INFO: Using GetCharABCWidths() to Calculate Text Extents

Version 3.1 of the Microsoft Windows graphical environment introduces a set of functions to manipulate TrueType fonts. An application can use the GetCharABCWidths() function to accurately determine the advance width, overhang, and underhang of a character. Because GetCharABCWidths() was designed for use only with TrueType fonts, it is less useful when it calculates the width of characters in a font in which the TrueType rasterizer simulates the bold attribute. This article describes a method to work around this limitation and also demonstrates a useful technique to calculate the bounding rectangle for a string of text that provides accurate results regardless of the font technology involved.

The TrueType rasterizer generates ABC character spacing to enable an application to position characters exactly. The "A" spacing is the distance added to the final position of the previous character (or to the beginning of the line) before placing the glyph. The "B" spacing is the width of the black part of the glyph. The "C" spacing is the distance to add to the current position to account for the white space to the right of the glyph. The total advance width for the glyph if given by A + B + C. Using the advance width of one character, an application can determine where to position the following character.

The GetCharABCWidths() function retrieves the widths, in logical units, of consecutive characters in a specified range from the current TrueType font. When the GetCharABCWidths() function returns a negative value for the "A" or "C" width for a character, that character includes underhang or overhang, respectively. The GetCharABCWidths function works only for TrueType fonts.

An application can use the GetCharWidth() function to determine the advance width of a character in any font. However, because GetCharWidth() has no mechanism to provide a negative "A" or "C" width, it cannot be used to place text exactly.

Under Windows 3.1, if the TrueType rasterizer simulates the bold attribute, the information returned by the GetCharABCWidths() function is less accurate. (For example, the standard fonts provided with Windows 3.1 include the WingDings font in standard weight only. If an application requests a bold version of WingDings, the TrueType rasterizer simulates the bold attribute by manipulating the original, normal-weight font.) When an application uses the GetCharABCWidths() function to determine the advance width of a manipulated TrueType font, the difference between the returned ABC width and the true ABC width is one for each character.

To work around this limitation, the application can add one to the ABC width of each character. However, to use this method, the application must determine that the rasterizer has simulated the bold attribute. The following function demonstrates determining that a TrueType font has bold enabled. Select a TrueType font into the Display Context (DC) before calling this function.

NOTE: If you are not using the MM_TEXT mapping mode, you will need to convert the value one from device coordinates to logical coordinates before adding the value to the character width. Note that this limitation occurs only in Windows 3.1 (the gde.EXE file dated March 10, 1992). Any application that uses the methods described in this article to work around this limitation must version-bind its code.

The following function demonstrates how to use the GetCharABCWidths() function to determine the bounding rectangle for a string of text. Although this function is very similar to the GetTextExtent() and GetTextExtentPoint() functions in Windows, it correctly accounts for overhang and underhang caused by negative "A" and "C" widths. This function also demonstrates how to version-bind the code that works around the limitation in Windows 3.1.