|Implementing footnotes in FOP|
Footnotes present difficulties for page layout primarily
because their point of invocation in the flow is different
from their point of appearance in the area tree. All of the
content lines of a footnote may appear on the same page as its
invocation point, all may appear on a following page, or the
lines may be split over a page or pages. (This characteristic
leads to another problem when a footnote overflows the last
page of flow content, but that difficulty will not be
discussed here.) This note considers some aspects of the
implementation of footnotes in a galley-based design.
In the structure described in the introduction to FOP galleys,
footnotes would be pre-processed as galleys themselves, but
they would remain attached as subtrees to their points of
invocation in the main text. Allocation to a
footnote-reference-area would only occur in the resolution
to Area nodes.
When footnotes are introduced, the communication between
galleys and layout manager, as mentioned above, would be
affected. The returned information would two b-p-d values:
the primary line-area b-p-d impact and the footnote b-p-d
impact. The distinction is necessary for two reasons; to
alert the layout manager to the first footnote of the page,
and because the footnote b-p-d will always impact the
main-reference-area b-p-d, whereas the primary inline-area
may not, e.g. in the case of multiple span-areas.
|Multiple columns and footnotes|
A possible method for multi-column layout and balancing
with footnotes, using a galley-based approach.
This note assumes a galley, as discussed elsewhere, flowing text with
footnotes and possibly other blocks into a possibly
multi-column area. The logic of flowing into multiple
columns is trivially applied to a single column. The galley
is manipulated within the context of the layout
Associated with the galley are two sets of data.
One contains the maps of all "natural" break-points and
the of all hyphenation break-points. This set is
constructed at the time of construction of the galley and
is a constant for a given galley. The second contains
dynamic data which represents one possible attempt to lay
out the galley. There may be multiple sets of such data
to reflect varying attempts. The data of this set are,
essentially, representations of line-areas, with the supporting
information necessary to determine these line-areas.
The line-area data includes the boundaries within the
galley of each line-area, the boundaries of each column
and the boundaries of the "page", or main area. When a
line-area boundary occurs at a hyphenation point, a
"virtual hyphen" is assumed and accounted for in the
i-p-d. As mentioned, individual footnote galleys will
hang from the parent galley. The associated data of the
footnote galleys is similar: a once-only break-points map,
and one or more line-area maps. No column boundaries are
required, but a page boundary is required at the end of
the last footnote or where a footnote breaks across a page
A number of b-p-d values are also maintained. For each
line-area, the b-p-d, the main area b-p-d increment, the
footnote b-p-d increment and the footnote's page-related
b-p-d increment are required. The main-area b-p-d
increments for any particular line-area are dependent on
the column position of the line-area. Total b-p-d's are
also kept: total footnote b-p-d, total main area b-p-d,
and totals for each column.
Figure 1 Columns before first footnote.
Figure 2 Adding a line area with first
Columns are balanced dynamically in the galley preliminary
layout. While the galley retains its basic linear
structure, the accompanying data structures accomplish
column distribution and balancing. As each line-area is
added, the columns are re-balanced. N.B.
This re-balancing involves only some of the dynamic data
associated with the participating galley(s). The data
structures associating breakpoints with the beginning and
end of individual line areas does not change in
re-balancing; only the association of line-area with column,
and, possibly, the various impact values for each line-area.
Figure 3 Adding a line area with next