gasp - Grid-fitting And Scan-conversion Procedure (OpenType 1.5)
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.
The 'gasp' table consists of a header followed by groupings of 'gasp' records:
gasp Table
| Type | Name | Description |
|---|---|---|
| USHORT | version | Version number (set to 1) |
| 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 four flags for the rangeGaspBehavior flags:
| Flag | Meaning |
|---|---|
| GASP_DOGRAY | Use grayscale rendering |
| GASP_GRIDFIT | Use gridfitting |
| GASP_SYMMETRIC_SMOOTHING | Use smoothing along multiple axes with ClearType® Only supported in version 1 gasp |
| GASP_SYMMETRIC_GRIDFIT | Use gridfitting with ClearType symmetric smoothing Only supported in version 1 gasp |
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 |
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
| 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 |