From 1bd8a62ef441b5465282b6a305b6b376de596f1c 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 | 619 +++++++++++++++++++++++--------------- 2 files changed, 372 insertions(+), 251 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 +

+ +

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

-

-version 2.1

+
+ +
-
-

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

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

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

+ III. Glyph metrics +

+
-

-III. Glyph metrics -

+ +

+ 1. Baseline, pens and layouts +

-

-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.

-

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:

-

Each layout uses a different convention for glyph placement:

- +
    +
  • +

    Left side bearing or bearingX

    -

-2. Typographic metrics and bounding boxes -

+

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.

+ -

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

+
  • +

    Top side bearing or bearingY

    -

    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 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

    +
    • -


    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. -

    +
  • +

    Advance width or advanceX

    -


    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.
    - +

    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.

    +
    • -

    -3. Bearings and Advances

    +
  • +

    Advance height advanceY

    -
      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: -

      the left side bearing: a.k.a. bearingX -

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

      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.

      + -


      the top side bearing: a.k.a. bearingY -

        this is 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
      +
    • +

      Glyph width

      -


      the advance width: a.k.a. advanceX -

        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.
      +

      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.

      +
      • -


      the advance height: a.k.a. advanceY -

        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.
      +
    • +

      Glyph height -


      the glyph width -

        this is simply 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.
      +

      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.

      +
      • -


      the glyph height -

        this is simply 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.
      +
    • +

      Right side bearing

      -


      the right side bearing -

        is 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.
      +

      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)
      +

      + advance_width - left_side_bearing - (xMax-xMin) +

      +
      • +
    -

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

    -

    +

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

    And here is another one for the vertical metrics : -

    -

    - +

    + horizontal glyph metrics +

    -

    -4. The effects of grid-fitting

    +

    And here is another one for the vertical metrics:

    -     +
      +
    • + 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. +
      • +
    -

    - 5. Text widths and bounding box :

    +

    This has some implications:

    -
      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.
          • -
        +
          +
        • + 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. +
          • -
            -
          • -The string's advance width isn't related to its bounding box's dimensions. -Especially if it contains beginning and terminal spaces or tabs.
            • -
          +
        • + 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. +
          • +
        -
          -
        • -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 -
    + +

    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. +
      • +
    + +

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