Flicker in GDI

This is a discussion on Flicker in GDI within the Windows Programming forums, part of the Platform Specific Boards category; I went through several tutorials covering double buffering and flickerless GDI animations. I just cant get them to work. Every ...

  1. #1
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    Question Flicker in GDI

    I went through several tutorials covering double buffering and flickerless GDI animations. I just cant get them to work. Every time i render, there is alot of flicker because I have to erase the entire screen and redraw everything on to it. switching to OpenGL or directX is not an option (due to time constriants etc).

    The way i traied it was to catch WM_ERASEBKGND and do my own erasing when i render. this still causes flicker regardless. Anyone have any explanation why this is happening to me??

    Here is some of the code I use:

    Code:
    	POINT MousePos;
    	PAINTSTRUCT Paint;
    	HDC CurrentDeviceContext;
    	HDC DeviceContextMem;
    	RECT EraseRect;
    	HBRUSH BackGrnd;
    
    	// Invalidate the Window
    	InvalidateRect(WindowHwnd, NULL, true);
    
    	// Start the Drawing Process
    	CurrentDeviceContext = BeginPaint(WindowHwnd, &Paint);
    
    	DeviceContextMem = CreateCompatibleDC(CurrentDeviceContext);
    
    	// Erase Background
    	memset(&EraseRect, 0, sizeof(RECT));
    	EraseRect.bottom = WindowHeight;
    	EraseRect.right = WindowWidth;
    	BackGrnd = CreateSolidBrush(RGB(255, 0, 255));
    	FillRect(CurrentDeviceContext, &EraseRect, BackGrnd);
    
    	GetCursorPos(&MousePos);
    
    	// Specify which primitive type is to be drawn 
    	for (int i = 0; i < NumPanel; i++)
    	{
    		if (PanelList[i])
    		{
    			PanelList[i]->Render(WindowHwnd, CurrentDeviceContext, DeviceContextMem);
    		}
    	}
    
    	DeleteDC(DeviceContextMem);
    
    	// Finish the drawing process
    	EndPaint(WindowHwnd, &Paint);

    Here is my "Panel" rendering code:

    Code:
    void Panel::Render(HWND Hwnd, HDC DevContext, HDC Context)
    {
    	HBITMAP OldBitmap;
    	BITMAP Bitmap;
    	BITMAPINFOHEADER BitmapInfo;
    	HBITMAP ImgMap;
    
    	if (Loaded && Renderable && ImgHeader.BitsPerPixel == 32)
    	{
    		// Clear our Bitmap
    		memset(&Bitmap, 0, sizeof(BITMAP));
    
    		// Create a struct ot hold the info about the image
    		BitmapInfo.biSize = sizeof(BitmapInfo);
    		BitmapInfo.biHeight = ImgHeader.Height;
    		BitmapInfo.biWidth = ImgHeader.Width;
    		BitmapInfo.biPlanes = 1;
    		BitmapInfo.biBitCount = ImgHeader.BitsPerPixel;
    		BitmapInfo.biCompression = BI_RGB;
    		BitmapInfo.biSizeImage = 0;
    		BitmapInfo.biXPelsPerMeter = BitmapInfo.biYPelsPerMeter = 10000;
    		BitmapInfo.biClrUsed = BitmapInfo.biClrImportant = 0;
    
    		// Create a DIB from our TGA data
    		ImgMap = CreateDIBitmap(DevContext, &BitmapInfo, CBM_INIT, 
    	            (KeyPressed ? Img.ClickData : Img.ImageData), (BITMAPINFO *) &BitmapInfo, DIB_RGB_COLORS);
    
    		// Set the position and Draw data
    		OldBitmap = (HBITMAP) SelectObject(Context, ImgMap);
    		BitBlt(DevContext, RasterX, RasterY, ImgHeader.Width, ImgHeader.Height, Context, 0, 0, MERGECOPY);
    
    		// Swap buffers
    		SelectObject(Context, OldBitmap);
    
    		// Delete our GDI Data
    		DeleteObject(ImgMap);
    	}
    }
    Last edited by Mastadex; 08-15-2006 at 12:38 PM. Reason: reduced horizontal extent of code
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  2. #2
    erstwhile
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    Assuming the first block of code is in response to a WM_PAINT, why are you using InvalidateRect there?

    What are you doing in response to a WM_ERASEBKGND message?

    Does your registered window class have a class brush?

    You also seem to be creating and allocating bitmaps with each render cycle; creating them once and destroying them when done will probably be more efficient.
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  3. #3
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    as an experiment removed the call to InvalidateRect()

    Kuphryn

  4. #4
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    Quote Originally Posted by Ken Fitlike
    Assuming the first block of code is in response to a WM_PAINT, why are you using InvalidateRect there?

    What are you doing in response to a WM_ERASEBKGND message?

    Does your registered window class have a class brush?

    You also seem to be creating and allocating bitmaps with each render cycle; creating them once and destroying them when done will probably be more efficient.

    Code:
    		case WM_ERASEBKGND:
    			// Erase Background
    			Rtrn = 1;
    			break;
    
    ....
    
    
    		case WM_PAINT:
    			Render(); // Calls the code posted above
    			break;

    Im not worried about efficiency just yet. I will do as you suggested, but once the problem has been fixed.
    The window is crated without using a background brush. I erase the background manually inside the render function.
    Last edited by Mastadex; 08-15-2006 at 12:42 PM.
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  5. #5
    and the hat of wrongness Salem's Avatar
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    Why would you need to erase the on-screen copy if you're just going to copy all over it with the new data?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  6. #6
    {Jaxom,Imriel,Liam}'s Dad Kennedy's Avatar
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    Just a thought: I had a similar problem and found that EVEN THOUGH I was using EndPaint I still needed to ValidateRect(hwnd, NULL) at the end of my WM_PAINT. -- it's worth the 1.2 minutes to try it.

  7. #7
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    Quote Originally Posted by Salem
    Why would you need to erase the on-screen copy if you're just going to copy all over it with the new data?
    Im not drawing to the whole screen all the time, else i wouldnt bother with erasing the background. the code where im erasing the background is basically just painting the window a certain color before i start painting the "panels" onto the screen.

    Ill try validaterect().
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  8. #8
    Yes, my avatar is stolen anonytmouse's Avatar
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    Haven't we already done this? As the Catch22 article says, the rule to avoid flicker is: You must never draw over the same pixel twice. I suggest you try:
    - Call ExcludeClipRect after painting every bitmap with the same coordinates.
    - Paint the background after the bitmaps have been painted. Having excluded them from the clipping region, they will not be painted over.

  9. #9
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    Ill try this.
    If it doesnt work, is there a way to use double buffering with GDI?

    EDIT: Got it working but the problem now is that my rendering is incredably slow. I might have to consider other options, such as double bufering.

    EDIT 2.0: I got double bufering to work. The problem was 1 line of code that was commented out during my debugging.
    Last edited by Mastadex; 08-17-2006 at 12:19 PM.
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