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Q74299: INFO: Calculating The Logical Height and Point Size of a Font

Article: Q74299
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.1,95; winnt:3.5,3.51,4.0
Operating System(s): 
Keyword(s): kbOSWinNT350 kbOSWinNT351 kbOSWinNT400 kbOSWin95 kbSDKWin16
Last Modified: 11-MAY-2001

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) 3.1 
- Microsoft Win32 Application Programming Interface (API), used with:
   - Microsoft Windows NT Server versions 3.5, 3.51, 4.0 
   - Microsoft Windows NT Workstation versions 3.5, 3.51, 4.0 
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SUMMARY
=======

To create a font in the Microsoft Windows graphical environment given only the
required point size, an application must calculate the logical height of the
font because the CreateFont() and CreateFontIndirect() functions use logical
units to specify height.

To describe a font to the user, an application can calculate a font's point size,
given its height. This article provides the formulas required to perform these
calculations for the MM_TEXT mapping mode. You will have to derive a new
equation to calculate the font size in another mapping mode.

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

To calculate the logical height, use the following formula:

                               Point Size * LOGPIXELSY
  height = Internal Leading + -------------------------
                                         72

LOGPIXELSY is the number of pixels contained in a logical inch on the device.
This value is obtained by calling the GetDeviceCaps() function with the
LOGPIXELSY index. The value 72 is significant because one inch contains 72
points.

The problem with this calculation is that there is no method to determine the
internal leading for the font because it has not yet been created. To work
around this difficulty, use the following variation of the formula:

           -(Point Size * LOGPIXELSY)
  height = --------------------------
                        72

This formula may also be written as follows:

     plf->lfHeight = -MulDiv (nPtSize, GetDeviceCaps (hdc, LOGPIXELSY), 72);

When an application calls the CreateFont() or CreateFontIndirect() functions and
specifies a negative value for the height parameter, the font mapper provides
the closest match for the character height rather than the cell height. The
difference between the cell height and the character height is the internal
leading, as demonstrated by the following diagram:

  ----------  <------------------------------
  |        |           |- Internal Leading  |
  | |   |  |  <---------                    |
  | |   |  |        |                       |- Cell Height
  | |---|  |        |- Character Height     |
  | |   |  |        |                       |
  | |   |  |        |                       |
  ----------  <------------------------------

The following formula computes the point size of a font:

               (Height - Internal Leading) * 72
  Point Size = --------------------------------
                          LOGPIXELSY

The Height and Internal Leading values are obtained from the TEXTMETRIC data
structure. The LOGPIXELSY value is obtained from the GetDeviceCaps function as
outlined above.

Round the calculated point size to the nearest integer. The Windows MulDiv()
function rounds its result and is an excellent choice to perform the previous
calculation.

Additional query words: 3.00 3.10 3.50 4.00 win16sdk TrueType

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Keywords          : kbOSWinNT350 kbOSWinNT351 kbOSWinNT400 kbOSWin95 kbSDKWin16 
Technology        : kbAudDeveloper kbSDKSearch kbWin32sSearch kbWin32API kbWinSDKSearch
Version           : WINDOWS:3.1,95; winnt:3.5,3.51,4.0
Issue type        : kbinfo

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