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).
+
+ 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:
+
+ 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.
+
+ 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).
-
+
+
+
+ |
+
+ grid-fitting scheme
+
+ |
+
+
+ advantages
+
+ |
+
+
+ disadvantages
+
+ |
+
-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.
+
+ |
+
+ explicit
+
+ |
-
-Note : Grid units are very often called "font units" or "EM units".
+
+ 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.
+ |
-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.
-
+
+ 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.
+ |
-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.
-
+
+ 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.
+ |
+
-
-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.
+
+ |
+
+ automatic
+
+ |
-
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.
+
+ 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.
+ |
-
-
The following table summarises the pros and cons of each scheme
-:
-
+
+ Quality. Often questionable at small sizes. Better with
+ anti-aliasing though.
-
-
-
-
-Grid-fitting scheme
-
- |
+ Speed. Depends on the grid-fitting algorithm.
-
-
-Pros
-
- |
+ Inconsistency. Results can vary between different
+ renderers, or even distinct versions of the same engine.
+
+
+
+
- |
-
-Cons
-
- |
-
+
-
-
-
-Explicit
-
- |
+
+
+
-
-
-Quality
-
excellence at small sizes is possible. This is
-very important for screen display.
-Consistency
-
all renderers produce the same glyph bitmaps.
-
- |
+
+
+
-