Rounding errors between GDI+ & GDI ...   

Hello,

Please consider the following code :

void Test (HDC hDC)
{
	// Set graphics mode to 72 dpi ...
	SetMapMode(hDC, MM_ISOTROPIC);
	SetWindowExtEx(hDC, 72, 72, NULL);
	SetViewportExtEx(hDC, GetDeviceCaps(hDC, LOGPIXELSX), 
GetDeviceCaps(hDC, LOGPIXELSY), NULL);
	SetGraphicsMode(hDC, GM_ADVANCED);

	// Draw a 2x2 inch rectangle using GDI
	HBRUSH hbOld = (HBRUSH)SelectObject(hDC, 
(HBRUSH)GetStockObject(NULL_BRUSH));
	Rectangle(hDC, 100, 100, 100 + 144, 100 + 144);
	SelectObject(hDC, hbOld);

	// Draw over a 2x2 inch rectangle using GDI+
	Graphics g(hDC);
	SolidBrush br(Color(200, 255, 128, 0));
	g.FillRectangle(&br, 100, 100, 144, 144);
}


It demonstrates that if you draw a GDI+ primitive over a GDI one, they 
do not perfectly overlap when the GDI mapping mode is not trivial. I 
guess that the Graphics objet performs some additionnal transformations 
to mimic the GDI mapping mode. Problem is: the result is not the same as 
drawing through the GDI internal matrix.

Any clue how to bypass this problem (without giving up the GDI 
transformations of course)?

Thanks,
Jean-Ed.

date: Wed, 16 Jul 2008 16:14:24 +0200   author:   Jean-Edouard Lachand-Robert

Re: Rounding errors between GDI+ & GDI ...   

I have also discovered that DPtoLP returns different values depending on the mapping
modes, so there are several serious problems of which you seem to have identified another.

GDI+ uses GM_ADVANCED mode, and that's where the errors occur.

See my essay on errors in Windows documentation at 

http://www.flounder.com/msdn_documentation_errors_and_omissions.htm#DPtoLP

				joe

On Wed, 16 Jul 2008 16:14:24 +0200, Jean-Edouard Lachand-Robert
 wrote:

>Hello,
>
>Please consider the following code :
>
>void Test (HDC hDC)
>{
>	// Set graphics mode to 72 dpi ...
>	SetMapMode(hDC, MM_ISOTROPIC);
>	SetWindowExtEx(hDC, 72, 72, NULL);
>	SetViewportExtEx(hDC, GetDeviceCaps(hDC, LOGPIXELSX), 
>GetDeviceCaps(hDC, LOGPIXELSY), NULL);
>	SetGraphicsMode(hDC, GM_ADVANCED);
>
>	// Draw a 2x2 inch rectangle using GDI
>	HBRUSH hbOld = (HBRUSH)SelectObject(hDC, 
>(HBRUSH)GetStockObject(NULL_BRUSH));
>	Rectangle(hDC, 100, 100, 100 + 144, 100 + 144);
>	SelectObject(hDC, hbOld);
>
>	// Draw over a 2x2 inch rectangle using GDI+
>	Graphics g(hDC);
>	SolidBrush br(Color(200, 255, 128, 0));
>	g.FillRectangle(&br, 100, 100, 144, 144);
>}
>
>
>It demonstrates that if you draw a GDI+ primitive over a GDI one, they 
>do not perfectly overlap when the GDI mapping mode is not trivial. I 
>guess that the Graphics objet performs some additionnal transformations 
>to mimic the GDI mapping mode. Problem is: the result is not the same as 
>drawing through the GDI internal matrix.
>
>Any clue how to bypass this problem (without giving up the GDI 
>transformations of course)?
>
>Thanks,
>Jean-Ed.

date: Thu, 31 Jul 2008 04:43:44 -0400   author:   Joseph M. Newcomer