Q145994: HOWTO: Calculate Dialog Units When Not Using the System Font

Article: Q145994
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,1.51,1.52,2.0,2.1,2.2,4.0,4.1,4.2,5.0,6.0
Operating System(s): 
Keyword(s): kbcode kbDlg kbFont kbMFC KbUIDesign kbVC200 kbVC400 kbVC500 kbVC600 kbGrpDSMFCATL kbDi
Last Modified: 10-JUN-2002

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225, versions 2.0, 2.1, 2.2, 4.0, 4.1, 4.2 
   - Microsoft Visual C++, versions 1.0, 1.5, 1.51, 1.52 
   - Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
   - Microsoft Visual C++.NET (2002) 
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SUMMARY
=======

GetDialogBaseUnits() always assumes the system font when calculating dialog
units. If you are using any font other than the system font, the return value
may not be correct for your dialog box. This article offers two alternatives you
can use to calculate dialog box units based on the current font.

MORE INFORMATION
================

The horizontal base unit is equal to the average width (in pixels) of the
characters in the system font. The Help for GetDialogBaseUnits() suggests the
following code for calculating the correct units:

  pixelX = (dialogunitX * baseunitX) / 4
  pixelY = (dialogunitY * baseunitY) / 8

However, if you aren't using the system font, you need to do some additional
calculations to get the right coordinates. Here are two methods:

Method One
----------

The vertical dialog box unit is equivalent to the character height. The
horizontal dialog box unit is equivalent to the average character width of the
dialog box's font. The average character width is calculated by finding the
average text extent of the alphabetic character set. This is necessary because
tmAveCharWidth is not precise. Use this code:

     void CAboutDlg::OnPaint()
     {
         CPaintDC dc(this); // device context for painting

         CFont* pFont = GetFont();
         CFont* oldFont = dc.SelectObject(pFont);
         TEXTMETRIC tm;
         dc.GetTextMetrics( &tm );
         int baseUnitY = tm.tmHeight;
         CSize size;
         size = dc.GetTextExtent(
             "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz", 52);
         int baseUnitX = (size.cx / 26 + 1) / 2;
         TRACE("baseUnitX = %d\n", baseUnitX);
         TRACE("baseUnitY = %d\n", baseUnitY);
         dc.SelectObject(oldFont);

     }

Method Two
----------

The alternative is even easier. Using MapDialogRect() with a coordinate of x = 4,
y = 8 will return the dialog box base units regardless of the dialog box's font.
However, in most situations, the programmer doesn't need to know the dialog box
base units. MapDialogRect() can be used directly to convert any dialog box units
directly to device coordinates. The following sample code shows how it can be
used to find the dialog box base units:

     void CAboutDlg::OnPaint()
     {
         CPaintDC dc(this); // device context for painting

         CRect rc( 0, 0, 4, 8 );
         MapDialogRect( &rc );
         int baseUnitY = rc.bottom;
         int baseUnitX = rc.right;
         TRACE("baseUnitX = %d\n", baseUnitX);
         TRACE("baseUnitY = %d\n", baseUnitY);

     }

REFERENCES
==========

For additional information, please see the following article in the Microsoft
Knowledge Base:

  Q125681 HOWTO: Calculate Dialog Base Units with Non-System-Based Font

Additional query words: 2.50 2.51 2.52 3.00 3.10 3.20

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Keywords          : kbcode kbDlg kbFont kbMFC KbUIDesign kbVC200 kbVC400 kbVC500 kbVC600 kbGrpDSMFCATL kbDialog 
Technology        : kbAudDeveloper kbMFC
Version           : :1.0,1.5,1.51,1.52,2.0,2.1,2.2,4.0,4.1,4.2,5.0,6.0
Issue type        : kbhowto

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