gasp — Grid-fitting and Scan-conversion Procedure Table (OpenType 1.8.3)

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, the rasterizer may apply default rules to decide how to render the glyphs on grayscale devices.

'gasp' Table Formats

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

'gasp' Header

Type	Name	Description
uint16	version	Version number (set to 1)
uint16	numRanges	Number of records to follow
GaspRange	gaspRanges[numRanges]	Sorted by ppem

The array of GaspRange records provides recommended behaviors for various ppem sizes.

GaspRange Record

Type	Name	Description
uint16	rangeMaxPPEM	Upper limit of range, in PPEM
uint16	rangeGaspBehavior	Flags describing desired rasterizer behavior.

There are four RangeGaspBehavior flags defined.

RangeGaspBehavior flags

Mask	Name	Description
0x0001	GASP_GRIDFIT	Use gridfitting
0x0002	GASP_DOGRAY	Use grayscale rendering
0x0004	GASP_SYMMETRIC_GRIDFIT	Use gridfitting with ClearType symmetric smoothing Only supported in version 1 'gasp'
0x0008	GASP_SYMMETRIC_SMOOTHING	Use smoothing along multiple axes with ClearType® Only supported in version 1 'gasp'
0xFFF0	Reserved	Reserved flags — set to 0

The set of bit flags may be extended in the future. The first two bit flags operate independently of the following two bit flags. If font smoothing is enabled, then the first two bit flags are used. If ClearType is enabled, then the following two bit flags are used. The seven 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
GASP_SYMMETRIC_GRIDFIT	0x0004	typically always enabled
GASP_SYMMETRIC_SMOOTHING	0x0008	larger screen sizes, typically ppem>15, most commonly used with the gridfit flag.
GASP_SYMMETRIC_SMOOTHING | GASP_SYMMETRIC_GRIDFIT	0x000C	larger screen sizes, typically ppem>15
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.

'gasp' Table and OpenType Font Variations

In a variable font, the threshold sizes at which rasterizer behaviors are changed may need to be adjusted for different variation instances. Variation data for adjusting the rangeMaxPPEM value of up to ten GaspRange records can be provided in the metrics variations (MVAR) table, referenced using value tags 'gsp0' to 'gsp9'. Note that the rasterizer behavior for a given GaspRange record cannot be changed for different variation instances; only the rangeMaxPPEM value can be adjusted.

The last GaspRange record in a 'gasp' table is assumed to have a rangeMaxPPEM value of 0xFFFF (effectively infinity). The rangeMaxPPEM value of the last record is never adjusted for different instances; the number of value records in the MVAR table that are associated with 'gasp' entries must never be more than numRanges minus one.

For general information on OpenType Font Variations, see the chapter, OpenType Font Variations Overview.

Sample 'gasp' table

Flag	Value	Font smoothing meaning	ClearType with symmetric smoothing meaning
version	0x0001
numRanges	0x0004
Range[0], Flag	0x0008 0x000A	ppem <= 8, grayscale only	ppem <= 8, symmetric ClearType only
Range[1], Flag	0x0010 0x0005	9 <= ppem <= 16, gridfit only	9 <= ppem <= 16, symmetric gridfit only
Range[2], Flag	0x0013 0x0007	17 <= ppem<= 19, gridfit and grayscale	17 <= ppem<= 19, symmetric gridfit
Range[3], Flag	0xFFFF 0x000F	20 <= ppem, gridfit and grayscale	20 <= ppem, symmetric gridfit and symmetric smoothing