINX module: In this tutorial, we will not use the original Win32 API LoadBitmap, SelectObject etc., but we will use the corresponding procedures from the WINX module: LoadBitmapID, SelectBitmap, etc. The reason for this is that a function like SelectObject() works for various GDI object types with type HGDIOBJ and its use would involve a lot of type CASTing when working with specific objects with types such as HBITMAP, HFONT, etc.

GDI Leaks

Once you're finished with an HDC, it's very important to release it (just how you do that depends on how you got it, which we'll talk about in a bit). GDI objects are limited in number. In versions of windows prior to Windows 95, they were not only incredably limited but also shared system wide, so that if one program used up too many, none of the rest would be able to draw anything! Fortunately this isn't the case any longer, and you could get away with using up quite a lot of resources in Windows 2000 or XP before anything too bad happened... but it's easy to forget to free GDI objects and they can quickly run your program out of GDI resources under Windows 9x. Theorethically you shouldn't be able to drain the system of GDI resources in NT systems (NT/2K/XP) but it still happens in extreme cases, or if you hit the right bug on the nose.

If your program runs fine for a few minutes and then starts drawing strangely or not at all, it's a good sign that you're leaking GDI resources. HDCs aren't the only GDI objects you need to be careful about releasing, but generally it's ok to keep things like bitmaps and fonts around for the entire lifetime of your program, since it's much more efficiant than reloading them each time you need them.

Also, an HDC can only contain one of each type of object (bitmap, font, pen...) at a time, and when you select a new one in it will return the last one. It's very important that you deal with this object properly. If you ignore it completely, it will be lost and they will pile up in memory causing GDI leaks. When an HDC is created, it's also created with some default objects selected into it... it's a good idea to store these when they are returned to you, and then when you are completed drawing with the HDC select them back into it. This will not only remove any of your own objects from the HDC (which is a good thing) but it will also cause the default objects to be properly disposed of when you release or destroy the HDC (a VERY good thing).

Displaying Bitmaps

OK, down to business. The simplest drawing operations on a window occure by handling WM_PAINT. When your window is first displayed, restored from being minimised, or uncovered from having another window on top of it, Windows sends the WM_PAINT message to the window to let it know that it needs to redraw it's contents. When you draw something on the screen it is NOT permanent, it's only there untill something else draws over it, and at that point you need to draw it again when the time comes.

    g_hbmBall : HBITMAP;
    | WM_CREATE  : g_hbmBall := LoadBitmapId(Instance, IDB_BALL);
                   IF g_hbmBall = NIL THEN
                     FUNC MessageBox(hwnd, "Could not load IDB_BALL!", "Error",
                                     MB_OK BOR MB_ICONEXCLAMATION);

The first step is of course loading the bitmap, this is quite simple with a bitmap resource, there are no significant differences from loading other resource types. Then we can get down to drawing...

VAR bm       : BITMAP;
    hbmOld   : HBITMAP;
    hdc      : HDC;
    hdcMem   : HDC;
    ps       : PAINTSTRUCT;
    | WM_PAINT   : hdc    := BeginPaint(hwnd, ps);
                   hdcMem := CreateCompatibleDC(hdc);

                   hbmOld := SelectBitmap(hdcMem, g_hbmBall);

                   FUNC GetBITMAP(g_hbmBall, bm);
                   FUNC BitBlt(hdc, 0, 0, bm.bmWidth, bm.bmHeight, hdcMem,
                               0, 0, SRCCOPY);

                   FUNC SelectBitmap(hdcMem, hbmOld);
                   FUNC DeleteDC(hdcMem);
                   FUNC EndPaint(hwnd, ps);
     (* ... *)

Getting the Window DC

To start off we declare a couple of variables we need. Notice that the first one is a BITMAP, not an HBITMAP. BITMAP is a struct that holds information about an HBITMAP which is the actual GDI object. We need a way to get the height and width of the HBITMAP so we use GetBITMAP().

GetBITMAP() is the WINX replacement for the original Win32 API GetObject() when the object is a bitmap.
FUNC GetBITMAP(g_hbmBall, bm); replaces
FUNC GetObject(CAST(HGDIOBJ,g_hbmBall), SIZE(bm), ADR(bm));.

The PAINTSTRUCT is a structure that contains information about the window being painted and what exactly is going on with the paint message. For most simple tasks, you can simply ignore the information it contains, but it's required for the call to BeginPaint(). BeginPaint() as it's name suggests is designed specifically for handling the WM_PAINT message. When not handling a WM_PAINT message you would use GetDC() which we will see in the timer animation examples in a while... but in WM_PAINT, it's important to use BeginPaint() and EndPaint().

BeginPaint() returns us an HDC that represents the HWND that we pass to it, the one that WM_PAINT is being handled for. Any drawing operation we perform on this HDC will immediately display on the screen.

Setting up a Memory DC for the Bitmap

As I mention above, in order to draw on or with bitmaps, we need to create a DC in memory... the easiest way to do that here is to CreateCompatibleDC() with the one we already have. This gives us a Memory DC that is compatible with the color depth and display properties of the HDC for the window.

Now we call SelectBitmap() to select the bitmap into the DC being careful to store the default bitmap so that we can replace it later on and not leak GDI objects.

SelectBitmap() is the WINX replacement for SelectObject() when the object is a bitmap.


Once we've got the dimentions of the bitmap filled into the BITMAP struct, we can call BitBlt() to copy the image from our Memory DC to the Window DC, thus displaying on the screen. As always, you can look up each parameter in MSDN, but in short they are: The destination, the position and size, the source and source position, and finally the Raster Operation (ROP code), which specifies how to do the copy. In this case, we want a simple exact copy of the source made, no fancy stuff.

BitBlt() is probably the all time happiest function in all of the Win32 API and is the staple diet of anyone learning to write games or other graphics applications in windows. It was probably the first API that I memorised all the parameters to.


At this point the bitmap should be on the screen, and we need to clean up after ourselves. The first thing to do is restore the Memory DC to the state it was when we got it, which means replacing our bitmap with the default one that we saved. Next we can delete it altogether with DeleteDC().

Finally we release the Window DC we got from BeginPaint() using EndPaint().

Destroying an HDC is a little confusing sometimes because there are at least 3 ways to do it depending on how you got it in the first place. Here's a list of the common methods of gaining an HDC, and how to release it when you're done.

  • GetDC() - ReleaseDC()
  • BeginPaint() - EndPaint()
  • CreateCompatibleDC() - DeleteDC()

And finally, at the termination of our program, we want to free any resources that we allocated. Technically speaking this isn't absolutely required, since modern Windows platforms are pretty good at freeing everything when your program exists, but it's always a good idea to keep track of your own objects because if get lazy and don't delete them they have a habit of getting loose. And no doubt, there are still bugs in windows especially older versions that won't clean up all of your GDI objects if you don't do a thorough job.

    | WM_DESTROY : FUNC DeleteBitmap(g_hbmBall);

Copyright © 1998-2011, Brook Miles. All rights reserved. Adapted for Modula-2 by Frank Schoonjans, with permission.