Double buffering in GDI -- easy?

This is a discussion on Double buffering in GDI -- easy? within the Game Programming forums, part of the General Programming Boards category; My SwapBuffers() function now looks like this: Code: void Bitmaps::SwapBuffers() { InvalidateRgn(hwnd, NULL, false); UpdateWindow(hwnd); } I clear the buffer ...

  1. #16
    Registered User
    Join Date
    Jan 2003
    Posts
    35
    My SwapBuffers() function now looks like this:

    Code:
    void Bitmaps::SwapBuffers()
    {
       InvalidateRgn(hwnd, NULL, false);
       UpdateWindow(hwnd);
    }
    I clear the buffer in the WM_PAINT case after blitting to ps.hdc. Guess I don't need the UpdateWindow in there, huh? I just tried it without & it works the same.

    But fill me in on the message pump idea. My message pump looks like this:

    Code:
             if (PeekMessage(&msg, 0, 0, 0, PM_NOREMOVE))
             {
                int result = GetMessage(&msg, 0, 0, 0);
                if (!result)
                {
                   gQuit=true;
                   break;
                }
    
                TranslateMessage(&msg);
                DispatchMessage(&msg);
             }
    I've tried out a number of different message pumps, and when they work they all lead to the same result. The one above used to work but maybe I need to do something different to handle all the paint messages?

  2. #17
    train spotter
    Join Date
    Aug 2001
    Location
    near a computer
    Posts
    3,859
    PeekMessage() will use 100% CPU as it keep asking the OS if there is a msg until it gets one. This will tend to make all the other apps slow down. The no use of no remove and a call to GetMessage() may be slower than using a single peekMessage() and removing them.

    Code:
    if (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE)) 
            { 
    			if(msg.message == WM_QUIT)	
    				break;
     			if(!TranslateAccelerator(msg.hwnd, hAccel, &msg))
    			{ 
    				TranslateMessage(&msg);						
    				DispatchMessage(&msg);				
    			}
            }
    DO NOT send a paint msg in your WM_PAINT handler!
    Can you say 'infinite loop'. Put a break point in there and see what I mean.

    >>InvalidateRgn(hwnd, NULL, false);
    I think you miss the point. The idea was you draw to a buffer( hdc-1). When the drawing is finished you call SwapBuffers.
    Try InvalidateRect() and use the same rect as in the BitBlt()

    SwapBuffers()
    1. copies the new buffer (hdc_1) to the one used to update the screen (hdc_2)
    2. calls for a paint in just the area that it has changed.


    the WM_PAINT handler then takes this hdc_2 and copies it to the one in the paint struct. Do not put any other code in the WM_PAINT. Another BitBlt here will slow the app down and clear your buffer (hdc_1) if some other event causes a paint ie user moves messagebox across your app. This is a bad thing if you have a multi threaded app and are drawing to the hdc as the paint handler clears it.
    "Man alone suffers so excruciatingly in the world that he was compelled to invent laughter."
    Friedrich Nietzsche

    "I spent a lot of my money on booze, birds and fast cars......the rest I squandered."
    George Best

    "If you are going through hell....keep going."
    Winston Churchill

  3. #18
    Registered User
    Join Date
    Jan 2003
    Posts
    35
    I hadn't been aware that double buffering is significantly slower than blitting directly into the front DC. The main cause of the problem (delayed reaction) seems to have been the number & size of the bitmaps I'm blitting to the screen. That may not be a complete explaination, but reducing the graphics load does solve the problem.

    I'd been lazy before and used 32x32 bitmaps of 4x4 dots (pellets; the game's based on Pac-Man). I changed it to draw the pellets using a pen, and the performance is much improved.

    It is still significantly slower than it was without double buffering, although the game is rather expensively designed overall. I may revert to no DB, but the better thing to do is redesign the game to be less blit-intensive. My goal has been to make the game run on slower computers. So basically my job now is to give the game a major tune-up, which it desperately needed anyway...

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