				 Font Metrics


				  Jim Fulton
			       MIT X Consortium



In X11, text is printed by specifying an "origin" in (x,y) relative to which
characters should be displayed.  The "baseline" of the font (the top-most row
of pixels in any descenders) is laid on top of the y-coordinate and characters
are drawn relative to this line.  Thus, characters that do not have descenders
but which touch the baseline will illuminate pixels in row (y-1), but not in
row y.

For each character in the font, the following information is provided:

    logical width		This is the horizontal distance from the
				origin of this character to next character.
				If the string "ab" where printed with its
				origin at (x,y) and "a" had a logical width of
				10 pixels, "b" would be printed at (x+10,y).
				Well-designed fonts include a certain amount
				of spacing in the logical width so that 
				characters don't bunch up.  This value will
				usually be positive for characters that are
				read from left-to-right, and negative for
				characters that are read right-to-left.

    left bearing		This is the directed, horizontal distance from
				the origin (x,y) at which a character is
				printed to the left edge of the glyph raster
				(which is the where the left-most pixel is
				drawn).  Counting from zero at the origin, this
				is the number of pixels to the left-most pixel.
				Depending on whether this pixel appears to the
				left, on, or to the right of the origin, this
				value will be negative, zero, or positive,
				respectively.

    right bearing		This is the directed, horizontal distance from
				the origin (x,y) at which a character is
				printed to the right edge of the glyph raster
				(which is one to the right of where the right-
				most pixel is drawn).  Counting from zero at
				the origin, this is the number of pixels to
				just beyond the right-most illuminated pixel.
				Depending on whether this pixel appears to the
				left, on, or to the right of the origin, this
				value will be negative, zero, or positive,
				respectively.

    ascent			This is the directed vertical distance from the
				origin (x,y) at which a character is printed to
				the top edge of the top-most pixel that is
				illuminated when this character is drawn.
				Counting from zero at the origin, this is the 
				number of pixels to the top-most pixel.
				Depending on whether the top-most pixel appears
				above, on, or below the origin, this value will
				be positive, zero, or negative, respectively
				(note the assymmetry between this and the other
				metrics).

    descent			This is the directed, vertical distance from
				the origin (x,y) at which a character is
				printed to the bottom edge of the glyph raster
				(which is one below where the bottom-most pixel
				is drawn).  Counting from zero at the origin,
				this is the number of pixels to just below the
				bottom-most pixel.  Depending on whether this
				pixel appears above, on, or below the baseline,
				this value will be negative, zero, or positive.


Fonts also have a font-ascent and a font-descent, defined the same as the
character ascents and descents, that is used for determining inter-line
spacing.  These two values are choosen by the font designer so that lines of
text that are vertically spaced by (font-ascent + font-descent) look
appropriate for the font.  

When text is drawn at a point (x,y), the server paints the first glyph (call
its metrics g, for short) such that:

    left-most pixel		is in column (x + g.lbearing)

    right-most pixel		is in column (x + g.rbearing - 1)

    top-most pixel		is in row (y - g.ascent)

    bottom-most pixel		is in row (y + g.descent - 1)


The origin is then advanced to by the logical width of the first character
(i.e. (x,y) becomes (x + g.width, y)), and the process is repeated for the
next character in the string being printed.

Normally, the only pixels which are touched are those that are set in the
character glyph.  However, applications (such as terminal emulators) that would
like a rectangle of "background color" painted behind each character can use a
special type of text printing, called ImageText.  In this case, the server
fills a rectangle before printing the character such that:

    left side			is in column x

    right side			is in column (x + logical_width - 1)

    top side			is in row (y - font_ascent)

    bottom side			is in row (y + font_descent - 1)


Since the size of the rectangle (logical_width by font_ascent+font_descent)
is determined by the font ascent and descent and not by the character font and
descent, a character printed with ImageText may "stick out" beyond the
rectangle as follows:

    lbearing < 0		some pixels on left will have no background

    rbearing >= width		some pixels on right will have no background

    ascent > font_ascent	some pixels on top will have no background

    descent >= font_descent	some pixels on bottom will have no background

This is particularly common for characters with accents.  Fonts that have 
been specially designed so that all of the characters have the same logical
width and illuminate only those pixels that would appear within the ImageText
background rectangle described above are called Character Cell fonts.  These
are the only fonts that are truly suitable for use with terminal emulators
and other programs that use ImageText.  Fonts in which all of the characters
are of the same width, but which stick out beyond the background rectangle are
called Monospaced fonts.  All other fonts are called Proportional fonts.  For
more information, see the X Logical Font Description Conventions (XLFD) 
document.

The following example will show how the string "T-p" would be printed in the
-adobe-helvetica-bold-r-normal--12-120-75-75-p-70-iso8859-1.  The glyphs
"T", "-", and "p" have the images and character metrics shown below

	########
	...##...	width 8
	...##...	lbearing 0, rbearing 8
	...##...	ascent 9, descent 0
	...##...
	...##...
	...##...
	...##...
	...##...


	#####		width 8
			lbearing 1, rbearing 6
			ascent 4, descent -3


	##.##.
	###.##		width 7
	##..##		lbearing 0, rbearing 6
	##..##		ascent 7, descent 3
	##..##
	###.##
	##.##.
	##....
	##....
	##....


Note that the width of "-" is 8 even though it only has 5 illuminated pixels.
In this case, one blank pixel to the left and two pixels to the right are 
considered to be part of the glyph.  This can seen clearly by padding the
images out to their logical widths and their proper heights (the font-ascent
and font-descent for this font are 11 and 3, respectively):


               +---     ........    ........    .......
               |        ........    ........    .......
               |        ########    ........    .......
               |        ...##...    ........    .......
               |        ...##...    ........    ##.##..
  font ascent  |        ...##...    ........    ###.##.
      (11)     |        ...##...    ........    ##..##.
               |        ...##...    .#####..    ##..##.
               |        ...##...    ........    ##..##.
               |        ...##...    ........    ###.##.
 baseline ____ |        ...##...    ........    ##.##..
               +===     ........    ........    ##.....
 font descent  |        ........    ........    ##.....
     (3)       |        ........    ........    ##.....
               +---
                        |           |           |
                        origin      origin      origin


Now, removing the illustration spaces between the images, we get:


               +---     .......................
               |        .......................
               |        ########...............
               |        ...##..................
               |        ...##...........##.##..
  font ascent  |        ...##...........###.##.
      (11)     |        ...##...........##..##.
               |        ...##....#####..##..##.
               |        ...##...........##..##.
               |        ...##...........###.##.
 baseline ____ |        ...##...........##.##..
               +===     ................##.....
 font descent  |        ................##.....
     (3)       |        ................##.....
               +---
                        |       |       |
                        origin  origin  origin


From this picture, we can compute the extents of the string as would be
returned by XTextExtents:

    width (total lit and unlit across)				23

    lbearing (number crossed to left lit pixel)			0

    rbearing (number crossed to beyond right lit pixel)		22

    ascent (number crossed to top lit pixel)			9

    descent (number crossed to below bottom lit pixel)		3

