Apple LaserWriter Pro 630 Envelope Feeder Specifications download pdf (Page 88)

CHAPTER 4

TrueType Fonts

TrueType Font Format

TrueType Font Deﬁnition 4

The third component is the actual TrueType font. The font has the minimum but essential

parts of a normal PostScript font: a font dictionary containing a font type, font matrix,

font bounding box, and an encoding vector. In addition, the font should contain a font

name, paint type, stroke width (for outline styles), the TrueType

'sfnt'

(font data) as it

exists in the

'sfnt'

resource on the host, and for Class B devices only, the TrueType

state information and a

BuildChar

procedure. The major bulk of the font is the

'sfnt'

font data.

The

FontType

entries for the three different classes of devices are as follows:

■

For Class A devices, such as the LaserWriter Pro printer, the TrueType font is

, and

the

BuildChar

procedure is therefore implicit. For example, based on the

FontType

entry, the font-rendering machinery will know where to ﬁnd, and how to execute, the

font data. The Type 42 font format is described in “TrueType Font Dictionary Entries,”

later in this chapter.

■

For Class B devices, the

FontType

entry is

, indicating that it is a user-deﬁned font

as understood by PostScript.

■

For Class C devices, the

FontType

entry is

The TrueType font has a

UniqueID

entry, a 24-bit number derived from the checksum in

the

'sfnt'

header. The presence of

UniqueID

in the font makes the PostScript font

cache operate more efﬁciently and avoids rerendering characters across jobs. Two

UniqueID

entries are given to the font, one for the hinted font (Class A or Class B) and

the other for the unhinted font (Class C). The

UniqueID

for a Class C font is further

restricted to be in the range between

4000000

and

4999999

. This range is reserved as

an open range by the Type 1 speciﬁcation. Only one ID is used on any particular printer.

Providing two

UniqueID

entries helps to avoid a situation where even though the

printer is capable of rendering hinted characters, it receives unhinted characters that

were stored in the font cache by a previous job. This could happen if the previous job did

not have enough memory to download TrueType code.

The

'sfnt'

array can contain any number of data strings, no single one of which can be

larger than 65,536 bytes. The

'sfnt'

array is divided into the required number of pieces

at arbitrary table or glyph boundaries within the

'sfnt'

. To guarantee word alignment

of the data, there is always one extra byte at the end of each string in the

'sfnt'

array.

The strings are internally linked or combined at run time to simulate a continuous string

of data. There is no loss in performance speed because of this restriction in string size.

The

'sfnt'

data exists in two forms: the actual

'sfnt'

data straight out of the

'sfnt'

resource and the unfolded glyph data (as Type 1

CharStrings

) for Class C devices. The

printer ignores the data it does not need so that the font size as it is stored on the printer

is not increased.

The entries in a TrueType font dictionary for a Class A or Class B printer are listed and

described in “TrueType Font Dictionary Entries,” later in this chapter. The font

dictionary for a Class C printer follows the Adobe Systems, Incorporated, Type 1 font

format speciﬁcation.

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Apple LaserWriter Pro 630 Envelope Feeder Specifications Page 88

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