INFO: Calculating The Logical Height and Point Size of a Font (74299)

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.