From 904c1e15bfdeac7a6ac53d1f51e8fc66634ca003 Mon Sep 17 00:00:00 2001 From: Werner Lemberg Date: Thu, 9 Nov 2000 08:01:18 +0000 Subject: [PATCH] Revised. --- docs/glyphs/glyphs-1.html | 4 + docs/glyphs/glyphs-2.html | 658 ++++++++++++++++++++------------------ 2 files changed, 347 insertions(+), 315 deletions(-) diff --git a/docs/glyphs/glyphs-1.html b/docs/glyphs/glyphs-1.html index df4bee7c3..4b409af02 100644 --- a/docs/glyphs/glyphs-1.html +++ b/docs/glyphs/glyphs-1.html @@ -56,6 +56,8 @@ +

+
@@ -166,6 +168,8 @@ they are usually expressed in pixels then.

+

+
+ - - - - FreeType Glyph Conventions + + + FreeType Glyph Conventions - -

-FreeType Glyph Conventions -

+

+ FreeType Glyph Conventions +

-

-version 2.1 -

+

+ Version 2.1 +

-

-Copyright 1998-2000 David Turner (david@freetype.org)
-Copyright 2000 The FreeType Development Team (devel@freetype.org) -

- -
- - - - - - - - - - - - - - - - - -
- -
- - - -
-Previous - -Contents - -Next -
- -

-II. Glyph Outlines -

- -

This section describes the way scalable representation of glyph images, - called outlines, are used by FreeType as well as client applications.

- -

-1. Pixels, Points and Device Resolutions : -

- -

Though it is a very common assumption when dealing with computer -graphics programs, the physical dimensions of a given pixel (be it for -screens or printers) are not squared. Often, the output device, be it a -screen or printer exhibits varying resolutions in the horizontal and vertical -directions, and this must be taken care of when rendering text. -

- -

It is thus common to define a device's characteristics through two numbers -expressed in dpi (dots per inch). For example, a printer with a -resolution of 300x600 dpi has 300 pixels per inch in the horizontal -direction, and 600 in the vertical one. The resolution of a typical computer -monitor varies with its size (a 15" and 17" monitors don't have the same -pixel sizes at 640x480), and of course the graphics mode resolution. -

- -

As a consequence, the size of text is usually given in points, -rather than device-specific pixels. Points are a simple physical -unit, where 1 point = 1/72th of an inch, in digital typography. As an -example, most roman books are printed with a body text which size is -chosen between 10 and 14 points.

- -

It is thus possible to compute the size of text in pixels from the size -in points through the following computation :

+

+ Copyright 1998-2000 David Turner (david@freetype.org)
+ Copyright 2000 The FreeType Development Team (devel@freetype.org) +

-

pixel_size = point_size * resolution / 72

+ + + +
-

Where resolution is expressed in dpi. Note that because the -horizontal and vertical resolutions may differ, a single point size -usually defines different text width and height in pixels.

+
+ + + + + + +
+ Previous + + Contents + + Next +
+
-

IMPORTANT NOTE: -
Unlike what is often thought, the "size of text in pixels" is not -directly related to the real dimensions of characters when they're displayed -or printed. The relationship between these two concepts is a bit more complex -and relate to some design choice made by the font designer. This is described -in more details the next sub-section (see the explanations on the EM square). -

+

+ + + +
+

+ II. Glyph Outlines +

+
+ +

This section describes the way scalable representation of glyph images, + called outlines, are used by FreeType as well as client applications.

+ + +

+ 1. Pixels, points and device resolutions +

+ +

Though it is a very common assumption when dealing with computer + graphics programs, the physical dimensions of a given pixel (be it for + screens or printers) are not squared. Often, the output device, be it a + screen or printer, exhibits varying resolutions in both horizontal and + vertical direction, and this must be taken care of when rendering + text.

+ +

It is thus common to define a device's characteristics through two + numbers expressed in dpi (dots per inch). For example, a + printer with a resolution of 300x600 dpi has 300 pixels per + inch in the horizontal direction, and 600 in the vertical one. The + resolution of a typical computer monitor varies with its size + (15" and 17" monitors don't have the same pixel sizes at + 640x480), and of course the graphics mode resolution.

+ +

As a consequence, the size of text is usually given in + points, rather than device-specific pixels. Points are a + simple physical unit, where 1 point = 1/72th of + an inch, in digital typography. As an example, most Roman books are + printed with a body text which size is chosen between 10 and + 14 points.

+ +

It is thus possible to compute the size of text in pixels from the + size in points with the following formula:

+ +
+ pixel_size = point_size * resolution / 72 +
+ +

The resolution is expressed in dpi. Since horizontal and + vertical resolutions may differ, a single point size usually defines a + different text width and height in pixels.

+ +

Unlike what is often thought, the "size of text in pixels" is not + directly related to the real dimensions of characters when they are + displayed or printed. The relationship between these two concepts is a + bit more complex and relate to some design choices made by the font + designer. This is described in more detail in the next sub-section (see + the explanations on the EM square).

+ +

+ 2. Vectorial representation +

-

-2. Vectorial representation : -

+

The source format of outlines is a collection of closed paths called + contours. Each contour delimits an outer or inner + region of the glyph, and can be made of either line + segments or Bézier arcs.

+ +

The arcs are defined through control points, and can be + either second-order (these are conic Béziers) or + third-order (cubic Béziers) polynomials, depending on + the font format. Note that conic Béziers are usually called + quadratic Béziers in the literature. Hence, each point + of the outline has an associated flag indicating its type (normal or + control point). And scaling the points will scale the whole + outline.

+ +

Each glyph's original outline points are located on a grid of + indivisible units. The points are usually stored in a font file as + 16-bit integer grid coordinates, with the grid origin's being at (0,0); + they thus range from -16384 to 16383. (Even though point + coordinates can be floats in other formats such as Type 1, we will + restrict our analysis to integer values for simplicity).

+ +

The grid is always oriented like the traditional mathematical + two-dimensional plane, i.e., the X axis from the left to the + right, and the Y axis from bottom to top.

+ +

In creating the glyph outlines, a type designer uses an imaginary + square called the EM square. Typically, the EM square can be + thought of as a tablet on which the character are drawn. The square's + size, i.e., the number of grid units on its sides, is very important for + two reasons:

+ +
    +
  • +

    It is the reference used to scale the outlines to a given text + dimension. For example, a size of 12pt at 300x300 dpi + corresponds to 12*300/72 = 50 pixels. This is the + size the EM square would appear on the output device if it was + rendered directly. In other words, scaling from grid units to + pixels uses the formula:

    + +

    + pixel_size = point_size * resolution / 72
    + pixel_coord = grid_coord * pixel_size / EM_size +

    +
    • +
  • +

    The greater the EM size is, the larger resolution the designer + can use when digitizing outlines. For example, in the extreme + example of an EM size of 4 units, there are only 25 point + positions available within the EM square which is clearly not + enough. Typical TrueType fonts use an EM size of 2048 units; + Type 1 PostScript fonts have a fixed EM size of 1000 grid + units but point coordinates can be expressed as floating values.

    +
    • +
+ +

Note that glyphs can freely extend beyond the EM square if the font + designer wants so. The EM is used as a convenience, and is a valuable + convenience from traditional typography.

+ +

Grid units are very often called font units or EM + units.

+ +

As said before, pixel_size computed in the above formula + does not relate directly to the size of characters on the screen. It + simply is the size of the EM square if it was to be displayed. Each + font designer is free to place its glyphs as it pleases him within the + square. This explains why the letters of the following text have not + the same height, even though they are displayed at the same point size + with distinct fonts: + +

+ Comparison of font heights +

+ +

As one can see, the glyphs of the Courier family are smaller than + those of Times New Roman, which themselves are slightly smaller than + those of Arial, even though everything is displayed or printed at a size + of 16 points. This only reflects design choices.

+ +

+ 3. Hinting and Bitmap rendering +

-

The source format of outlines is a collection of closed paths -called contours. Each contour delimits an outer or inner region -of the glyph, and can be made of either line segments or bezier -arcs.

+

The outline as stored in a font file is called the "master" outline, + as its points coordinates are expressed in font units. Before it can be + converted into a bitmap, it must be scaled to a given size/resolution. + This is done through a very simple transformation, but always creates + undesirable artifacts, e.g. stems of different widths or heights in + letters like "E" or "H".

-

The arcs are defined through control points, and can be either -second-order (these are "conic" beziers) or third-order ("cubic" beziers) polynomials, depending on -the font format. Note that conic beziers are usually called "quadratic" -beziers in the literature. Hence, each point of the outline has an -associated flag indicating its type (normal or control point). -And scaling the points will scale the whole outline. -

+

As a consequence, proper glyph rendering needs the scaled points to + be aligned along the target device pixel grid, through an operation + called grid-fitting, and often hinting. One of its + main purposes is to ensure that important widths and heights are + respected throughout the whole font (for example, it is very often + desirable that the "I" and the "T" have their central vertical line of + the same pixel width), as well as to manage features like stems and + overshoots, which can cause problems at small pixel sizes.

-

Each glyph's original outline points are located on a grid of indivisible -units. The points are usually stored in a font file as 16-bit integer grid -coordinates, with the grid origin's being at (0,0); they thus range from --16384 to 16383. (even though point coordinates can be floats in other -formats such as Type 1, we'll restrict our analysis to integer ones, driven -by the need for simplicity..). -

+

There are several ways to perform grid-fitting properly; most + scalable formats associate some control data or programs with each glyph + outline. Here is an overview:

-

IMPORTANT NOTE: -
The grid is always oriented like the traditional mathematical 2D -plane, i.e. the X axis from the left to the right, and the Y axis from -bottom to top.

+
    +
  • +

    explicit grid-fitting

    -

    In creating the glyph outlines, a type designer uses an imaginary square -called the "EM square". Typically, the EM square can be thought of as a -tablet on which the character are drawn. The square's size, i.e., the number -of grid units on its sides, is very important for two reasons:

    +

    The TrueType format defines a stack-based virtual machine, for + which programs can be written with the help of more than + 200 opcodes (most of these relating to geometrical operations). + Each glyph is thus made of both an outline and a control program to + perform the actual grid-fitting in the way defined by the font + designer.

    +
    • +
  • +

    implicit grid-fitting (also called hinting)

    -
      -

    • -it is the reference used to scale the outlines to a given text dimension. -For example, a size of 12pt at 300x300 dpi corresponds to 12*300/72 = 50 -pixels. This is the size the EM square would appear on the output device -if it was rendered directly. In other words, scaling from grid units to -pixels uses the formula:

      +

      The Type 1 format takes a much simpler approach: Each glyph + is made of an outline as well as several pieces called + hints which are used to describe some important features of + the glyph, like the presence of stems, some width regularities, and + the like. There aren't a lot of hint types, and it is up to the + final renderer to interpret the hints in order to produce a fitted + outline.

      +
      • +
    • +

      automatic grid-fitting

      -

      pixel_size = point_size * resolution / 72 -
      pixel_coordinate = grid_coordinate * pixel_size / EM_size -

      +

      Some formats simply include no control information with each + glyph outline, apart metrics like the advance width and height. It + is then up to the renderer to "guess" the more interesting features + of the outline in order to perform some decent grid-fitting.

      +
      • +
    +

    The following table summarises the pros and cons of each scheme.

    -

  • -the greater the EM size is, the larger resolution the designer can use -when digitizing outlines. For example, in the extreme example of an EM -size of 4 units, there are only 25 point positions available within the -EM square which is clearly not enough. Typical TrueType fonts use an EM -size of 2048 units (note: with Type 1 PostScript fonts, the EM size is -fixed to 1000 grid units. However, point coordinates can be expressed in -floating values). -

    • -
+
+ + + + + + -

Note that glyphs can freely extend beyond the EM square if the font -designer wants so. The EM is used as a convenience, and is a valuable -convenience from traditional typography.

+ + -
-

Note : Grid units are very often called "font units" or "EM units".

+ -

As one can see, the glyphs of the Courier family are smaller than those -of Times New Roman, which themselves are slightly smaller than those of -Arial, even though everything is displayed or printed  at a size of -16 points. This only reflect design choices. -

+ + + + -

-3. Hinting and Bitmap rendering -

+
-

As a consequence, proper glyph rendering needs the scaled points to -be aligned along the target device pixel grid, through an operation called -"grid-fitting", and often "hinting". One of its main purpose is to ensure -that important widths and heights are respected throughout the whole font -(for example, it is very often desirable that the "I" and the "T" have -their central vertical line of the same pixel width), as well as manage -features like stems and overshoots, which can cause problems at small pixel -sizes. -

+ + -
-
explicit grid-fitting : -
The TrueType format defines a stack-based virtual machine, -for which programs can be written with the help of more than 200 opcodes -(most of these relating to geometrical operations). Each glyph is thus -made of both an outline and a control program, its purpose being to perform -the actual grid-fitting in the way defined by the font designer.
+
+ -


implicit grid-fitting (also called hinting) : -

The Type 1 format takes a much simpler approach : each glyph -is made of an outline as well as several pieces called "hints" which are -used to describe some important features of the glyph, like the presence -of stems, some width regularities, and the like. There aren't a lot of -hint types, and it's up to the final renderer to interpret the hints in -order to produce a fitted outline.
+ -
-


The following table summarises the pros and cons of each scheme -:

- + - +

- - +
+
+
+ grid-fitting scheme +
+ 		+
+ advantages +
+ 		+
+ disadvantages +
+
+
+ explicit +
+ 		+

Quality. Excellent results at small sizes are possible. + This is very important for screen display.

-

NOTE: -
As said before, the pixel_size computed in  the above formula -does not relate directly to the size of characters on the screen. It simply -is the size of the EM square if it was to be displayed directly. Each font -designer is free to place its glyphs as it pleases him within the square. -This explains why the letters of the following text have not the same height, -even though they're displayed at the same point size with distinct fonts -: -

-

+

Consistency. All renderers produce the same glyph + bitmaps.

+ 		+

Speed. Intepreting bytecode can be slow if the glyph + programs are complex.

+

Size. Glyph programs can be long.

+

Technicity. + It is extremely difficult to write good hinting + programs. Very few tools available.

+
+
+ implicit +
+ 		+

Size. Hints are usually much smaller than explicit glyph + programs.

-

The outline as stored in a font file is called the "master" -outline, as its points coordinates are expressed in font units. Before -it can be converted into a bitmap, it must be scaled to a given -size/resolution. This is done through a very simple transform, but always -creates undesirable artifacts, e.g. stems of different widths or heights -in letters like "E" or "H". -

+

Speed. + Grid-fitting is usually a fast process.

+ 		+

Quality. Often questionable at small sizes. Better with + anti-aliasing though.

-

There are several ways to perform grid-fitting properly, for example -most scalable formats associate some control data or programs with each -glyph outline. Here is an overview :

+

Inconsistency. Results can vary between different + renderers, or even distinct versions of the same engine.

+
+
+ automatic +
+ 		+

Size. No need for control information, resulting in + smaller font files.

-


automatic grid-fitting : -

Some formats simply include no control information with each -glyph outline, apart metrics like the advance width and height. It's then -up to the renderer to "guess" the more interesting features of the outline -in order to perform some decent grid-fitting.
- - +

Speed. Depends on the grid-fitting algorithm. Usually + faster than explicit grid-fitting.

+ 		+

Quality. Often questionable at small sizes. Better with + anti-aliasing though.

-
- - +

Speed. Depends on the grid-fitting algorithm.

- +

Inconsistency. Results can vary between different + renderers, or even distinct versions of the same engine.

+ + +
-
-
Grid-fitting scheme
-
-		 -
-
Pros
-
-
+
-		 -
-
Cons
-
-
-
-
Explicit
-
-
+ + + + + +
+ Previous + + Contents + + Next +
+
-
 -
-
Quality -
excellence at small sizes is possible. This is -very important for screen display. -

Consistency -
all renderers produce the same glyph bitmaps.

-
-
+ -
 -
-
Speed -
intepreting bytecode can be slow if the glyph -programs are complex. -

Size -
glyph programs can be long -

Technicity -
it is extremely difficult to write good hinting -programs. Very few tools available.

-
-
-
-
Implicit
-
-		 -
-
Size -
hints are usually much smaller than explicit -glyph programs. -

Speed -
grid-fitting is usually a fast process

-
-		 -
-
Quality -
often questionable at small sizes. Better with -anti-aliasing though. -

Inconsistency -
results can vary between different renderers, -or even distinct versions of the same engine.

-
-
-
-
Automatic
-
-		 -
-
Size -
no need for control information, resulting in -smaller font files. -

Speed -
depends on the grid-fitting algo.Usually faster -than explicit grid-fitting.

-
-		 -
-
Quality -
often questionable at small sizes. Better with -anti-aliasing though -

Speed -
depends on the grid-fitting algo. -

Inconsistency -
results can vary between different renderers, -or even distinct versions of the same engine.

-
-
- - -
- - - -
-Previous - -Contents - -Next -
- -