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<!DOCTYPE document PUBLIC "-//APACHE//DTD Documentation V1.3//EN" "http://forrest.apache.org/dtd/document-v13.dtd">

<document>
  <header>
    <title>Apache™ FOP Design: Layout Managers</title>
    <subtitle>Break Possibility Proposal</subtitle>
    <version>$Revision: 1298724 $</version>
    <authors>
      <person name="Karen Lease" email="klease@club-internet.fr"/>
    </authors>
  </header>

    <body>
<section id="intro">
  <title>Introduction</title>
<p>
As explained in <link href="layout.html">Layout</link>,
the hierarchy of Layout Managers is responsible for building and placing
areas. Each Layout Manager is responsible for creating and filling
areas of a particular type, either inline or block. This document
explains one potential algorithm for this process. It is based on the
the generation of <em>break possibilities</em> (BP for short). The
Layout Managers (LM for short), will generate one or more BP and
choose the best one. The BP is then used to generate the corresponding
areas.
</p>
</section>
<section>
  <title>Anatomy of a Break Possibility</title>
<p>A break possibility is represented by the BreakPoss class. A
BreakPoss contains size information in the stacking direction and in
the
non-stacking direction (at least for inline areas, it must have both). Flags
indicating various conditions (ISFIRST, ISLAST, CAN_BREAK_AFTER,
FORCE_BREAK_AFTER, ANCHORS etc). A BreakPoss contains a reference to
the top-level LayoutManager which generated it.
</p>
<p>A BreakPoss contains an object implementing
the BreakPoss.Position interface. This object is specific to the layout
manager which created the BreakPoss. It should indicate where the
break occurs and allow the LM to
create an area corresponding to the BP. A higher level LM Position
must somehow reference or wrap the Position returned by its child LM in its
BreakPoss object. The layout manager  modifies the flags and dimension
information in the BP to reflect its own requirements. For example an
inline FO layout manager might add space-start, space-end, border and
padding values to the stacking or non-stacking dimensions. It might also
modify the flags based its on keep properties.</p>
</section>
<section>
  <title>Turning Break Possibilities into Areas</title>
<p>Once break possibilities have been generated, the galley-level
layout manager selects the best one
and passes it back to the LayoutManager which generated it to create
the area. A LayoutManager is responsible for
storing enough information in its Position objects to be able to
create the corresponding areas.</p>
  </section>
<section>
  <title>A walk-through</title>
<p>Layout Managers are created from the top down. First the
page sequence creates a PageLM and a FlowLM. The PageLM will manage
finding the right page model (with help from the PageSequenceMaster)
and managing the balancing act between before-floats, footnotes and
the normal text flow. The FlowLM will
manage the normal content in the main flow. We can think of it as a
<em>galley</em> manager.
</p>
<p>In general, each LM asks its child LMs to return sucessive
break possibilities. It passes some
information to the child in a flags object and it gets back
a break possibility which contains the size in
the stacking direction as well as information about such things as
anchors, break conditions and span conditions which can change the
reference area environment. This process continues down to the lowest
level of the layout manager hierarchy which corresponds to atomic
inline-level FOs such as characters or graphics.
</p>
<p>
Each layout manager will repeatedly call getNextBreakPoss on its current
child LM until the child returns a BP with the ISLAST
flag set. Then the layout manager moves on to its next child LM (ie,
it asks the next child FO to generate a layout manager.) Galley level
layout managers which are Line and Flow will return to their parent
layout managers either when they have finished their content or when
they encounter a a BP which will fill one of their areas.
</p>
<p>The break possibilities are generated from the bottom up.
All inline content must first be broken into
lines which are then stacked into block areas. This is done by the
LineLayoutManager, which creates line areas.
The LineLM asks its child LM to generate a break possibility, which
represents a place where the line can end. This
initially means each potential line-end (primarily spaces or forced
linefeeds and a few other potential line-end characters such as hard
hyphens.) The text LM returns an object which stores the size in the
stacking direction as a MinOptMax triplet
and a <em>cost</em>, which is based on how well this break
would satisfy the constraints. The Text LM keeps track of its position in
the text content and returns the total size of the text area it would
create if it were to break at a given point. The returned BP
object also contains information about whether the break is forced
(linefeed) or whether this is the last area which can be generated by
the LM (ISLAST flag). If a textFO ends on a non-break character, the
ISLAST flag is set, but the CAN_BREAK_AFTER flag isn't, since we don't
know if there is any following text in another inline object for
example.
</p>
<p>Variable size content is taken into account from
the bottom up. Each LM returns a range of sizes in the stacking
direction, based on property values. For text, this comes from
variable word-space values or letter-space values. For other inline
objects, it may include variable space-start and space-end values
(after calculation of the entire sequence of space specifiers at a
particular break possibility.)</p>
<p>The main constraint for laying out
lines is the available inline-progression-dimension (IPD) for the line
area to be created. This
depends on the IPD of the reference area ancestor, on the indents of the
containing fo:block, and on any side-floats which may be intruding on
this line.</p>
<note>See below <link href="#getRefIPD">Getting the Reference
IPD</link>
for discussion of how the reference area IPD is
transmitted to the Line LM.</note>
<p>For now, let's assume that only the LineLM knows about the IPD
available to it. Therefore only it can make a decision about which BP
is the best one; the lower level inline layout managers can only
return potential break points.</p>
<note>There are certainly optimizations to this model which can be
examined later.</note>
<p>So the Line LM will ask its child LM(s) for break possibilities until
it gets back a BP whose stacking dimension <em>could</em> fill the
line. This means that the BP.stackdim.max &gt;= LineIPD.min. It can look
for further BP, perhaps one whose stackdim.opt is closer to the
LineIPD.opt. If it isn't happy with the choice of break possibilities,
it can go past the end of the line to the next one, and then try to
find a hyphenation point between the last one which fits and the first
one which doesn't. If no possibility is found whose min/max values
enclose the available IPD, some constraint will be violated (and
reported in the log.) The actual strategy is up to the Line LM and
should be able to be easily replaced without changing the architecture
(Strategy pattern).
</p>
<p>The definition of a good break possibility depends on the
properties at the block and inline level which govern things such as
wrapping behavior and justification mode. For example, if lines are
not to be wrapped, only an explicit linefeed can serve as a BP. If
lines are wrapped but not justified then there is no requirement to
completely fill the IPD on each line, but a sophisticated layout
manager will try to achieve "aesthetic rag".
</p>
<p>Note that no areas have actually been created yet. Once the LineLM
has found a potential break point for the inline content, it can
calculate the total size of the line area which would be created. The
size in the IPD is determined by the Line LM based on the chosen BP.
The size of the line area in the the block-progression-dimension
depends on the size of the text (or other inline content). These
values are set by the inline-level LM
in their returned BP (in terms of ascender and descender heights with
respect to the baseline). The LineLM adds spacing implied by the
current line-stacking strategy and line-height property values. It
stores a reference to the chosen inline BP and "wraps" that in its own
Position object which it stores in the BP it returns to its parent LM
(the block layout manager).
</p><p>The block LM now has a potential break position after its
first line. It assigns that possibility a cost, based on widow, orphan
and keep properties. It can also calculate the total size of the block
area it would create, were it to end the area after this line. It does
this by adding any padding and border (taking into account
conditionality). It also calculates space-before and space-after
values, or contributes to building up a sequence of such values.
With this information, the block LM creates a new BP (or
updates the existing one). It stores a Position object in this
BP which wraps the returned BP from its child Line LM.
It returns the new BP to its parent and so on, back up to the
FlowLM.</p>
<p>Obviously there is more complicated logic involved when dealing
with lists and tables. These cases need to be walked through in detail.</p>
<p>The FlowLM sees if the returned stacking dimension will still
fit in its available block-progression-dimension (BPD). It repeatedly calls
getNextBreakPoss on its
child LMs until it reaches the maximum BPD for the flow reference area
or until there is no more content to lay out. If one child LM is
finished, it moves on to the next until the last child LM has returned
a BP with the ISLAST flag set. If any child LM returns a
BP with a FORCE_BREAK_BEFORE or SPAN flag set, the FlowLM will
force layout of any pending break possibilities and return to its
parent (the PageLM) in order to handle the break or span condition.</p>
<p>If the returned BP has any new before-float or footnote anchors in
it (ANCHOR flag in the
BP), the FlowLM will also return to the PageLM. The PageLM must then
try to find space to place the floats, possibly asking the FlowLM for
help if the body contains multiple columns.</p>
</section>
<section>
  <title>Some issues</title>
<p>Following are a few remarks on specific issues.</p>
<section>
  <title>Where Line Layout Managers are created</title>
<p>If the first child FO in a block FO is an inline-level FO
such as text, the block LM creates an intermediate level LineLM
to layout the
sequence of inline content into Lines. Note that the whole sequence of
inline FOs is managed by a single instance of LineLM. The LineLM
becomes the parent to the various inline-level LM created by each
individual inline FO.
Since an fo:block can have both block and inline content, its LM
may create a sequence of intermixed BlockLM and LineLM.</p>
</section>
<section id="getRefIPD">
  <title>Getting the reference IPD</title>
<p>When the layout process starts, with the FlowLM asking its first
child LM for a break possibility, the IPD isn't known, since we don't
know whether
the first FO might be spanning, or on which page it might start. (Of
course, if all page masters in the sequence have the same region-body IPD
and all have only a single column, the IPD will never change
and could already be calculated before starting layout.)
The FlowLM gets its
first child LM and calls its getNextBreakPoss method. That is a child LM for
some block-level FO. For now, suppose it's an fo:block. The BlockLM
will create its first child LM, which may be another block-level LM in
the case of nested blocks or a LineLM as explained above. (Question:
do we need a START flag for layout status?)
</p>
<p>We keep calling getNextBreakPoss on lower level layout managers until we
get down to the inline level or to a level which cannot have break-before
properties, such as a list-item-label. At that point, we assume we are
going to have to layout some actual content. But we can't do that yet
since we don't know the inline-progression-dimension. So we return a
BP object which has 0 size in the stacking dimension, but which
has flags set to signal to
higher-level layout managers what needs to be done. If it has a break-before
property or a span property, it stores these in the BP. If
no reference IPD is yet defined, it sets a flag to get that. It then
returns to its parent. The parent LM will inspect the BP object
returned. In general, it "wraps" it with information about its own
needs. If the returned BP is not actually returning any potential
areas, the LM can still add information about its own break or span
requirements. This return path continues back up to the PageLM. It
will then check break and span requirements and create a new page
if necessary using the appropriate page-master. At that point, the
reference IPD for the main
flow is known and is set in the flags object used for
the next getNextBreakPoss call to the lower level LM.
</p><p>Using this information, the BlockLM parent can now calculate
the available IPD for its LineLM child, based on its indents.
(If there are any
side-floats information about the intrusion must be passed down by the
FlowLM to lower level managers.) The LineLM can now generate a series
of BreakPoss objects, which it passes back to its parent LM.
</p>
</section>
<section>
  <title>Hyphenation</title>
<p>
The LineLM is responsible for initiating hyphenation if it is allowed
by the properties and if no satisfactory BP can be found without
hyphenating. The hyphenation manager is passed two break
possibilities, one whose IPD is less than the desired line area IPD
and one whose IPD is greater. These break possibilities might have
been generated by different inline-level layout managers (text + a
wrapper with a color change for example), though
frequently they represent two positions in a single text run.
If hyphenation is successful, a new BP is
returned. The LineLM may look for several intermediate BP
based on the "cost" of the returned possibilities. If no intermediate
BP is found, the line will be "short", the white-space stretch will be
exceeded, or perhaps the content will be overflowed or clipped,
depending on various property settings.</p>
</section>
<section>
  <title>Optimizing</title>
<p>It obviously seems inefficient to go down to the lowest level
LM and back up to the FlowLM for every possible line-break
decision. It seems like it would be possible to optimize by letting
the lower level layout managers run until they had exceeded the
current limit in
the stacking direction. They would then return control to the "galley"
level (LineLM or FlowLM) which would fine-tune the break decision by
asking the lower level LM to find a previous BP which would fit. At
the inline level, this means hyphenation as described above.</p>
<p>Another interesting question is at what point pending break
possibilities can be turned into areas.The idea is to wait until we
are sure we won't have to redo the breaking. This depends on the
sophistication of the layout strategy. For example, if a
linebreak can be considered final if the line is full and there are no
anchors on the line, we could create the LineArea at that point. But
if we are willing to change a previous line-end decision to get a
better overall composition of a whole group of lines (to prevent multiple
hyphens for example), we might wait until the LineLM had finished
laying out all its material and then make all the Lines at once.</p>
</section>
</section>
    </body>
</document>