Thread: 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);
	}
}

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.

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.

Why would you need to erase the on-screen copy if you're just going to copy all over it with the new data?

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().

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.