An introduction to glyphs
Description: Glyphs in the FreeType engine
Keywords: FreeType TrueType library engine glyphs baseline bearing bounding box

http://www.freetype.org

Glyph Hell

An introduction to glyphs, as used and defined in the FreeType engine

Introduction

This article discusses in great detail the definition of glyph metrics, per se the TrueType specification, and the way they are managed and used by the FreeType engine. This information is crucial when it comes to rendering text strings, either in a conventional (i.e. Roman) layout, or with vertical or right-to-left ones. Some aspects like glyph rotation and transformation are explained too.

Comments and corrections are highly welcome, and can be sent to the FreeType developers list.

I. An overview of font files

In TrueType, a single font file is used to contain information related to classification, modeling and rendering of text using a given typeface. This data is located in various independent `tables', which can be sorted in four simple classes, as described below:

- Face Data

We call face data the amount of information related to a given typeface, independently of any particular scaling, transformation, and/or glyph index. This usually means some typeface-global metrics and attributes, like family and styles, PANOSE number, typographic ascenders and descenders, as well as some very TrueType specific items like the font `programs' found in the fpgm and prep tables, the gasp table, character mappings, etc.

In FreeType, a face object is used to model a font file's face data.

- Instance Data

  We call instance a given pointsize/transformation, at a given device resolution (e.g. 8pt at 96x96dpi, or 12pt at 300x600dpi, etc). Some tables found in the font files are used to produce instance-specific data, like the cvt table, or the prep program. Though they are often part of the face data, their processing results in information called instance data.

  In FreeType, it is modeled through an instance object, which is always created from an existing face object.

- Glyph Data

  We call glyph data the piece of information related to specific glyphs. This includes the following things that are described in more details in the next sections:

  - The glyph's vectorial representation, also called its outline.

  - Various metrics, like the glyph's bounding box, its bearings and advance values.

  - TrueType specifies a specific instruction bytecode, used to associate each glyph with a small program, called the glyph code. Its purpose is to grid-fit the outline to any target instance, in order to produce excellent output at small pixel sizes.

  The FreeType engine doesn't map each glyph to a single structure, as this would waste memory for no good reason. Rather, a glyph object is a container, created from any active face, which can be used to load and/or process any font glyph at any instance (or even no instance at all). Of course, the glyph properties (outline, metrics, bitmaps, etc.) can be extracted independently from an object once it has been loaded or processed.

- Text and Layout Data

  Finally, there is a last class of data that doesn't really fit in all others, and that can be called text data. It comprises information related to the grouping of glyphs together to form text. Simple examples are the kerning table, which controls the spacing between adjacent glyphs, as well as some of the extensions introduced in TrueType Open, OpenType, and TrueType GX like glyph substitution (ligatures, vertical representations), baseline management, justification, etc.

  This article focuses on the basic TrueType tables, and hence, will only talk about kerning, as FreeType doesn't support OpenType nor GX (yet). [Support for TrueType Open is already partially available.]

II. Glyph Outlines

TrueType is a scalable font format; it is thus possible to render glyphs at any scale, and under any affine transform, from a single source representation. However, simply scaling vectorial shapes exhibits at small sizes (where `small' refers here to anything smaller than at least 150 pixels) a collection of un-harmonious artifacts, like widths and/or heights degradations.

Because of this, the format also provides a complete programming language used to design small programs associated to each glyph. Its role is to align the point positions on the pixel grid after the scaling. This operation is hence called grid-fitting, or even hinting.

1. 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 and/or second-order beziers (also called conic beziers or quadratics).

It is described internally as a series of successive points, with each point having an associated flag indicating whether it is `on' or `off' the curve. These rules are applied to decompose the contour:

- Two successive `on' points indicate a line segment joining them.

- One `off' point amidst two `on' points indicates a conic bezier, the `off' point being the control point, and the `on' ones the start and end points.

- Finally, two successive `off' points forces the rasterizer to create (only during bitmap rendering) a virtual `on' point amidst them, at their exact middle. This greatly facilitates the definition of successive Bezier arcs.

                                      *              # on
                                                     * off
                                   __---__
      #-__                      _--       -_
          --__                _-            -
              --__           #               \
                  --__                        #
                      -#
                               Two `on' points
       Two `on' points       and one `off' point
                                between them

                    *
      #            __      Two `on' points with two `off'
       \          -  -     points between them.  The point
        \        /    \    marked `0' is the middle of the
         -      0      \   `off' points, and is a `virtual
          -_  _-       #   on' point where the curve passes.
            --             It does not appear in the point
                           list.
            *

Each glyph's original outline points are located on a grid of indivisible units. The points are stored in the font file as 16-bit integer grid coordinates, with the grid origin's being at (0,0); they thus range from -16384 to 16383.

In creating the glyph outlines, a type designer uses an imaginary square called the EM square. Typically, the EM square encloses the capital letter `M' and most other letters of a typical roman alphabet. 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 instance. For example, a size of 12pt at 300x300dpi 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_coordinate = grid_coordinate * 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 (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 this. 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.

IMPORTANT NOTE:

Under FreeType, scaled pixel positions are all expressed in the 26.6 fixed float format (made of a 26-bit integer mantissa, and a 6-bit fractional part). In other words, all coordinates are multiplied by 64. The grid lines along the integer pixel positions, are multiples of 64, like (0,0), (64,0), (0,64), (128,128), etc., while the pixel centers lie at middle coordinates (32 modulo 64) like (32,32), (96,32), etc.

2. Hinting and Bitmap rendering

   As said before, simply scaling outlines to a specific instance always creates undesirable artifacts, like stems of different widths or heights in letters like `E' or `H'. Proper glyph rendering needs that the scaled points are aligned along the pixel grid (hence the name grid-fitting), and that important widths and heights are respected throughout the whole font (for example, it is very often desirable that the letters `I' and `T' have their central vertical line of the same pixel width).

   Type 1 PostScript font files include with each glyph a small series of distances called hints, which are later used by the type manager to try grid-fitting the outlines as cleverly as possible. On one hand, it has the consequence that upgrading your font engine can enhance the visual aspects of all fonts of your system; on the other hand, the quality of even the best version of Adobe's Type Manager isn't always very pleasing at small sizes (notwithstanding font smoothing).

   TrueType takes a radically different approach: Each glyph has an associated `program', designed in a specific geometrical language, which is used to align explicitly each outline point to the pixel grid, preserving important distances and metrics. A stack-based low-level bytecode is used to store it in the font file, and is interpreted later when rendering the scaled glyphs.

   This means that even very complex glyphs can be rendered perfectly at very small sizes, as long as the corresponding glyph code is designed correctly. Moreover, a glyph can loose some of its details, like serifs, at small sizes to become more readable, because the bytecode provides interesting features.

   However, this also have the sad implication that an ill-designed glyph code will always render junk, whatever the font engine's version, and that it's very difficult to produce quality glyph code. There are about 200 TrueType opcodes, and no known `high-level language' for it. Most type artists aren't programmers at all and the only tools able to produce quality code from vectorial representation have been distributed to only a few font foundries, while tools available to the public, e.g. Fontographer, are usually expensive though generating average to mediocre glyph code.

   All this explains why an enormous number of broken or ugly `free' fonts have appeared on the TrueType scene, and that this format is now mistakenly thought as `crap' by many people. Funnily, these are often the same who stare at the `beauty' of the classic `Times New Roman' and `Arial/Helvetica' at 8 points.

   Once a glyph's code has been executed, the scan-line converter converts the fitted outline into a bitmap (or a pixmap with font-smoothing).

III. Glyph metrics

1. Baseline, Pens and Layouts

The baseline is an imaginary line that is used to `guide' glyphs when rendering text. It can be horizontal (e.g. Roman, Cyrillic, Arabic, etc.) or vertical (e.g. Chinese, Japanese, etc). Moreover, to render text, a virtual point, located on the baseline, called the pen position, is used to locate glyphs.

Each layout uses a different convention for glyph placement:

- With horizontal layout, glyphs simply `rest' on the baseline. Text is rendered by incrementing the pen position, either to the right or to the left.

  The distance between two successive pen positions is glyph-specific and is called the advance width. Note that its value is always positive, even for right-to-left oriented alphabets, like Arabic. This introduces some differences in the way text is rendered.

  IMPORTANT NOTE:

  The pen position is always placed on the baseline in TrueType, unlike the convention used by some graphics systems, like Windows, to always put the pen above the line, at the ascender's position.

- With vertical layout, glyphs are centered around the baseline:

2. Typographic metrics and bounding boxes

   A various number of face metrics are defined for all glyphs in a given font. Three of them have a rather curious status in the TrueType specification; they only apply to horizontal layouts:

   - The ascent

     This is the distance from the baseline to the highest/upper grid coordinate used to place an outline point. It is a positive value, due to the grid's orientation with the y axis upwards.

   - The descent

     The distance from the baseline to the lowest grid coordinate used to place an outline point. This is a negative value, due to the grid's orientation.

   - The linegap

     The distance that must be placed between two lines of text. The baseline-to-baseline distance should be computed as

     ascent - descent + linegap

     if you use the typographic values.

   The problem with these metrics is that they appear three times in a single font file, each version having a slightly different meaning:

   1. The font's horizontal header provides the ascent, descent and linegap fields, which are used to express the designer's intents, rather than the real values that may be computed from all glyphs in the outline. These are used by the Macintosh font engine to perform font mapping (i.e. font substitution).

   2. The OS/2 table provides the usWinAscent and usWinDescent fields. These values are computed for glyphs of the Windows ANSI charset only, which means that they are wrong for any other glyph. Note that usWinDescent is always positive (i.e. looks like `-descent').

   3. The OS/2 table provides the typoAscender, typoDescender and typoLinegap values, which hopefully concern the whole font file. These are the correct system-independent values!

   All metrics are expressed in font units. If you want to use any of the two first versions of these metrics, the TrueType specification contains some considerations and computing tips that might help you.

   Other, simpler metrics are:

   - The glyph's bounding box, also called bbox

     This is an imaginary box that encloses any glyph (usually as tightly as possible). It is represented by four fields, namely xMin, yMin, xMax, and yMax, that can be computed for any outline. In FreeType, their values can be in font units (if measured in the original outline) or in 26.6 pixel units (if measured on scaled outlines).

     Note that if it wasn't for grid-fitting, you wouldn't need to know a box's complete values, but only its dimensions to know how big is a glyph outline/bitmapa. However, correct rendering of hinted glyphs needs the preservation of important grid alignment on each glyph translation/placement on the baseline, which is why FreeType always returns the complete glyph outline.

     Note also that the font's header contains a global font bounding box in font units which should enclose all glyphs in a font. This can be used to pre-compute the maximum dimensions of any glyph at a given instance.

   - The internal leading

     This concept comes directly from the world of traditional typography. It represents the amount of space within the `leading' which is reserved for glyph features that lay outside of the EM square (like accentuation). It usually can be computed as

     internal_leading = ascent - descent - EM_size

   - The external leading

     This is another name for the linegap.

3. Bearings and Advances

   Each glyph has also distances called bearings and advances. Their definition is constant, but their values depend on the layout, as the same glyph can be used to render text either horizontally or vertically.

   1. The left side bearing: a.k.a. bearingX

      This is the horizontal distance from the current pen position to the glyph's left bounding box edge. It is positive for horizontal layouts, and most generally negative for vertical one.

   2. The top side bearing: a.k.a. bearingY

      This is the vertical distance from the baseline to the top of the glyph's bounding box. It is usually positive for horizontal layouts, and negative for vertical ones

   3. The advance width: a.k.a. advanceX

      This is the horizontal distance the pen position must be incremented (for left-to-right writing) or decremented (for right-to-left writing) by after each glyph is rendered when processing text. It is always positive for horizontal layouts, and null for vertical ones.

   4. The advance height: a.k.a. advanceY

      This is the vertical distance the pen position must be decremented by after each glyph is rendered. It is always null for horizontal layouts, and positive for vertical layouts.

   5. The glyph width

      The glyph's horizontal extent. More simply, it is (bbox.xMax - bbox.xMin) for unscaled font coordinates. For scaled glyphs, its computation requests specific care, described in the grid-fitting chapter below.

   6. The glyph height

      The glyph's vertical extent. More simply, it is (bbox.yMax - bbox.yMin) for unscaled font coordinates. For scaled glyphs, its computation requests specific care, described in the grid-fitting chapter below.

   7. The right side bearing

      Only used for horizontal layouts to describe the distance from the bbox's right edge to the advance width. It is in most cases a non-negative number. The FreeType library doesn't provide this metric directly, as it isn't really part of the TrueType specification. It can be computed simply as

      advance_width - left_side_bearing - (xMax-xMin)

   Finally, if you use `ABC widths' under Windows and OS/2, the following relations apply:

         A = left side bearing
         B = width
         C = right side bearing

         A+B+C = advance width

4. The effects of grid-fitting

   All these metrics are stored in font units in the font file. They must be scaled and grid-fitted properly to be used at a specific instance. This implies several things:

   - First, a glyph program not only aligns the outline along the grid pixel, it also processes the left side bearing and the advance width. Other grid-fitted metrics are usually available in optional TrueType tables if you need them.

   - A glyph program may decide to extend or stretch any of these two metrics if it has a need for it. This means that you cannot assume that the fitted metrics are simply equal to the scaled one plus or minus a liberal distance < 1 pixel (i.e., less than 64 fractional pixel units). For example, it is often necessary to stretch the letter `m' horizontally at small pixel sizes to make all vertical stems visible, while the same glyph can be perfectly `square' at larger sizes.

   - Querying the fitted metrics of all glyphs at a given instance is very slow, as it needs to load and process each glyph independently. For this reason, some optional TrueType tables are defined in the specification, containing pre-computed metrics for specific instances (the most commonly used, like 8, 9, 10, 11, 12, and 14 points at 96dpi, for example). These tables aren't always present in a TrueType font.

     If you don't need the exact fitted value, it's much faster to query the metrics in font units, then scale them to the instance's dimensions.

   IMPORTANT NOTE:

   Another very important consequence of grid-fitting is the fact that moving a fitted outline by a non-integer pixel distance will simply ruin the hinter's work, as alignments won't be preserved. The translated glyph will then look `ugly' when converted to a bitmap!

   In other words, each time you want to translate a fitted glyph outline, you must take care of only using integer pixel distances (the x and y offsets must be multiples of 64, which equals to 1.0 in the 26.6 fixed float format). If you don't care about grid-fitting (typically when rendering rotated text), you can use any offset you want and use sub-pixel glyph placement.

IV. Text processing

This section demonstrates how to use the concepts previously defined to render text, whatever the layout you use.

1. Writing simple text strings

We will start by generating a simple string with a Roman alphabet. The layout is thus horizontal, left to right.

For now, we will assume all glyphs are rendered in a single target bitmap. The case of generating individual glyph bitmaps, then placing them on demand on a device is presented in a later chapter of this section.

Rendering the string needs to place each glyph on the baseline; this process looks like the following:

1. Place the pen to the cursor position. The pen is always located on the baseline. Its coordinates must be grid-fitted (i.e. multiples of 64)!

         pen_x = cursor_x;
         pen_y = cursor_y;

2. Load the glyph outline and its metrics. Using the flag TTLOAD_DEFAULT will scale and hint the glyph:

         TT_Load_Glyph( instance,
                        glyph,
                        glyph_index,
                        TTLOAD_DEFAULT );

         TT_Get_Glyph_Metrics( glyph, &metrics );
         TT_Get_Glyph_Outline( glyph, &outline );

3. The loader always places the glyph outline relative to the imaginary pen position (0,0). You thus simply need to translate the outline by the vector:

         ( pen_x, pen_y )

   To place it on its correct position, you can use the call

         TT_Translate_Outline( outline, pen_x, pen_y );

4. Render the outline in the target bitmap, the glyph will be surimposed on it with a binary `or' operation (FreeType never creates glyph bitmaps by itself, it simply renders glyphs in the arrays you pass to it. See the API reference for a complete description of bitmaps and pixmaps).

         TT_Get_Outline_Bitmap( outline, &target_bitmap );

   IMPORTANT NOTE:

   If you don't want to access the outline in your code, you can also use the API function TT_Get_Glyph_Bitmap() which does the same as the previous lines:

         TT_Get_Glyph_Outline( glyph, &outline );
         TT_Translate_Outline( outline, x_offset, y_offset );
         TT_Get_Outline_Bitmap( outline, &target_bitmap );
         TT_Translate_Outline( outline, -x_offset, -y_offset );

   is equivalent to:

         TT_Get_Glyph_Bitmap( glyph,
                              x_offset,
                              y_offset,
                              &target_bitmap );

5. Now advance the pen to its next position. The advance is always grid-fitted when the glyph was hinted:

         pen_x += metrics.advance;

   The advance being grid-fitted, the pen position remains aligned on the grid.

6. Start over on item 2 until string completion. That's it!

2. Writing right-to-left and vertical text

   Generating strings for different layouts is very similar. Here are the most important differences.

   - For right-to-left text (like Arabic)

     The main difference here is that, as the advance width and left side bearings are oriented against the flow of text, the pen position must be decremented by the advance width, before placing and rendering the glyph. Other than that, the rest is strictly similar.

   - For vertical text (like Chinese or Japanese)

     In this case, the baseline is vertical, which means that the pen position must be shifted in the vertical direction. You need the vertical glyph metrics to do that (using the TT_Get_Big_Glyph_Metrics() function).

     Once you get these, the rest of the process is very similar. The glyph outline is placed relative to an imaginary origin of (0,0), and you should translate it to the pen position before rendering it.

     The big difference is that you must decrement pen_y, rather than increment pen_x (this is for the TrueType convention of y oriented upwards).

           pen_y -= metrics.advance;

3. Generating individual glyph bitmaps and using them to render text

   Loading each glyph when rendering text is slow, and it's much more efficient to render each one in a standalone bitmap to place it in a cache. Text can then be rendered fast by applying simple blit operations on the target device.

   To be able to render text correctly with the bitmaps, you must record and associate with them its fitted bearings and advances. Hence the following process:

   1. Generate the bitmaps.

      - Load the glyph and get its metrics.

              TT_Load_Glyph( instance,
                             glyph,
                             glyph_index,
                             TTLOAD_DEFAULT );

              TT_Get_Glyph_Metrics( glyph, &metrics );

        The bbox is always fitted when calling TT_Get_Glyph_Metrics() on a hinted glyph. You can then easily compute the glyph's dimension in pixels as:

              width  = (bbox.xMax - bbox.xMin) / 64;
              height = (bbox.yMax - bbox.yMin) / 64;

        NOTE 1:
        The fitted bounding box always contains all the dropouts that may be produced by the scan-line converter. This width and height are thus valid for all kinds of glyphs).

        NOTE 2:
        If you want to compute the dimensions of a rotated outline's bitmap, compute its bounding box with TT_Get_Outline_BBox(), then grid-fit the bbox manually:

              #define  FLOOR(x)    ((x) & -64)
              #define  CEILING(x)  (((x)+63) & -64)

              xMin = FLOOR(xMin);
              yMin = FLOOR(yMin);
              yMin = CEILING(xMax);
              yMax = CEILING(yMax);

        then compute width and height as above.

      - Create a bitmap of the given dimension, e.g.:

              bitmap.width  = width;
              bitmap.cols   = (width+7) & -8;
              bitmap.rows   = height;
              bitmap.flow   = TT_Flow_Up;
              bitmap.size   = bitmap.cols * bitmap.rows;
              bitmap.buffer = malloc( bitmap.size );

      - Render the glyph into the bitmap.

        Don't forget to shift it by (-xMin, -yMin) to fit it in the bitmap:

              /* Note that the offsets must be grid-fitted to */
              /* preserve hinting!                            */
              TT_Get_Glyph_Bitmap( glyph,
                                   &bitmap,
                                   -bbox.xMin,
                                   -bbox.yMin );

   2. Store the bitmap with the following values:

            bearingX / 64 = left side bearing in pixels
            advance / 64  = advance width/height in pixels

      When your cache is set up, you can then render text using a scheme similar to the ones describe in 1. and 2., with the exception that now pen positions and metrics are expressed in pixel values. We are done!

            pen_x = cursor_x;
            pen_y = cursor_y;

            while ( glyph_to_render )
            {
              access_cache( glyph_index, metrics, bitmap );

              blit_bitmap_to_position
               ( pen_x + bearingX,
                 pen_y (+ bearingY depending on orientation ) );

              pen_x += advance;
            }

4. Device-independent text rendering

   The previously described rendering processes all align glyphs on the baseline according to metrics fitted for the display's distance. In some cases, the display isn't the final output, and placing the glyphs in a device-independent way is more important than anything.

   A typical case is a word processor which displays text as it should appear on paper when printed. As you've probably noticed, the glyphs aren't always spaced uniformly on the screen as you type them, sometimes the space between an `m' and a `t' is too small, some other it is too large, etc.

   These differences are simply due to the fact that the word processor aligns glyphs in an device-independent way, using original metrics in font units to do it, then scale them as it can to display text on screen, usually at a very smaller resolution than your printer's one.

   Device-independence is a crucial part of document portability, and it is very saddening to see that most professional word processors don't do it correctly. For example, MS Word uses the fitted metrics of the printer's resolution, rather than the originals in font units.

   This is great to get sure that your text prints very well on your printer, but it also implies that someone printing the exact same document on a device with different output resolutions (e.g. bubble-jet vs. laser printers) may encounter trouble.

   As the differences in advances accumulate on one line, they can sum to the width of one or more glyphs in extreme cases, which is enough to `overflow' the automatic justification algorithm. This may add additional lines of printed text, or even remove some. Moreover, supplemental lines can produce unexpected page breaks and `blank' pages. This can be extremely painful when working with large documents, as this `feature' may require you to redesign completely your formatting to re-print it.

   In conclusion, if you want portable document rendering, never hesitate to use and apply device-independent terms! For example, a simple way to produce text would be:

   1. Get a scale to convert from your device-independent units to 26.6 pixels.

   2. Get another scale to convert from original font units to device-independent units.

   3. Perform pen placement and advances in device-independent units.

   4. To render each glyph, compute the pen's rounded position, as well as the rounded glyph left side bearing, both expressed in 26.6 pixels (don't use the fitted metrics). You will then be able to place the glyph and/or blit its bitmap.

5. Kerning glyphs

   An interesting effect that most people appreciate is kerning. It consists of modifying the spacing between two successive glyphs according to their outlines. For example, the letters `T' and a `y' can be easily moved closer, as the top of the `y' fits nicely under the `T's upper right bar.

   To perform kerning, the TrueType specification provides a specific table (its tag being `kern'), with several storage formats. This section doesn't explain how to access this information; however, you can have a look at the standard extension called `ttkern.h' which comes with FreeType.

   The kerning distance between two glyphs is a value expressed in font units which indicates whether their outline can be moved together or apart when one follows the other. The distance isn't reflexive, which means that the kerning for the glyph pair (`T',`y') isn't the same as the one for (`y',`T').

   The value is positive when the glyphs must be moved apart, and negative when they must be moved closer. You can implement kerning simply by adding its scaled and rounded value to the advance width when moving the pen position. Here an example for horizontal kerning:

         #define ROUND( x )  ( (x + 32) & -64 )

         scaled_kerning = kerning * imetrics.x_scale / 0x10000;

         pen_x += metrics.advance + ROUND( scaled_kerning );

6. Rotated and stretched/slanted text

   In order to produce rotated glyphs with FreeType, one must understand a few things:

   - The engine doesn't apply specific transformations to the glyphs it loads and processes (other than the simpler resolution-base scaling and grid-fitting). If you want to rotate glyphs, you will have to load their outline, then apply the geometric transformations that please you (a number of APIs are there to help you to do it easily).

   - Even if the glyph loader hints `straight' glyphs, it is possible to inform the font and glyph programs that you're going to later transform the resulting outlines. Two flags can be passed to the bytecode interpreter:

     - The `rotated' flag indicates that you are going to rotate the glyphs in a non-trivial direction (i.e., on neither of the two coordinate axis). You are advised not to set it when writing 90 degrees-rotated text for example.

     - The `stretched' flag indicates that you are going to apply a transformation that will distort distances. While rotations and symmetries keep distances constant, slanting and stretching do modify them.

   These flags can be interpreted by the glyph code to toggle certain processings which vary from one font to the other. However, most of the TrueType fonts that were tested with FreeType, if not all of them, simply change the dropout-mode when any of these flags is set, and/or disable hinting when rotation is detected. We advise you to never set these flags, even when rotating text. For what it's worth, hinted rotated text is no uglier than un-hinted one.

   You can use the function TT_Set_Instance_Transform_Flags() to set them. Then, rendering can be done with the following calls:

         /* set the flags */
         TT_Set_Instance_Transforms( instance,
                                     rotated,
                                     stretched );

         /* load a given glyph */
         TT_Get_Glyph_Outline( instance,
                               glyph,
                               index,
                               TTLOAD_DEFAULT );

         /* access its outline */
         TT_Get_Glyph_Outline( instance, &outline );

         /* in order to transform it */
         TT_Transform_Outline( outline, &matrix );
         /* and/or */
         TT_Translate_Outline( outline,
                               x_offset, y_offset );

         /* to render it */
         TT_Get_Outline_Bitmap( outline, &bitmap );

   Here is an example, assuming that the following variables

         TT_Matrix  matrix;        /* 2x2 matrix */
         TT_Pos     x_off, y_off;  /* corrective offsets */

   define a transformation that can be correctly applied to a glyph outline which have been previously placed relative to the imaginary point position (0,0) with bearings preserved. Rendering text can now be done as follows:

   1. Initialize the pen position; when rotating, it is extremely well advised to use sub-pixel placement as you don't care about hinting.

            pen_x = cursor_x;
            pen_y = cursor_y;

   2. Transform the glyph as needed, then translate it to the current pen position:

            TT_Transform_Outline( outline, &matrix );
            TT_Translate_Outline( outline,
                                  pen_x + x_off,
                                  pen_y + y_off );

      (Note that the transformation offsets have been included in the translation.)

   3. Render the bitmap, as it has now been placed correctly.

   4. To change the pen position, transform the vector (0,advance) with your matrix, and add it:

            vec_x = metrics.advance;
            vec_y = 0;
            TT_Transform_Vector( &vec_x, &vec_y, &matrix );
            pen_x += vec_x;
            pen_y += vec_y;

   5. Start over at 2. until completion.

   IMPORTANT NOTE:

   Do not grid-fit the pen position before rendering your glyph when rendering rotated text. If you do, your transformed baseline won't be preserved on each glyph, and the text will look like it's `hopping' randomly. This is particularly visible at small sizes.

   Sub-pixel precision placement is very important for clean rotated text.

7. Font-smoothing, a.k.a. gray-levels rendering

   The FreeType engine's scan-line converter (the component also called the rasterizer) is able to convert a vectorial glyph outline into either a normal bitmap, or an 8-bit pixmap (a.k.a. colored bitmaps on some systems). This last feature is called gray-level rendering or font-smoothing, because it uses a user-supplied palette to produce anti-aliased versions of the glyphs.

   Its principle is to render a bitmap which is twice as large than the target pixmap, then simply filtering it using a 2x2 summation.

   NOTE:

   FreeType's scan-line converter doesn't use or need an intermediate second bitmap. Rather, filtering is performed in a single pass during the sweep (see the file `raster.txt' for more information about it).

   You'll notice that, as with Windows 95, FreeType's rasterizer only grays those parts of the glyph which need it, i.e., diagonals and curves, while keeping horizontal and vertical stems straight `black'. This greatly improves the legibility of text, while avoiding the `blurry' look anti-aliased fonts typically found with Adobe's Type Manager or Acrobat.

   There are thus five available gray-levels, ranging from 0 to 4, where level 0 and level 4 are the background and foreground colors, respectively, and where levels 1, 2, 3 are intermediate. For example, to render black text on a white background, one can use a palette like:

   palette[0] = white (background)
   palette[1] = light gray
   palette[2] = medium gray
   palette[3] = dark gray
   palette[4] = black (foreground)

   To set the engine's gray-level palette, simply use the API function TT_Set_Raster_Palette() after initialization. It expects an array of 5 chars which will be used to render the pixmaps.

   Note that the rasterizer doesn't create bitmaps or pixmaps. Rather, it simply renders glyphs in the arrays you pass to it. The generated glyph bitmaps are simply `or'-ed to the target (with 0 being the background as a convention); in the case of pixmaps, pixels are simply written to the buffer, in spans of four aligned bytes.

   NOTE:

   The raster isn't able to superpose `transparent' glyphs on the target pixmap. This means that you should always call the API functions TT_Get_Glyph_Pixmap() and TT_Get_Outline_Pixmap() with an empty map, and perform the superposition yourself.

   This can be more or less tricky, depending on the palette you are using and your target graphics resolution. One of the components found in the test directory, called `display.c', has large comments on the way it implements it for the test programs. You are encouraged to read the test program sources to understand how one can take advantage of font smoothing.

   Pixmap surimposition is too system-specific a feature to be part of the FreeType engine. Moreover, not everybody needs it!

   Finally, the question of sur-imposing anti-aliased colored text on any texture, since being even more tricky, is left as an exercise to the reader ;-) If this topic really interests you, the FreeType mailing list may host some helpful enthusiasts ready to answer your questions. Who knows :-)

8. Other interesting text processes

   - Glyph substitution

     Substitution is used to replace one glyph by another when some specific condition is met in the text string. Its most common examples are ligatures (like replacing the `f' followed by `i' by the single glyph `fi' if available in the font), as well as positional selection as performed in the Arabic script (for those not aware of this, each letter of the Arabic alphabet can be written differently according to its position on words: starting, ending, intermediate, or isolated).

     The base TrueType format doesn't define any table for glyph substitution. However, GX, TrueType Open, and OpenType provide (incompatible) extensions to perform it. Of course, it isn't supported by the engine, but an extension could be easily written to access the required tables.

     [Support for TrueType Open is already partially available.]

   - Justification

     ...

To be continued...