From fed760b1bf633c3c3d97c704a0956afddd49d890 Mon Sep 17 00:00:00 2001 From: Werner Lemberg Date: Thu, 9 Nov 2000 16:23:23 +0000 Subject: [PATCH] Revised. --- docs/glyphs/glyphs-2.html | 4 +- docs/glyphs/glyphs-3.html | 705 ++++++++++++++++++++++---------------- 2 files changed, 415 insertions(+), 294 deletions(-) diff --git a/docs/glyphs/glyphs-2.html b/docs/glyphs/glyphs-2.html index 70ece6487..06e9d98e1 100644 --- a/docs/glyphs/glyphs-2.html +++ b/docs/glyphs/glyphs-2.html @@ -62,7 +62,7 @@

- II. Glyph Outlines + II. Glyph mutlines

@@ -363,7 +363,7 @@ -

+

+ - - - - 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 -
- -

-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, Korean, etc). Moreover, to render -text, a virtual point, located on the baseline, called the "pen position" -or "origin", 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. -

    - -

    - - -
  • with a 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.

- -

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.
-Other, simpler metrics are: -

the glyph's bounding box, also called "bbox" -

    this is an imaginary box that encloses all glyphs from the font, as -tightly as possible. It is represented by four fields, namely xMin, -yMin, -xMax, -and yMax, that can be computed for any outline. Their values can -be in font units (if measured in the original outline) or in fractional/integer -pixel units (when 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/bitmap. However, correct rendering of hinted glyphs needs the preservation -of important grid alignment on each glyph translation/placement on the -baseline.

-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 line gap.
- - -

-3. Bearings and Advances

- - - -

-4. The effects of grid-fitting

- -
    Because hinting aligns the glyph's control points to the pixel grid, -this process slightly modifies the dimensions of character images in ways -that differ from simple scaling. -

    For example, the image of the lowercase "m" letter sometimes fits a -square in the master grid. However, to make it readable at small pixel -sizes, hinting tends to enlarge its scaled outline in order to keep its -three legs distinctly visible, resulting in a larger character bitmap. -

    The glyph metrics are also influenced by the grid-fitting process. Mainly -because : -
      -

      -
    • -The image's width and height are altered. Even if this is only by one pixel, -it can make a big difference at small pixel sizes
      • - -
    • -The image's bounding box is modified, thus modifying the bearings
      • - -
    • -The advances must be updated. For example, the advance width must be incremented -when the hinted bitmap is larger than the scaled one, to reflect the augmented -glyph width.
      • -
    - -


    Note also that : -
      -

      -
    • -Because of hinting, simply scaling the font ascent or descent might not -give correct results. A simple solution consists in keeping the ceiling -of the scaled ascent, and floor of the scaled descent.
      • -
    - -
      -
    • -There is no easy way to get the hinted glyph and advance widths of a range -of glyphs, as hinting works differently on each outline. The only solution -is to hint each glyph separately and record the returned values. Some formats, -like TrueType, even include a table of pre-computed values for a small -set of common character pixel sizes.
      • -
    - -
      -
    • -Hinting depends on the final character width and height in pixels, which -means that it is highly resolution-dependent. This property makes correct -WYSIWYG layouts difficult to implement.
      • -
    - -


    IMPORTANT NOTE: -
    Performing 2D transforms on glyph outlines is very easy with FreeType. -However, when using translation on a hinted outlines, one should aways -take care of  exclusively using integer pixel distances (which -means that the parameters to the FT_Translate_Outline API should all be -multiples of 64, as the point coordinates are in 26.6 fixed float format). -

    Otherwise, the translation will simply ruin the hinter's work, -resulting in a very low quality bitmaps. -
      -
     

- -

- 5. Text widths and bounding box :

- -
    As seen before, the "origin" of a given glyph corresponds to the position -of the pen on the baseline. It is not necessarily located on one of the -glyph's bounding box corners, unlike many typical bitmapped font formats. -In some cases, the origin can be out of the bounding box, in others, it -can be within it, depending on the shape of the given glyph. -

    Likewise, the glyph's "advance width" is the increment to apply to the -pen position during layout, and is not related to the glyph's "width", -which really is the glyph's bounding width. -
      -

    The same conventions apply to strings of text. This means that : -
      -

      -
        -
      • -The bounding box of a given string of text doesn't necessarily contain -the text cursor, nor is the latter located on one of its corners.
        • -
      - -
        -
      • -The string's advance width isn't related to its bounding box's dimensions. -Especially if it contains beginning and terminal spaces or tabs.
        • -
      - -
        -
      • -Finally, additional processing like kerning creates strings of text whose -dimensions are not directly related to the simple juxtaposition of individual -glyph metrics. For example, the advance width of "VA" isn't the sum of -the advances of "V" and "A" taken separately.
        • -
      -
    -
- - -
- - - -
-Previous - -Contents - -Next -
- -
+ + +
+ +
+ + + + + + +
+ Previous + + Contents + + Next +
+
+ +

+ + + +
+

+ 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, Korean, etc). Moreover, to + render text, a virtual point, located on the baseline, called the pen + position or origin, 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.

    + +

    The pen position is always placed on the baseline.

    + +

    + horizontal layout +

    +
    • +
  • +

    With a vertical layout, glyphs are centered around the + baseline:

    + +

    + vertical layout +

    +
    • +
+ + + +

+ 2. Typographic metrics and bounding boxes +

+ +

A various number of face metrics are defined for all glyphs in a + given font.

+ +
    +
  • +

    Ascent

    + +

    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.

    +
    • + +
  • +

    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.

    +
    • + +
  • +

    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.

    +
    • +
+ +

Other, simpler metrics are:

+ +
    +
  • +

    The glyph's bounding box, also called bbox

    + +

    This is an imaginary box that encloses all glyphs from the font, + usually as tightly as possible. It is represented by four fields, + namely xMin, yMin, xMax, and + yMax, that can be computed for any outline. Their values + can be in font units (if measured in the original outline) or in + fractional/integer pixel units (when 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/bitmap. However, correct rendering of hinted + glyphs needs the preservation of important grid alignment on each + glyph translation/placement on the baseline.

    +
    • + +
  • +

    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 +

    +
    • + +
  • +

    External leading

    + +

    This is another name for the line gap.

    +
    • +
+ + + +

+ 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: + +
    +
  • +

    Left side bearing or bearingX

    + +

    The horizontal distance from the current pen position to the + glyph's left bbox edge. It is positive for horizontal layouts, and + in most cases negative for vertical one.

    +
    • + +
  • +

    Top side bearing or bearingY

    + +

    The vertical distance from the baseline to the top of the glyph's + bbox. It is usually positive for horizontal layouts, and negative + for vertical ones

    +
    • + +
  • +

    Advance width or advanceX

    + +

    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.

    +
    • + +
  • +

    Advance height advanceY

    + +

    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.

    +
    • + +
  • +

    Glyph width

    + +

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

    +
    • + +
  • +

    Glyph height + +

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

    +
    • + +
  • +

    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:

    + +

    + advance_width - left_side_bearing - (xMax-xMin) +

    +
    • +
+ +

Here is a picture giving all the details for horizontal metrics: + +

+ horizontal glyph metrics +

+ +

And here is another one for the vertical metrics:

+ +

+ vertical glyph metrics +

+ + + +

+ 4. The effects of grid-fitting +

+ +

Because hinting aligns the glyph's control points to the pixel grid, + this process slightly modifies the dimensions of character images in + ways that differ from simple scaling.

+ +

For example, the image of the lowercase "m" letter sometimes fits a + square in the master grid. However, to make it readable at small pixel + sizes, hinting tends to enlarge its scaled outline in order to keep its + three legs distinctly visible, resulting in a larger character + bitmap.

+ +

The glyph metrics are also influenced by the grid-fitting process: + +

    +
  • + The image's width and height are altered. Even if this is only by + one pixel, it can make a big difference at small pixel sizes. +
    • +
  • + The image's bounding box is modified, thus modifying the bearings. +
    • +
  • + The advances must be updated. For example, the advance width must + be incremented if the hinted bitmap is larger than the scaled one, + to reflect the augmented glyph width. +
    • +
+ +

This has some implications:

+ +
    +
  • + Because of hinting, simply scaling the font ascent or descent might + not give correct results. A possible solution is to keepthe ceiling + of the scaled ascent, and floor of the scaled descent. +
    • + +
  • + There is no easy way to get the hinted glyph and advance widths of a + range of glyphs, as hinting works differently on each outline. The + only solution is to hint each glyph separately and record the + returned values. Some formats, like TrueType, even include a table + of pre-computed values for a small set of common character pixel + sizes. +
    • +
  • + Hinting depends on the final character width and height in pixels, + which means that it is highly resolution-dependent. This property + makes correct WYSIWYG layouts difficult to implement. +
    • +
+ + + +

Performing 2D transformations on glyph outlines is very easy with + FreeType. However, when using translation on a hinted outlines, one + should aways take care of exclusively using integer pixel + distances (which means that the parameters to the + FT_Translate_Outline() API should all be multiples + of 64, as the point coordinates are in 26.6 fixed float + format).

+ +

Otherwise, the translation will simply ruin the hinter's + work, resulting in a very low quality bitmaps!

+ + + + +

+ 5. Text widths and bounding box +

+ +

As seen before, the "origin" of a given glyph corresponds to the + position of the pen on the baseline. It is not necessarily located on + one of the glyph's bounding box corners, unlike many typical bitmapped + font formats. In some cases, the origin can be out of the bounding box, + in others, it can be within it, depending on the shape of the given + glyph.

+ +

Likewise, the glyph's "advance width" is the increment to apply to + the pen position during layout, and is not related to the glyph's + "width", which really is the glyph's bounding width. + +

The same conventions apply to strings of text. This means that: + +

    +
  • + The bounding box of a given string of text doesn't necessarily + contain the text cursor, nor is the latter located on one of its + corners. +
    • + +
  • + The string's advance width isn't related to its bounding box + dimensions. Especially if it contains beginning and terminal spaces + or tabs. +
    • +
  • + Finally, additional processing like kerning creates strings of text + whose dimensions are not directly related to the simple + juxtaposition of individual glyph metrics. For example, the advance + width of "VA" isn't the sum of the advances of "V" and "A" taken + separately. +
    • +
+ +

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