gasp - Grid-fitting And Scan-conversion Procedure

This table contains information which describes the preferred rasterization techniques for the typeface when it is rendered on grayscale-capable devices. This table also has some use for monochrome devices, which may use the table to turn off hinting at very large or small sizes, to improve performance.

At very small sizes, the best appearance on grayscale devices can usually be achieved by rendering the glyphs in grayscale without using hints. At intermediate sizes, hinting and monochrome rendering will usually produce the best appearance. At large sizes, the combination of hinting and grayscale rendering will typically produce the best appearance.

If the 'gasp' table is not present in a typeface, TrueType will apply default rules to decide how to render the glyphs on grayscale devices.

The 'gasp' table consists of a header followed by groupings of 'gasp' records:

gasp Table

Type	Name	Description
USHORT	version	Version number (set to 0)
USHORT	numRanges	Number of records to follow
GASPRANGE	gaspRange[numRanges]	Sorted by ppem

Each GASPRANGE record looks like this:

Type	Name	Description
USHORT	rangeMaxPPEM	Upper limit of range, in PPEM
USHORT	rangeGaspBehavior	Flags describing desired rasterizer behavior.

There are two flags for the rangeGaspBehavior flags:

Flag	Meaning
GASP_GRIDFIT	Use gridfitting
GASP_DOGRAY	Use grayscale rendering

The set of bit flags may be extended in the future.The four currently defined values of rangeGaspBehavior would have the following uses:

Flag	Value	Meaning
GASP_DOGRAY	0x0002	small sizes, typically ppem<9
GASP_GRIDFIT	0x0001	medium sizes, typically 9<=ppem<=16
GASP_DOGRAY\| GASP_GRIDFIT	0x0003	large sizes, typically ppem>16
(neither)	0x0000	optional for very large sizes, typically ppem>2048

The records in the gaspRange[] array must be sorted in order of increasing rangeMaxPPEM value. The last record should use 0xFFFF as a sentinel value for rangeMaxPPEM and should describe the behavior desired at all sizes larger than the previous record's upper limit. If the only entry in 'gasp' is the 0xFFFF sentinel value, the behavior described will be used for all sizes.

Sample 'gasp' table

Field	Value	Meaning
version	0x0000
numRanges	0x0003
Range[0], Flag	0x0008 0x0002	ppem<=8, grayscale only
Range[1], Flag	0x0010 0x0001	9<=ppem<=16, gridfit only
Range[2], Flag	0xFFFF 0x0003	16<ppem, gridfit and grayscale