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Fonts

This section describes the terminology used throughout the Font Manager documentation to refer to the individual elements of a font, different types of fonts, and the different functions a font can have. If you are already familiar with font terminology on the Macintosh, you can skip this section.

In this section:

Characters, Character Codes, and Glyphs
Kinds of Fonts
Identifying Fonts
Font Measurements
Font Rendering

Characters, Character Codes, and Glyphs

The smallest element in any character set is a character, which is a symbol that represents the concept of, for example, a lowercase “b”, the number “2”, or the arithmetic operation “+”. You do not ever see a character on a display device. What you actually see on a display device is a glyph, the visual representation of the character. One glyph can represent one character, such as a lowercase ”b”; more than one character, such as the “fi” ligature, which is a single glyph that could represent two characters; or a nonprinting character, such as the space character.

The Font Manager identifies an individual character by a character code and provides the glyph for that character to QuickDraw. Character codes for most character sets are single byte values between $00 and $FF; however, the character codes for some large character sets, such as the Japanese character set, are two bytes long. A font designer must supply a missing-character glyph for characters that are not included in the font. QuickDraw displays this glyph whenever the user presses a key for a character that is not in the font.

Although most fonts assign the same glyphs to character code values $00 to $7F, there are differences in which glyphs are assigned to the remaining character codes. For example, the glyph assigned to byte value $F0 () in the Apple Standard Roman character set is not typically included in a font defined for a computer that does not use the Mac OS. Different regions of the world require different glyphs for their typography, which make it impossible for any one standard to be complete.

Character-encoding schemes were developed to manage the assignment of different glyphs to character codes in different fonts. An encoding scheme names each character and then maps that name into a character code in each font. PostScript fonts that use an encoding scheme that differs from the standard Apple encoding scheme can specify their glyph assignments in the encoding table of the font family resource described in the style mapping table. See Inside Macintosh: Text located on the Apple developer website:

http://developer.apple.com/documentation/mac/Text/Text-2.html

The Font Manager handles two types of glyphs: bitmapped glyphs and glyphs from outline fonts. A bitmapped glyph is a bitmap (a collection of bits arranged in rows and columns) designed at a fixed point size for a particular display device, such as a monitor or a printer. For example, after deciding that a glyph for a screen font should be so many pixels tall and so many pixels wide, a font designer carefully chooses the individual pixels that constitute the bitmapped glyph. A pixel is the smallest dot the screen can display. The font stores the bitmapped glyph as a picture for the display device.

A glyph from an outline font is a model of how a glyph should look. A font designer uses lines and curves rather than pixels to draw the glyph. The outline, a mathematical description of a glyph from an outline font, has no designated point size or display device characteristic (such as the size of a pixel) attached to it. The Font Manager uses the outline as a pattern to create bitmaps at any size for any display device.

Kinds of Fonts

Each glyph has some characteristics that distinguish it from other glyphs that represent the same character: for example, the shape of the oval, the design of the stem, or whether or not the glyph has a serif. If all the glyphs for a particular character set share certain characteristics, they form a type face, which is a distinctly designed collection of glyphs. Each typeface has its own name, such as New York, Geneva, or Symbol. The same typeface can be used with different hardware, such as typesetting machines, monitors, or laser printers.

A style is a specific variation in the appearance of a glyph that can be applied consistently to all the glyphs in a typeface. Styles available in the Mac OS include plain, bold, italic, underline, outline, shadow, condensed, and extended. QuickDraw can add styles, such as bold or italic to bitmaps, or a font designer can design a font in a specific style (for instance, Courier Bold).

A font refers to a complete set of glyphs in a specific typeface and style. And, in the case of bitmapped fonts, a specific size. Bitmapped fonts are fonts of the bitmapped font ('NFNT') resource type or 'FONT' resource type that provide an individual bitmap for each glyph in each size and style. Courier plain 10-point, Courier bold 10-point, and Courier plain 12-point, for example, are considered three different fonts. If the user requests a font that is not available in a particular size, QuickDraw can alter a bitmapped font at a different size to created the required glyphs. However, this generated bitmap often appears to be irregular in some way.

Outline fonts are fonts of the outline font ('sfnt') resource type that consist of glyphs in a particular typeface and style with no size restriction. The Font Manager can generate thousands of point sizes from the same outline font. For example, a single outline Courier font can produce Courier 10-point, Courier 12-point, and Courier 200-point.

Identifying Fonts

When multiple fonts of the same typeface are present in system software, the Font Manager groups them into font families. Each font in a font family can be bitmapped or outline. Bitmapped fonts in the same family can be different styles or sizes. For example, an outline plain font for Geneva and two bitmapped fonts for Geneva plain 12-point and Geneva italic 12-point might make up one font family named Geneva, to which a user could subsequently add other sizes or styles.

A font has a font name, which is stored as a string such as “Geneva” or “New York”. The font name is usually the same name as the typeface from which it was derived. If a font is not in plain style, its style becomes part of the font’s name and distinguishes it from the plain style of that font: for example “Palatino” and “Palatino Bold”.

A font family reference is a reference to an opaque structure that represents a collection of fonts with the same design characteristics. You can use the font family reference to refer to font families while you application is running. However, if you need to refer a font or font family every time your application launches (that is, across reboots), you should refer to the font or font family by name, and not by the font family reference.

Font Measurements

Font designers use specific terms for the measurements of different parts of a glyph, whether outline or bitmapped. “Figure 2-1” shows the terms used for the most frequently used measurements.

As shown in “Figure 2-1,” the bounding box of a glyph is the smallest rectangle that entirely encloses the pixels of the glyph. The glyph origin is where QuickDraw begins drawing the glyph. Notice that there is some white space between the glyph origin and the visible beginning of the glyph: this is the left-side bearing of the glyph. The left-side bearing value can be negative, which lessens the spacing between adjacent characters. The advance width is the full horizontal measurement of the glyph as measured from its glyph origin to the glyph origin of the next glyph on the line, including the white space on both sides.

If all of the glyph images in the font were superimposed using a common glyph origin, the smallest rectangle that would enclose the resulting image is the font rectangle.

The glyphs of a fixed-width font all have the same advance width. Fixed-width fonts are also know as monospaced fonts. In Courier, a fixed-width font, the uppercase “M” has the same width as the lowercase “i”. In a proportional font, different glyphs may have different widths, so the uppercase “M” is wider than the lowercase “i”.

Most glyphs in a font appear to sit on the base line, an imaginary horizontal line. The ascent line is an imaginary horizontal line chosen by the font’s designer that aligns approximately with the tops of the uppercase letters in the font, because these are the tallest commonly used glyphs in a font. The ascent line is the same distance from the base line for all glyphs in the font. The descent line is an imaginary horizontal line that usually aligns with the bottoms of descenders (the tails on the glyphs such as “p” or “g”), and it is the same distance from the base line for every glyph in the font. The ascent and descent lines are part of the font designer’s recommendations about line spacing as measured from base line to base line. All of these lines are horizontal because Roman text is read from left to right, in a straight horizontal line.

For bitmapped fonts, the ascent line marks the maximum y-value and the descent line marks the minimum y-value used for the font. The y-value is the location on the vertical axis of each indicated line: the minimum y-value is the lowest location on the vertical axis and the maximum y-value is the highest location on the vertical axis. For outline fonts, a font designer can create individual glyphs that extend above the ascent line or below the descent line. The integral sign in “Figure 2-2,” for example, is much taller than the uppercase “M”. In this case, the maximum y-value is more important than the ascent line for determining the proper line spacing for a line containing both of these glyphs. You can have the Font Manager reduce such oversized glyphs so that they fit between the ascent and descent lines. See “Preserving the Shapes of Glyphs” for details.

Font size (or point size) indicates the size of a font’s glyphs as measured from the base line of one line of text to the base line of the next line of single-spaced text. In the United States, font size is traditionally measured in points, and there are 72.27 traditional points per inch. However, QuickDraw and the PostScript language define 1 point to be 1/72 of an inch, so there are exactly 72 points per inch on the Macintosh.

All bitmaps must fit on the QuickDraw coordinate plane. On a 72-dpi display device, fonts have an upper size limit of 32,767 points.

There is not a strict typographical standard for defining a point size. It is often, but not always, the sum of the ascent, descent, and leading values for a font. Point size is used by a font designer to indicate the size of a font relative to other fonts in the same family. Glyphs from fonts with the same point size are not necessarily of the same height. This means that a 12-point font can exceed the measurement of 12 points from the base line of one line of text to the base line of the next.

Leading (pronounced “LED-ing”) is the amount of blank vertical space between the descent line of one line of text drawn using a font and the ascent line of the next line of single-spaced text drawn in the same font. The Font Manager returns the font’s suggested leading, which is in pixels, in the FontMetrics function for both outline and bitmapped fonts. QuickDraw returns similar information in the GetFontInfo function. Although the designer specifies a recommended leading value for each font, you can always change that value if you need more or less space between the lines of text in your application. The line spacing for a font can be calculated by adding the value of the leading to the distance from the ascent line to the descent line of a single line of text.

Although each glyph has a specific advance width and left-side bearing measurement assigned to it, you can change the amount of white space that appears between glyphs. Kerning is the process of drawing part of a glyph so that is overlaps another glyph. The period in the top portion of “Figure 2-3” stands apart from the uppercase “Y”. In the bottom portion of the figure, the word and the period have been kerned: the period has been moved under the right arm of the “Y” and the glyphs of the word are closer. Kerning data (the distances by which pairs of specified glyphs should be moved closer together) is stored in the kerning tables of the different font resources.

Font Rendering

This section describes how TrueType outline fonts are rendered by a font rendering engine. Although PostScript fonts are rendered using different mathematical algorithms, the two rendering methods are similar in that both use mathematical equations to estimate contours. For information on PostScript font rendering, see the PostScript Language Reference Manual by Adobe Systems, Inc., available through Addison-Wesley Publishing Company.

TrueType outline fonts are stored in an outline font ('sfnt') resource as a collection of outline points. (Do not confuse these outline points with the points that determine point size, or the Point data type, which specifies a location in the QuickDraw coordinate plane.) A font rendering engine calculates lines and curves between the points, sets the bits that make the bitmap, and then sends the bitmap to QuickDraw for display.

There are two types of outline points: on-curve points define the endpoints of lines and off-curve points determine the curvature of the line between the on-curve points. Two consecutive on-curve points define a straight line. To draw a curve, the font rendering engine needs a third point that is off the curve and between the two on-curve points.

A font rendering engine uses this parametric Bézier equation to draw the curves of the glyph from an outline font:

where t ranges between 0 and 1 as the curve moves from point A to point C. A and C are on-curve points; B s an off-curve point.

“Figure 2-4” shows two Bézier curves. The positions of on-curve points A and C remain constant, while off-curve point B shifts. The curve changes in relation to the position of point B.

A font designer can use any number of outline points to create a glyph outline. These points must be numbered consecutively along the contours of the glyph, because a font rendering engine draws lines on curves sequentially. This process produces a glyph such as the lower case “b” in “Figure 2-5.”

There are several groups of points in “Figure 2-5” that include two consecutive off-curve points. For instance, points 2 and 3 are both off-curve. In this case, the font rendering engine interpolates an on-curve point midway between the two off-curve points, thereby defining two Bézier curves, as shown in “Figure 2-6.” Note that this additional on-curve point is used for creation of the glyph only; the font rendering engine does not alter the outline font resource’s list of points.

When the font rendering engine has finished drawing a closed loop, it has completed one contour of the outline. The font designer groups the points in the outline font resource into contours. In “Figure 2-5,” the font rendering engine draws the first contour in the glyph from point 0 to point 17, and the second contour from point 18 to the end, creating the glyph in “Figure 2-7.”

At this stage the glyph does not have a fixed point size. Remember that point size is measured as the distance from the base line of one line of text to the base line of the next line of single-spaced text. The font rendering engine has the measurements of the outline relative to the base line and ascent line, so it can correlate the measurements with the requested point size and calculate how large the outline should be for that point size.

The font rendering engine uses the contours to determine the boundaries of the bitmap for this glyph when it is displayed. For example, the Macintosh computer’s screen is a grid made of pixels. The font rendering engine fits the glyph, scaled for the correct size, to this grid. If the center of one section of this grid (comparable to a pixel or a printer dot) falls on a contour or within two contours, the font rendering engine sets the bit for the bitmap.

Because there are two contours for the glyph in “Figure 2-7” the font rendering engine begins with pixels at the boundary marked by contour 1 and stops when it gets to contour 2. Some glyphs need only one contour, such as the uppercase “I” in some fonts. Others, such as the Å glyph, have three or more contours.

If the pixels (or dots) are tiny in proportion to the outline (when resolution is high or the point size of the glyph is large), they fill out the outline smoothly, and any pixels that jut out from the contours are not noticeable. If the display device has a low resolution or the point size is small, the pixels are large in relation to the outline. You can see in “Figure 2-8” that the outline has produced an unattractive bitmap. There are gaps and blocky areas that would not be found in the high-resolution version of the same glyph.

Because the size of the pixels or dots used by the display device cannot change, the outline should adapt in order to produce a better bitmap. To achieve this end, font designers include instructions in the outline font resources that indicate how to change the shape of the outline under various conditions, such as low resolution or small point size. The lowercase “b” outline in “Figure 2-9” is the same one depicted in “Figure 2-8,” except that the font rendering engine has applied the instructions to the figure and produces a better bitmapped glyph. These instructions are equivalent to “move these points here” or “change the angle formed by these points.” A font designer includes programs consisting of these instructions in certain outline font resource tables, where the font rendering engine finds them and executes them under specified conditions. Most applications do not need to use instructions; however, if you want to know more about them, see the TrueType Font Format Specification (available through http://developer.apple.com/fonts/).

In the case of the Mac OS, once the Font Manager has produced the outline according to the design and instructions, it creates a bitmap and sends the bitmap to QuickDraw, which draws it on the screen. The Font Manager then saves the bitmapped glyph in memory (caches it) and uses it the next time the user requests this glyph in this font at this point size.

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