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http://www.freetype.org/freetype2/docs/glyphs/glyphs-3.html

II. 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.
  
• 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.

• 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 ones.
  
• 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:
And here is another one for the vertical 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 keep 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.

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_Outline_Translate() 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.