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From Apache Wiki <wikidi...@apache.org>
Subject [Xmlgraphics-fop Wiki] Update of "LineLayout/AlignmentHandling" by ManuelMall
Date Sun, 02 Oct 2005 15:37:28 GMT
Dear Wiki user,

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The following page has been changed by ManuelMall:
http://wiki.apache.org/xmlgraphics-fop/LineLayout/AlignmentHandling

The comment on the change is:
First attemtp at document inline alignment issues and their resolution

New page:
Part of the line building process is the correct alignment of inline children of a line area.
The rules and properties governing this process are described in various places in the XSL-FO
specification. In particular [http://www.w3.org/TR/xsl/slice4.html#area-inline section 4.6]
and [http://www.w3.org/TR/xsl/slice7.html#area-alignment section 7.13] are relevant. However
there are a number of open, unclear or User Agent specific areas in the specification and
this page attempts to document to approach taken by FOP in those cases.

= Baseline Tables =
The specification assumes the existence of baseline tables in the font definitions. However,
most common fonts formats, especially True Type and PFM, do not carry this information. For
these fonts FOP needs to calculate the various baselines based on the basic font information
which consists of the ''ascender height'', the ''descender depth'' and the ''x height''. These
fonts are all assumed to have their alignment point on the alphabetic baseline, that is the
3 values mentioned are given relative to the fonts alphabetic baseline. This seems to be true
even for non Western scripts as inspection of some sample font files for scripts that would
be expected to use ideographic and hanging baselines shows. FOP uses these simple formulas
to calculate the position of the following baselines relative to the alphabetic baseline:
 * ideographic = ''descender depth''
 * alphabetic = 0
 * central = (''ascender height'' - ''descender depth'') / 2
 * middle = ''x height'' / 2
 * text-after-edge = ''descender depth''
 * text-before-edge = ''ascender height''

== Hanging and mathematical baselines ==
There are no obvious formulas to calculate the position of these baselines. At the time of
writing FOP puts the hanging baseline at 80% of the ''ascender height'' and the mathematical
baseline at 50%.

'''NOTE:''' It may be desirable to have this configurable in the user configuration or be
able to set these baseline values in the font configuration.

== before-edge and after-edge baselines ==
These are not baselines defined in a font but computed baselines. According to the spec these
baselines are only defined for line areas. However, any alignment between two areas is done
by aligning the alignment baseline of the parent area with the matching alignment point of
the child area. That alignment point is defined as the intersection of the corresponding baseline
with the start-edge. This will only work for the before-edge and after-edge alignment baselines
if these baselines are defined on the child area to be aligned. FOP therefore takes the approach
that the before-edge and after-edge baselines for inline areas coincide with the before and
after edges of the allocation rectangles of the inline area. For most inline areas these are
identical to the text-before-edge and text-after-edge baselines.

== Baselines for areas generated from external-graphic and inline-foreign-object ==
For these areas we cannot use font data to calculate the baseline positions. FOP treats these
areas as if they are defined using a font with ''ascender height'' == ''height of area'',
''descender depth'' == 0 and ''x height'' == ''ascender height'' / 2 and then uses the same
formulas as above.

== Baseline co-ordinate systems and scaling ==
Internally FOP implements directly only the ''scaled-baseline-table''. This is partly because
the font system delivers all font data already scaled to the font size. The origin is always
the position of the ''dominant-baseline'' as identified by the ''dominant-baseline-identifier''.
Positive baseline values are from the dominant-baseline towards the before-edge.

== Implementation notes ==
FOP defines a {{{ScaledBaselineTable}}} interface, a {{{ScaledBaselineTableFactory}}} and
(at the time of writing) a single implementation of the interface, the {{{BasicScaledBaselineTable}}}.
All of those are contained within the {{{layoutmgr.inline}}} package. This architecture should
allow to easily add other baseline table implementations, especially for fonts which contain
baseline information, e.g. Open Type, at a later date.

== Baselines for vertical writing modes ==
While the interfaces defined do support baseline table construction for different writing
modes the current implemenation supports horizontal writing modes only.

= Baseline shifts =
The spec leaves a number of issues unclear with respect to baseline shifts.

== Baseline shift and a change of dominant baseline ==
When a baseline shift is combined with a change of the dominant baseline it is unclear if
the shift is applied first and then the new baseline table calculated or if the new baseline
table is calculated first and then the shift applied. Here is an example:
{{{
Some text
  <fo:inline font-size=".5em" 
    dominant-baseline="hanging" 
    baseline-shift=".25em">small and up
  </fo:inline>
}}}
Is the ".25em" shift between the alphabetic baseline of the parent and the alphabetic baseline
of the child or the hanging baseline of the child? FOP takes the view that a baseline-shift
first moves the dominant baseline and then establishes the new dominant baseline.

== Implicit baseline shifts when rescaling ==
When a baseline table is rescaled an implicit baseline shift can happen a situation the spec
appears to ignore. Take this example:
{{{
Some text
  <fo:inline font-size=".5em" 
    alignment-baseline="central" 
    dominant-baseline="reset-size">small and up
  </fo:inline>
}}}
Because the alignment baseline is different to the dominant baseline and the baseline table
gets rescaled we have an implict shift of the dominant baseline. The above example is logically
equivalent to:
{{{
Some text
  <fo:inline font-size=".5em" 
    baseline-shift="xxx">small and up
  </fo:inline>
}}}
with "xxx" being the shift amount required to align the "central" baselines. FOP calculates
and maintains these implict baseline shifts.

== Subscripts and Superscripts ==
Sub and Super scripts are special forms of baseline shifts. The spec implies that each font
has a subscript and a superscript shift amount. The FOP fonts don't have that information.
Initial tests with using a fixed shift factor for superscripts and subscripts (different factor
for super and sub though) appear to give unsatisfactory results especially if different font
sizes are involved between the main element and the subscript element. Some further testing
with CSS and browsers seem to indicate that they take both font sizes into account. Therefore
FOP at the time of wrting uses the following method to determine the shift values: Assuming
the dominant baseline is alphabetic a superscript element is aligned such that its alphabetic
baseline coincides with the ''x height'' of the parent, that is a superscript "starts" at
the top of the lowercase "x", i.e. similar but not identical to "x^2^". Assuming the dominant
baseline is alphabetic a subscript element is aligned such tha
 t its middle baseline aligns with the parents alphabetic baseline, that for a subscript character
"x" the middle of "x" aligns with the alphabetic baseline of the parent, i.e. similar but
not identical to "2,,x,,". There may be further fine tuning required to achieve visually more
appealing results, e.g. both superscripts and subscripts may have to be lowered a fraction.

'''NOTE:''' It may be desirable to have this configurable in the user configuration or be
able to set these values in the font configuration.

= Overconstraint alignment =
With respect to line areas there can be conflicting or overconstraint alignment specifications.
I couldn't find a reference to that situation in the specification. Example:
{{{
<fo:block font-size="32pt">Big
  <fo:inline font-size=".25em"
      dominant-baseline="reset-size"
      alignment-baseline="before-edge">top
    <fo:inline font-size="2em">alpha
    </fo:inline>
  </fo:inline>
</fo:block>
}}}
Here we want "top" at the before-edge of the line area but the nested inline "alpha" wants
to be aligned at the alphabetic baseline of "top". Because "alpha" uses a bigger font than
"top" these two conditions cannot be satisfied simultaneously. It is unclear how this is to
be resolved.

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