A document with printer portability is formatted identically on all output devices under Windows—all monitors and all printers. Although TrueType allows the same font to be used on all output devices, this does not guarantee that line breaks will be the same on all devices. For line breaks to match, applications must take advantage of TrueType design metrics. These design metrics allow an application to compute the fractional portion of the spacing at the ends of lines and make up the difference in the interword spacing. This computation reduces the round-off error from a half-pixel per character to a half-pixel per line, preserving line breaks in all cases.
Applications must cooperate in order to guarantee the printer portability enabled by TrueType technology, because different devices may have different resolutions. Even when fonts are portable across printers, glyphs designed or rasterized for different resolutions must have different pixel widths. For applications that use the TextOut function, for example, different character widths can lead to accumulated round-off errors that change line breaks and paragraph placement.
Applications that lay out a document at the highest printer resolution attempt to distribute any difference in character resolutions in white spaces. This method is not always successful; for example, it fails when all glyphs are one pixel larger at 600 dots per inch (DPI) than at 300 DPI. In this case, fonts with a width of 45 at 600 DPI would have a width of 23 at 300 DPI, a width of 11 at 150 DPI, and so on. There could easily be insufficient white space to absorb the glyphs at the lower resolutions if line breaks were being preserved, because the glyphs become larger in relation to the resolution of the device. In this case, the characters would have to overlap to preserve the line breaks. Even if all the character widths exactly doubled when changing from a resolution of 300 DPI to 600 DPI, the line breaks might not be the same if an application justified text—that is, aligned it on both the left and right. It is possible that another half-pixel of white space at the lower resolution would allow one more word on the line. At the higher resolution, the half-pixel would become a full pixel and the line breaks would change. (Similar device-resolution problems occur in the vertical direction.) TrueType exposes the design width of characters to help applications maintain line breaks. For information about design widths, see Section 18.4.8.2, “Design Widths.”
Different printers, or even different production runs of the same printer, can have different limits for their printable areas. If a document has been laid out up to the margins of one printer, it may not format identically on a different printer. If glyphs are in contact with the margins on the first printer, parts of the glyph may be beyond the printable area on the second printer. Depending on the printer, the glyph will either be clipped or dropped completely.
Prior to the introduction of TrueType, sophisticated desktop-publishing and word-processing applications were forced to “reflow” the entire document whenever a user selected a different printer. Applications can now use TrueType font metrics to solve this problem. For a description of using these metrics, see Section 18.4.8.2, “Using Portable TrueType Metrics.”
Printer portability can potentially downgrade font performance, quality, or both, depending on such factors as the type of connection between the computer and printer, the speed of the computer, the memory in the printer and the computer, the number of fonts being used, differing resolutions between the screen and printer, and the number of characters used in each font. Documents that are fully portable between printers necessarily cannot take advantage of the specialized features of a particular printer.
GDI cannot perform text operations to printer-compatible memory device contexts. This means that it is not possible to build a bitmap describing a page to be printed and then send the completed bitmap to the printer.