mozilla-release/layout/doc/DD-SpaceManager.html

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<h1><font color="#cc0000">Gecko Layout Detailed Design Document</font></h1>
 
<h1>Space Manager Detailed Design</h1>
 
<h2>Overview</h2>
<p>
  The Space Manager and related classes and structures are an important of
 the Gecko Layout system, specifically Block Layout. &nbsp;See the High Level
 Design document for an overview of the Space Manager, and as an introduction
 to the classes, structures and algorithms container in this, the Detailed
 Design Document.
</p>
 

<hr width="100%" size="2"> 
<h2>nsSpaceManager</h2>
<p>
   The Space Manager is the central class is a group of classes that manage
 the occupied and available space that exists in horizontal bands across
a  canvas. &nbsp;The primary goal of the Space Manager is to provide information
 about those bands of space to support the CSS notion of floated elements.
</p>
 
<p>
  There are three important parts to the Space Manager API: the parts that 
deal with the coordinate space of the Space Manager, the parts that deal with
the regions managed by the Space Manager, and the parts that manage float
impact intervals.
</p>
 
<p>
  The class nsSpaceManager is declared in the file <a href="http://lxr.mozilla.org/seamonkey/source/layout/base/src/nsSpaceManager.h">
  nsSpaceManger.h</a>
  . &nbsp;The class is only used in the layout module and cannot be exported
 outside of that module (nor does it need to be). &nbsp;It is not a general
 purpose class, and is not intended to be subclasses<font color="#cc0000">
 .</font>
</p>
 
<p>
  Here is the class declaration, taken from the source file as of 01.08.02
</p>
 

<pre>/**
 * Class for dealing with bands of available space. The space manager
 * defines a coordinate space with an origin at (0, 0) that grows down
 * and to the right.
 */
class nsSpaceManager {
public:
  nsSpaceManager(PresShell* aPresShell, nsIFrame* aFrame);
  ~nsSpaceManager();

  void* operator new(size_t aSize);
  void operator delete(void* aPtr, size_t aSize);

  static void Shutdown();

  /*
   * Get the frame that's associated with the space manager. This frame
   * created the space manager, and the world coordinate space is
   * relative to this frame.
   *
   * You can use QueryInterface() on this frame to get any additional
   * interfaces.
   */
  nsIFrame* GetFrame() const { return mFrame; }

  /**
   * Translate the current origin by the specified (dx, dy). This
   * creates a new local coordinate space relative to the current
   * coordinate space.
   */
  void Translate(nscoord aDx, nscoord aDy) { mX += aDx; mY += aDy; }

  /**
   * Returns the current translation from local coordinate space to
   * world coordinate space. This represents the accumulated calls to
   * Translate().
   */
  void GetTranslation(nscoord&amp; aX, nscoord&amp; aY) const { aX = mX; aY = mY; }

  /**
   * Returns the y-most of the bottommost band or 0 if there are no bands.
   *
   * @return  PR_TRUE if there are bands and PR_FALSE if there are no bands
   */
  PRBool YMost(nscoord&amp; aYMost) const;

  /**
   * Returns a band starting at the specified y-offset. The band data
   * indicates which parts of the band are available, and which parts
   * are unavailable
   *
   * The band data that is returned is in the coordinate space of the
   * local coordinate system.
   *
   * The local coordinate space origin, the y-offset, and the max size
   * describe a rectangle that's used to clip the underlying band of
   * available space, i.e.
   * {0, aYOffset, aMaxSize.width, aMaxSize.height} in the local
   * coordinate space
   *
   * @param   aYOffset the y-offset of where the band begins. The coordinate is
   *            relative to the upper-left corner of the local coordinate space
   * @param   aMaxSize the size to use to constrain the band data
   * @param   aBandData [in,out] used to return the list of trapezoids that
   *            describe the available space and the unavailable space
   * @return  NS_OK if successful and NS_ERROR_FAILURE if the band data is not
   *            not large enough. The 'count' member of the band data struct
   *            indicates how large the array of trapezoids needs to be
   */
  nsresult GetBandData(nscoord       aYOffset,
                       const nsSize&amp; aMaxSize,
                       nsBandData&amp;   aBandData) const;

  /**
   * Add a rectangular region of unavailable space. The space is
   * relative to the local coordinate system.
   *
   * The region is tagged with a frame
   *
   * @param   aFrame the frame used to identify the region. Must not be NULL
   * @param   aUnavailableSpace the bounding rect of the unavailable space
   * @return  NS_OK if successful
   *          NS_ERROR_FAILURE if there is already a region tagged with aFrame
   */
  nsresult AddRectRegion(nsIFrame*     aFrame,
                         const nsRect&amp; aUnavailableSpace);

  /**
   * Resize the rectangular region associated with aFrame by the specified
   * deltas. The height change always applies to the bottom edge or the existing
   * rect. You specify whether the width change applies to the left or right edge
   *
   * Returns NS_OK if successful, NS_ERROR_INVALID_ARG if there is no region
   * tagged with aFrame
   */
  enum AffectedEdge {LeftEdge, RightEdge};
  nsresult ResizeRectRegion(nsIFrame*    aFrame,
                            nscoord      aDeltaWidth,
                            nscoord      aDeltaHeight,
                            AffectedEdge aEdge = RightEdge);

  /**
   * Offset the region associated with aFrame by the specified amount.
   *
   * Returns NS_OK if successful, NS_ERROR_INVALID_ARG if there is no region
   * tagged with aFrame
   */
  nsresult OffsetRegion(nsIFrame* aFrame, nscoord dx, nscoord dy);

  /**
   * Remove the region associated with aFrane.
   *
   * Returns NS_OK if successful and NS_ERROR_INVALID_ARG if there is no region
   * tagged with aFrame
   */
  nsresult RemoveRegion(nsIFrame* aFrame);

  /**
   * Clears the list of regions representing the unavailable space.
   */
  void ClearRegions();

  /**
   * Methods for dealing with the propagation of float damage during
   * reflow.
   */
  PRBool HasFloatDamage()
  {
    return !mFloatDamage.IsEmpty();
  }

  void IncludeInDamage(nscoord aIntervalBegin, nscoord aIntervalEnd)
  {
    mFloatDamage.IncludeInterval(aIntervalBegin + mY, aIntervalEnd + mY);
  }

  PRBool IntersectsDamage(nscoord aIntervalBegin, nscoord aIntervalEnd)
  {
    return mFloatDamage.Intersects(aIntervalBegin + mY, aIntervalEnd + mY);
  }

#ifdef DEBUG
  /**
   * Dump the state of the spacemanager out to a file
   */
  nsresult List(FILE* out);

  void SizeOf(nsISizeOfHandler* aHandler, uint32_t* aResult) const;
#endif

private:
  // Structure that maintains information about the region associated
  // with a particular frame
  struct FrameInfo {
    nsIFrame* const mFrame;
    nsRect          mRect;       // rectangular region
    FrameInfo*      mNext;

    FrameInfo(nsIFrame* aFrame, const nsRect&amp; aRect);
#ifdef NS_BUILD_REFCNT_LOGGING
    ~FrameInfo();
#endif
  };

  // Doubly linked list of band rects
  struct BandRect : PRCListStr {
    nscoord   mLeft, mTop;
    nscoord   mRight, mBottom;
    int32_t   mNumFrames;    // number of frames occupying this rect
    union {
      nsIFrame*    mFrame;   // single frame occupying the space
      nsVoidArray* mFrames;  // list of frames occupying the space
    };

    BandRect(nscoord aLeft, nscoord aTop,
             nscoord aRight, nscoord aBottom,
             nsIFrame*);
    BandRect(nscoord aLeft, nscoord aTop,
             nscoord aRight, nscoord aBottom,
             nsVoidArray*);
    ~BandRect();

    // List operations
    BandRect* Next() const {return (BandRect*)PR_NEXT_LINK(this);}
    BandRect* Prev() const {return (BandRect*)PR_PREV_LINK(this);}
    void      InsertBefore(BandRect* aBandRect) {PR_INSERT_BEFORE(aBandRect, this);}
    void      InsertAfter(BandRect* aBandRect) {PR_INSERT_AFTER(aBandRect, this);}
    void      Remove() {PR_REMOVE_LINK(this);}

    // Split the band rect into two vertically, with this band rect becoming
    // the top part, and a new band rect being allocated and returned for the
    // bottom part
    //
    // Does not insert the new band rect into the linked list
    BandRect* SplitVertically(nscoord aBottom);

    // Split the band rect into two horizontally, with this band rect becoming
    // the left part, and a new band rect being allocated and returned for the
    // right part
    //
    // Does not insert the new band rect into the linked list
    BandRect* SplitHorizontally(nscoord aRight);

    // Accessor functions
    PRBool  IsOccupiedBy(const nsIFrame*) const;
    void    AddFrame(const nsIFrame*);
    void    RemoveFrame(const nsIFrame*);
    PRBool  HasSameFrameList(const BandRect* aBandRect) const;
    int32_t Length() const;
  };

  // Circular linked list of band rects
  struct BandList : BandRect {
    BandList();

    // Accessors
    PRBool    IsEmpty() const {return PR_CLIST_IS_EMPTY((PRCListStr*)this);}
    BandRect* Head() const {return (BandRect*)PR_LIST_HEAD(this);}
    BandRect* Tail() const {return (BandRect*)PR_LIST_TAIL(this);}

    // Operations
    void      Append(BandRect* aBandRect) {PR_APPEND_LINK(aBandRect, this);}

    // Remove and delete all the band rects in the list
    void      Clear();
  };


  FrameInfo* GetFrameInfoFor(nsIFrame* aFrame);
  FrameInfo* CreateFrameInfo(nsIFrame* aFrame, const nsRect&amp; aRect);
  void       DestroyFrameInfo(FrameInfo*);

  void       ClearFrameInfo();
  void       ClearBandRects();

  BandRect*  GetNextBand(const BandRect* aBandRect) const;
  void       DivideBand(BandRect* aBand, nscoord aBottom);
  PRBool     CanJoinBands(BandRect* aBand, BandRect* aPrevBand);
  PRBool     JoinBands(BandRect* aBand, BandRect* aPrevBand);
  void       AddRectToBand(BandRect* aBand, BandRect* aBandRect);
  void       InsertBandRect(BandRect* aBandRect);

  nsresult   GetBandAvailableSpace(const BandRect* aBand,
                                   nscoord         aY,
                                   const nsSize&amp;   aMaxSize,
                                   nsBandData&amp;     aAvailableSpace) const;

  nsIFrame* const mFrame;     // frame associated with the space manager
  nscoord         mX, mY;     // translation from local to global coordinate space
  BandList        mBandList;  // header/sentinel for circular linked list of band rects
  FrameInfo*      mFrameInfoMap;
  nsIntervalSet   mFloatDamage;

  static int32_t sCachedSpaceManagerCount;
  static void* sCachedSpaceManagers[NS_SPACE_MANAGER_CACHE_SIZE];

  nsSpaceManager(const nsSpaceManager&amp;);  // no implementation
  void operator=(const nsSpaceManager&amp;);  // no implementation
};

</pre>
 
<h3>Public API</h3>
 
<h4>Life Cycle:</h4>
<p>
  The Constructor requires a Presentation Shell, used for arena allocations
 mostly, and a frame that this Space Manager is rooted on. &nbsp;The coordinate
 space of this Space Manager is relative to the frame passed in to the constructor.
</p>
 
<pre>  nsSpaceManager(PresShell* aPresShell, nsIFrame* aFrame);
  ~nsSpaceManager();
</pre>
<p>
  Operators 'new' and 'delete' are overridden to support a recycler. &nbsp;Space
 Manager instances come and go pretty frequently, and this recycler prevents
 excessive heap allocations and the performance penalties associated with
it. The #define NS_SPACE_MANAGER_CACHE_SIZE is used to control the number
of Space Manager instances that can be present in the recycler, currently
4. &nbsp;If more than NS_SPACE_MANAGER_CACHE_SIZE are allocated at a time,
then standard allocation is used.
</p>
 
<pre>
  void* operator new(size_t aSize);
  void operator delete(void* aPtr, size_t aSize);

</pre>
<p>
 A Static method is used to shutdown the Space Manager recycling. &nbsp;This 
method deletes all of the Space Mangers inthe recycler,and prevents further 
recycling. &nbsp;It is meant to be called only when the layout module is being
terminated.
</p>
 
<pre>  static void Shutdown();

</pre>
 
<h4>Origin / Coordinate Space Translation</h4>
 
<pre>  /**
   * Translate the current origin by the specified (dx, dy). This
   * creates a new local coordinate space relative to the current
   * coordinate space.
   */
  void Translate(nscoord aDx, nscoord aDy) { mX += aDx; mY += aDy; }

  /**
   * Returns the current translation from local coordinate space to
   * world coordinate space. This represents the accumulated calls to
   * Translate().
   */
  void GetTranslation(nscoord&amp; aX, nscoord&amp; aY) const { aX = mX; aY = mY; }

  /**
   * Returns the y-most of the bottommost band or 0 if there are no bands.
   *
   * @return  PR_TRUE if there are bands and PR_FALSE if there are no bands
   */
  PRBool YMost(nscoord&amp; aYMost) const;
</pre>
 
<h4>Region Management</h4>
 
<pre>  /**
   * Returns a band starting at the specified y-offset. The band data
   * indicates which parts of the band are available, and which parts
   * are unavailable
   *
   * The band data that is returned is in the coordinate space of the
   * local coordinate system.
   *
   * The local coordinate space origin, the y-offset, and the max size
   * describe a rectangle that's used to clip the underlying band of
   * available space, i.e.
   * {0, aYOffset, aMaxSize.width, aMaxSize.height} in the local
   * coordinate space
   *
   * @param   aYOffset the y-offset of where the band begins. The coordinate is
   *            relative to the upper-left corner of the local coordinate space
   * @param   aMaxSize the size to use to constrain the band data
   * @param   aBandData [in,out] used to return the list of trapezoids that
   *            describe the available space and the unavailable space
   * @return  NS_OK if successful and NS_ERROR_FAILURE if the band data is not
   *            not large enough. The 'count' member of the band data struct
   *            indicates how large the array of trapezoids needs to be
   */
  nsresult GetBandData(nscoord       aYOffset,
                       const nsSize&amp; aMaxSize,
                       nsBandData&amp;   aBandData) const;

  /**
   * Add a rectangular region of unavailable space. The space is
   * relative to the local coordinate system.
   *
   * The region is tagged with a frame
   *
   * @param   aFrame the frame used to identify the region. Must not be NULL
   * @param   aUnavailableSpace the bounding rect of the unavailable space
   * @return  NS_OK if successful
   *          NS_ERROR_FAILURE if there is already a region tagged with aFrame
   */
  nsresult AddRectRegion(nsIFrame*     aFrame,
                         const nsRect&amp; aUnavailableSpace);

  /**
   * Resize the rectangular region associated with aFrame by the specified
   * deltas. The height change always applies to the bottom edge or the existing
   * rect. You specify whether the width change applies to the left or right edge
   *
   * Returns NS_OK if successful, NS_ERROR_INVALID_ARG if there is no region
   * tagged with aFrame
   */
  enum AffectedEdge {LeftEdge, RightEdge};
  nsresult ResizeRectRegion(nsIFrame*    aFrame,
                            nscoord      aDeltaWidth,
                            nscoord      aDeltaHeight,
                            AffectedEdge aEdge = RightEdge);

  /**
   * Offset the region associated with aFrame by the specified amount.
   *
   * Returns NS_OK if successful, NS_ERROR_INVALID_ARG if there is no region
   * tagged with aFrame
   */
  nsresult OffsetRegion(nsIFrame* aFrame, nscoord dx, nscoord dy);

  /**
   * Remove the region associated with aFrane.
   *
   * Returns NS_OK if successful and NS_ERROR_INVALID_ARG if there is no region
   * tagged with aFrame
   */
  nsresult RemoveRegion(nsIFrame* aFrame);

  /**
   * Clears the list of regions representing the unavailable space.
   */
  void ClearRegions();
</pre>
 
<h4>Float Impact</h4>
 
<pre>  /**
   * Methods for dealing with the propagation of float damage during
   * reflow.
   */
  PRBool HasFloatDamage()
  {
    return !mFloatDamage.IsEmpty();
  }

  void IncludeInDamage(nscoord aIntervalBegin, nscoord aIntervalEnd)
  {
    mFloatDamage.IncludeInterval(aIntervalBegin + mY, aIntervalEnd + mY);
  }

  PRBool IntersectsDamage(nscoord aIntervalBegin, nscoord aIntervalEnd)
  {
    return mFloatDamage.Intersects(aIntervalBegin + mY, aIntervalEnd + mY);
  }
</pre>
 
<h4>Debug Only Methods</h4>
 
<pre>  /**
   * Dump the state of the spacemanager out to a file
   */
  nsresult List(FILE* out);

  void SizeOf(nsISizeOfHandler* aHandler, uint32_t* aResult) const;

</pre>
 
<h4>Unused / Obsolete Methods</h4>
 
<pre>  /*
   * Get the frame that's associated with the space manager. This frame
   * created the space manager, and the world coordinate space is
   * relative to this frame.
   *
   * You can use QueryInterface() on this frame to get any additional
   * interfaces.
   */
   nsIFrame* GetFrame() const { return mFrame; }

</pre>
 
<h3>Implementation Notes</h3>
 
<h4></h4>
 
<h4>Algorithm 1: GetBandData</h4>
<p>
GetBandData is used to provide information to clients about what space if
available and unavailable in a band of space. &nbsp;The client provides a
vertical offset, the yOffset, that corresponds to the band that is of interest.
&nbsp;This will be the y offset of the frame that is being reflowed. &nbsp;The
caller also provides a collection of BandData objects (an array) and the
number of items that the collection can handle. &nbsp;If the collection is
too small, then an error is returned and the count is updated to indicate
the size required.
</p>

<p>
The algorithm to provide the band data is as follows:
</p>
<ul>
  <li>Get a &nbsp;vertical offset in world coordinates (instead of frame-relative
coordinates) by adding the y-origin of the SpaceManager to the y offset passed
in</li>
  <li>If the (adjusted) y value passed in is greater than the greatest band
being managed, then all space is available so a single trapezoid is returned,
marked as available and sized to the maximum size value (passed in).</li>
  <li>If the (adjusted) y offset intersects a band, then gather the band
data:</li>
  <ul>
    <li>walk the internal bandData list from head to tail</li>
    <li>for each band data entry, see if the top of the band is greater than
the (adjusted) y offset requested</li>
    <li>if it is, then band is below the offset requested, so the area between
the band and the y offset is available - create a trapezoid with that region
and return it.</li>
    <li>if the (adjusted) y offset is between the band top and bottom, then
get the available space for the band by calling GetBandAvailableSpace</li>
    <li>otherwise, move to the next band</li>
  </ul>
</ul>
<h5>GetBandAvailableSpace:</h5>
This method is called from GetBandData only. It walks all of the bands in
the space manager and determines which bands intersect with the band passed
in, and if within those bands there are regions that are available or occupied.

<ul>
  <li>First, walk all of the bands until a band that is to the right of the
desired offset is located</li>
  <li>Starting at that band, &nbsp;walk the remaining bands:</li>
  <ul>
    <li>if the current band is to the right of the requested band, then there
is available space.&nbsp;</li>
    <ul>
      <li>if there is more room in the bandData collection, then add a trapezoid
to the bandData collection such that it is marked as available and has a
rect that represents the space between the reference band tna dht band being
examined</li>
      <li>if there is no more room in the BandData collection, estimate the
number of entries requires as the current count + twice the number of bands
below the reference band, plus two. &nbsp;Return an error code so the caller
can reallocate the collection and try again.</li>
    </ul>
    <li>check the size of the collection again, if there is no room left
then estimate the number of items requires as the current count + twice the
number of bands below the band in question plus one.&nbsp;</li>
    <li>create a new trapezoid in the band collection that has a region corresponding
to the reference band rect, marked as occupied by either a single or multiple
frames.</li>
    <li>move to the next band</li>
  </ul>
  <li>after walking all of the band data, se if we have reached the right
edge of the band.&nbsp;</li>
  <ul>
    <li>If not, then check for space in the band collection</li>
    <ul>
      <li>if there is no room left, then set the count to the current count
plus 1 and return an error.</li>
      <li>otherwise, create another entry in the band collection, marked
as available, and with a rect corresponding to the area remainin in the band
(eg. from the right edge of the last band rect to the right edge of the band).</li>
    </ul>
  </ul>
</ul>
 
<h4>Algorithm 2: AddRectRegion</h4>
Clients call into this method to notify the Space Manager that a new frame
is occupying some space.

<ul>
  <li>First, try to get frame info for the frame. If it is found, return
an error since the frame is already associated with a region in the Space
Manager.</li>
  <li>Next, create a rect from the occupied space passed in by the caller,
transforming it first to world-coordinates by adding the Space Manager's
offset to the occupied space rect passed in.</li>
  <li>Create a new Frame Info instance for the frame and rect, returning
an error if allocation fails.</li>
  <li>Check if the occupied space rect (adjusted) is empty, if so, return
an error &nbsp;(<font color="#cc0000">NOTE: this could be done earlier, or
prevented by the caller</font>)</li>
  <li>Allocate a new BandRect instance with the rect and frame as constructor
arguments, and insert it into the collection via InsertBandRect</li>
</ul>
<h5>InsertBandRect:</h5>
Internal method to insert a band rect into the BandList in the correct location.
There are several cases it has to handle, as specified in the source file
comments:

<pre>// When comparing a rect to a band there are seven cases to consider.
// 'R' is the rect and 'B' is the band.
//
//      Case 1              Case 2              Case 3              Case 4
//      ------              ------              ------              ------
// +-----+             +-----+                      +-----+             +-----+
// |  R  |             |  R  |  +-----+    +-----+  |     |             |     |
// +-----+             +-----+  |     |    |  R  |  |  B  |             |  B  |
//          +-----+             |  B  |    +-----+  |     |    +-----+  |     |
//          |     |             |     |             +-----+    |  R  |  +-----+
//          |  B  |             +-----+                        +-----+
//          |     |
//          +-----+
//
//
//
//      Case 5              Case 6              Case 7
//      ------              ------              ------
//          +-----+    +-----+  +-----+    +-----+
//          |     |    |  R  |  |  B  |    |     |  +-----+
//          |  B  |    +-----+  +-----+    |  R  |  |  B  |
//          |     |                        |     |  +-----+
//          +-----+                        +-----+
// +-----+
// |  R  |
// +-----+
//
</pre>
<ul>
  <li>First, check for the easiest case, where there are no existing band
rects, or the band rect passed in is below the bottommost rect. In this case,
just append the band rect and return.</li>
  <li>Starting at the head of the list of bandRects, check for intersection
with the rect passed in:</li>
  <ul>
    <li>case #1: the rect is totally above the current band rect, so insert
a new band rect before the current bandRect</li>
    <li>cases #2 and #7: the rect is partially above the band rect, so it
is divided into two bandRects, one entirely above the band, and one containing
the remainder of the rect. &nbsp;Insert the part that is totally above the
bandRect before the current bandRect, as in case #1 above, and adjust the
other band rect to exclude the part already added.</li>
    <li>case #5: the rect is totally below the current bandRect, so just
skip to the next band</li>
    <li>case #3 and #4: rect is at least partially intersection with the
bandRect, so divide the current band into two parts, where the top part is
above the current rect. &nbsp;Move to the new band just created, which is
the next band.</li>
    <li>case #6: the rect shares the bottom and height with the bandRect,
so just add the rect to the band.</li>
    <li>case #4 and #7: create a new rect for the part that overlaps the
bandRect, and add it to the current bandRect (similar to case #6) and then
move on to the next band, removing that part from the rect passed in. &nbsp;If
no more bands, append the rect passed in to the end of the bandRect list.</li>
  </ul>
</ul>
<i>This algorithm is pretty confusing - basically what needs to happen is
that rects and bands need to be divided up so that complicated cases like
#2, #4, and #7, are reduced to simpler cases where the rects is totally above,
below, or between a band rect. &nbsp;From the current implementation, it
might be worth verifying that the final result of the inserts is a correctly
ordered liest of bandRects (debug mode only).</i>

 
<h4>Algorithm 3: RemoveRegion</h4>
 When a float is removed, the Space Manager is notified by a call to RemoveRegion,
passing in the frame that is being removed.

<ul>
  <li>Get the FrameInfo for the frame passed in. If not found, an error is
returned.</li>
  <li>If the rect for the frame is not empty, then visit each band in the
bandList:</li>
  <ul>
    <li>for each rect in the band:

    </li>
  </ul>
  <ul>
    <ul>
      <li>if the bandRect is occupied by the frame, either remove the frame
from the bandRect (if there are other frames sharing it) and remember that
it was shared</li>
      <li>otherwise simply remove the bandRect (no other frames share it).</li>
      <li>if the bandRect was shared, then try to coalesce adjacent bandRects</li>
      <ul>
        <li>if the previous bandRect is directly next to the current bandRect,
and they have the same frame list, then make the current bandRect cover the
previous bandRect's full region (adjust the left edge to be that of the previous
bandRect) and remove the previous bandRect.</li>
      </ul>
    </ul>
  </ul>
  <ul>
    <li>if the current band or prior band had a rect occupied byu the frame,
then try to join the two bands via JoinBands</li>
  </ul>
  <li>Finally, destroy the frameInfo for the frame.

  </li>
</ul>
  
<br>
 
<hr width="100%" size="2"> 
<h2>Cross-Component Algorithms</h2>

<br>
 

<hr width="100%" size="2"> 
<h2>Tech Notes</h2>
<ul>
  <li>

  </li>
 
</ul>
 

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