PRB: Average & Maximum Char Widths Different for TT Fixed Font

• Microsoft Windows Software Development Kit (SDK) for Windows version 3.1
• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.5 and 3.51
      - Microsoft Windows 95 version 4.0
  

SYMPTOMS

The tmAveCharWidth and tmMaxCharWidth fields of the TEXTMETRIC structure are not equal for a fixed-pitch TrueType (TT) font such as Courier New.

RESOLUTION

This is by TrueType design. The tmMaxCharWidth denotes the maximum possible ink width of the font rather than maximum cell width of the font.

MORE INFORMATION

TrueType fonts use ABC character spacing. The "A" width is the distance that is added to the current position before placing the TrueType glyph. The "B" width is the width of the black part (ink width) of the TT glyph. The "C" width is the distance from the end of the "B" width to the beginning of the next character. Advanced width (cell width) is equal to A+B+C.

The physical TT fonts that are passed to drivers have just the "B" part of the characters, so all fonts at the level of the driver appear to be proportional width fonts. The tmMaxCharWidth is the least width in which the "B" part of all characters will fit. The tmAveCharWidth is the average advance width (A+B+C). For a fixed-pitch TT font such as Courier New, the A+B+C width is the same for all characters; however, the maximum width as defined above can be different.

tmMaxCharWidth is greater than tmAveCharWidth only if A+C is negative. This is possible for a fixed-pitch font.

Please see section 18.2.4.1 of the Windows 3.1 SDK "Guide to Programming" for more information about ABC character spacing.