gasp: Grid-fitting and Scan Conversion Procedure

gasp: Grid-fitting and Scan Conversion Procedure

The 'gasp' table allows a TrueType font to define the best combinations of hinting and gray scale rendering (font smoothing) to apply at different font sizes.

Requirements

If a client requests gray scale, the TrueType rasterizer will generate a gray scale bitmap directly from the glyph outline data, even if an embedded bitmap exists at the requested size. Thus a font that includes embedded bitmaps should use the 'gasp' table to turn off gray scale at the sizes covered by the bitmaps.
Windows 95 can read up to 8 ranges in the 'gasp' table.

Recommendations

The best settings for the ranges depend on whether the font is hinted or not, and whether the font includes bitmaps or not. For example, in a Far East font containing hints but no bitmaps, the 'gasp' table might include settings for three ranges:

Typical size range	Description	'gasp' setting
less than 12 ppem	At very small screen sizes, there are not enough pixels for hinting to represent the character shape regularly. Gray scale rendering produces a fuzzy but often legible character.	0x0002 (hints off, gray on)
12-18 ppem	At intermediate screen sizes, hinting can create regular, easily legible shapes without the use of gray scale.	0x0001 (hints on, gray off)
greater than 18 ppem	At large screen sizes, gray scale can improve hinting to smooth out diagonals, curves, and subtle features.	0x0003 (hints on, gray on)