Amiga ISO Collection/AmigaUtilCD2.iso/Programming/Misc/TRSICAT.LZX/CATS_CD2_TRSI/Inc&AD1.3/Text_Autodocs/layers.doc

Text File | 1992-09-12 | 23.2 KB | 914 lines

TABLE OF CONTENTS

layers.library/BeginUpdate
layers.library/BehindLayer
layers.library/CreateBehindLayer
layers.library/CreateUpfrontLayer
layers.library/DeleteLayer
layers.library/DisposeLayerInfo
layers.library/EndUpdate
layers.library/FattenLayerInfo
layers.library/InitLayers
layers.library/InstallClipRegion
layers.library/LockLayer
layers.library/LockLayerInfo
layers.library/LockLayers
layers.library/MoveLayer
layers.library/MoveLayerInFrontOf
layers.library/NewLayerInfo
layers.library/ScrollLayer
layers.library/SizeLayer
layers.library/SwapBitsRastPortClipRect
layers.library/ThinLayerInfo
layers.library/UnlockLayer
layers.library/UnlockLayerInfo
layers.library/UnlockLayers
layers.library/UpfrontLayer
layers.library/WhichLayer


layers.library/BeginUpdate                           layers.library/BeginUpdate

    NAME
    BeginUpdate -- Prepare to repair damaged layer.

    SYNOPSIS
    result = BeginUpdate( l )
    d0                    a0

    BOOLEAN result;
    struct Layer *l;

    FUNCTION
    Convert damage list to ClipRect list and swap in for
    programmer to redraw through. This routine simulates
    the ROM library environment. The idea is to only render in the
    "damaged" areas, saving time over redrawing all of the layer.
    The layer is locked against changes made by the layer library.

    INPUTS
    l - pointer to a layer

    RESULTS
    result - TRUE if damage list converted to ClipRect list sucessfully.
         FALSE if list conversion aborted. (probably out of memory)

    BUGS
    If BeginUpdate returns FALSE, programmer must abort the attempt to
    refresh this layer and instead call EndUpdate( l, FALSE ) to restore
    original ClipRect and damage list.

    SEE ALSO
    EndUpdate, graphics/layers.h, graphics/clip.h


layers.library/BehindLayer                           layers.library/BehindLayer

    NAME
    BehindLayer -- Put layer behind other layers.

    SYNOPSIS
    result = BehindLayer( dummy, l )
    d0              a0     a1

    BOOLEAN result;
    LONG dummy;
    struct Layer *l;

    FUNCTION
    Move this layer to the most behind position swapping bits
    in and out of the display with other layers.
    If other layers are REFRESH then collect their damage lists and
    set the LAYERREFRESH bit in the Flags fields of those layers that
    may be revealed.  If this layer is a backdrop layer then
    put this layer behind all other backdrop layers.
    If this layer is NOT a backdrop layer then put in front of the
    top backdrop layer and behind all other layers.

    Note: this operation may generate refresh events in other layers
       associated with this layer's Layer_Info structure.

    INPUTS
    dummy - unused
    l - pointer to a layer

    RESULTS
    result - TRUE    if operation successful
         FALSE   if operation unsuccessful (probably out of memory)

    BUGS

    SEE ALSO
       graphics/layers.h, graphics/clip.h


layers.library/CreateBehindLayer               layers.library/CreateBehindLayer

    NAME
    CreateBehindLayer -- Create a new layer behind all existing layers.

    SYNOPSIS
    result = CreateBehindLayer(li,bm,x0,y0,x1,y1,flags [,bm2])
    d0                         a0 a1 d0 d1 d2 d3   d4  [ a2 ]

    struct Layer *result;
    struct Layer_Info *li;
    struct BitMap *bm;
    LONG x0,y0,x1,y1;
    LONG flags;
    struct BitMap *bm2;

    FUNCTION
    Create a new Layer of position and size (x0,y0)->(x1,y1)
    Make this layer of type found in flags.
    If SuperBitMap, use bm2 as pointer to real SuperBitMap,
    and copy contents of Superbitmap into display layer.
    If this layer is a backdrop layer then place it behind all
    other layers including other backdrop layers. If this is
    not a backdrop layer then place it behind all nonbackdrop
    layers.

    Note: when using SUPERBITMAP, you should also set LAYERSMART flag.

    INPUTS
    li - pointer to LayerInfo structure
    bm - pointer to common BitMap used by all Layers
    x0,y0 -    upper left hand corner of layer
    x1,y1 -    lower right hand corner of layer
    flags -    various types of layers supported as bit sets.
        (for bit definitions, see graphics/layers.h )
    bm2 - pointer to optional Super BitMap

    RESULTS
    result - pointer to Layer structure if successful
         NULL if not successful

    BUGS

    SEE ALSO
    DeleteLayer, graphics/layers.h, graphics/clip.h, graphics/gfx.h


layers.library/CreateUpfrontLayer             layers.library/CreateUpfrontLayer

    NAME
    CreateUpfrontLayer -- Create a new layer on top of existing layers.

    SYNOPSIS
    result = CreateUpfrontLayer(li,bm,x0,y0,x1,y1,flags [,bm2])
    d0                a0 a1 d0 d1 d2 d3   d4  [ a2 ]

    struct Layer *result;
    struct Layer_Info *li;
    struct BitMap *bm;
    LONG x0,y0,x1,y1;
    LONG flags;
    struct BitMap *bm2;

    FUNCTION
    Create a new Layer of position and size (x0,y0)->(x1,y1)
    and place it on top of all other layers.
    Make this layer of type found in flags
    if SuperBitMap, use bm2 as pointer to real SuperBitMap.
    and copy contents of Superbitmap into display layer.

    Note: when using SUPERBITMAP, you should also set LAYERSMART flag.

    INPUTS
    li - pointer to LayerInfo structure
    bm - pointer to common BitMap used by all Layers
    x0,y0 -    upper left hand corner of layer
    x1,y1 - lower right hand corner of layer
    flags -    various types of layers supported as bit sets.
    bm2 - pointer to optional Super BitMap

    RESULTS
    result - pointer to Layer structure if successful
         NULL if not successful

    BUGS

    SEE ALSO
       DeleteLayer, graphics/layers.h, graphics/clip.h, graphics/gfx.h


layers.library/DeleteLayer                           layers.library/DeleteLayer

    NAME
    DeleteLayer -- delete layer from layer list.

    SYNOPSIS
    result = DeleteLayer( dummy, l )
    d0                    a0,    a1

    BOOLEAN result;
    LONG dummy;
    struct Layer *l;

    FUNCTION
    Remove this layer from the list of layers.  Release memory
    associated with it.  Restore other layers that may have been
    obscured by it.  Trigger refresh in those that may need it.
    If this is a superbitmap layer make sure SuperBitMap is current.
    The SuperBitMap is not removed from the system but is available
    for program use even though the rest of the layer information has
    been deallocated.

    INPUTS
    dummy - unused
    l - pointer to a layer

    RESULTS
    result - TRUE if this layer successfully deleted from the system
         FALSE if layer not deleted. (probably out of memory )

    BUGS

    SEE ALSO
    graphics/layers.h, graphics/clip.h


layers.library/DisposeLayerInfo                 layers.library/DisposeLayerInfo

    NAME
    DisposeLayerInfo -- Return all memory for LayerInfo to memory pool

    SYNOPSIS
    DisposeLayerInfo(li)
             a0

    struct Layer_Info *li;

    FUNCTION
    return LayerInfo and any other memory attached to this LayerInfo
    to memory allocator.

    Note: if you wish to delete the layers associated with this Layer_Info
        structure, remember to call DeleteLayer() for each of the layers
        before calling DisposeLayerInfo().

    INPUTS
    li - pointer to LayerInfo structure

    EXAMPLE

    -- delete the layers associated this Layer_Info structure --

    DeleteLayer(li,simple_layer);
    DeleteLayer(li,smart_layer);

    -- see documentation on DeleteLayer about deleting SuperBitMap layers --
    my_super_bitmap_ptr = super_layer->SuperBitMap;
    DeleteLayer(li,super_layer);

    -- now dispose of the Layer_Info structure itself --
    DisposeLayerInfo(li);


    BUGS

    SEE ALSO
    DeleteLayer, graphics/layers.h


layers.library/EndUpdate                               layers.library/EndUpdate

    NAME
    EndUpdate -- remove damage list and restore state of layer to normal.

    SYNOPSIS
    EndUpdate( l, flag )
           a0  d0

    struct Layer *l;
    USHORT flag;

    FUNCTION
    After the programmer has redrawn his picture he calls this
    routine to restore the ClipRects to point to his standard
    layer tiling. The layer is then unlocked for access by the
    layer library.

    Note: use flag = FALSE if you are only making a partial update.
        You may use the other region functions (graphics functions such as
        OrRectRegion, AndRectRegion, and XorRectRegion ) to clip adjust
        the DamageList to reflect a partial update.

    INPUTS
    l - pointer to a layer
    flag - use TRUE if update was completed. The damage list is cleared.
           use FALSE if update not complete. The damage list is retained.

    EXAMPLE

    -- begin update for first part of two-part refresh --
    BeginUpdate(my_layer);

    -- do some refresh, but not all --
    my_partial_refresh_routine(my_layer);

    -- end update, false (not completely done refreshing yet) --
    EndUpdate(my_layer, FALSE);

    -- begin update for last part of refresh --
    BeginUpdate(my_layer);

    -- do rest of refresh --
    my_complete_refresh_routine(my_layer);

    -- end update, true (completely done refreshing now) --
    EndUpdate(my_layer, TRUE);

    BUGS

    SEE ALSO
    BeginUpdate, graphics/layers.h, graphics/clip.h


layers.library/FattenLayerInfo                   layers.library/FattenLayerInfo

    NAME
    FattenLayerInfo -- convert 1.0 LayerInfo to 1.1 LayerInfo
    OBSOLETE OBSOLETE OBSOLETE OBSOLETE OBSOLETE

    SYNOPSIS
    OBSOLETE OBSOLETE OBSOLETE OBSOLETE OBSOLETE
    FattenLayerInfo(li)
            a0

    struct Layer_Info *li;
    OBSOLETE OBSOLETE OBSOLETE OBSOLETE OBSOLETE

    FUNCTION
    V1.1 software and any later releases need to have more info in the
    Layer_Info structure. To do this in a 1.0 supportable manner requires
    allocation and deallocation of the memory whenever most
    layer library functions are called.  To prevent unnecessary
    allocation/deallocation FattenLayerInfo will preallocate the
    necessary data structures and fake out the layer library into
    thinking it has a LayerInfo gotten from NewLayerInfo.
    NewLayerInfo is the approved method for getting this structure.
    When a program needs to give up the LayerInfo structure it
    must call ThinLayerInfo before freeing the memory. ThinLayerInfo
    is not necessary if New/DisposeLayerInfo are used however.

    INPUTS
    li - pointer to LayerInfo structure

    BUGS

    SEE ALSO

    NewLayerInfo, ThinLayerInfo, DisposeLayerInfo, graphics/layers.h


layers.library/InitLayers                             layers.library/InitLayers

    NAME
    InitLayers -- Initialize Layer_Info structure
    OBSOLETE OBSOLETE OBSOLETE OBSOLETE OBSOLETE

    SYNOPSIS
    OBSOLETE OBSOLETE OBSOLETE OBSOLETE OBSOLETE
    InitLayers(li)
           a0

    struct Layer_Info *li;
    OBSOLETE OBSOLETE OBSOLETE OBSOLETE OBSOLETE

    FUNCTION
    Initialize Layer_Info structure in preparation to use
    other layer operations on this list of layers.
    Make the Layers unlocked (open), available to layer operations.

    INPUTS
    li - pointer to LayerInfo structure

    BUGS

    SEE ALSO
    NewLayerInfo, DisposeLayerInfo, graphics/layers.h


layers.library/InstallClipRegion               layers.library/InstallClipRegion

    NAME
    InstallClipRegion -- Install clip region in layer

    SYNOPSIS
    oldclipregion = InstallClipRegion( l,  region )
    d0                   a0  a1

    struct Region *oldclipregion;
    struct Layer *l;
    struct Region *region;

    FUNCTION
    Installs a transparent Clip region in the layer. All
    subsequent graphics calls will be clipped to this region.
    You MUST remember to call InstallClipRegion(l,NULL) before
    calling DeleteLayer(l) or the Intuition function CloseWindow()
     if you have installed a non-NULL ClipRegion in l.

    INPUTS
    l - pointer to a layer
    region - pointer to a region

    RESULTS
    oldclipregion - The pointer to the previous ClipRegion that
        was installed. Returns NULL if no previous ClipRegion installed.

        Note: If the system runs out of memory while computing the
        resulting ClipRects the LAYERS_CLIPRECTS_LOST bit will
        be set in l->Flags.

    BUGS
    If the system runs out of memory during normal layer operations,
    the ClipRect list may get swept away and not restored.
    As soon as there is enough memory and the layer library
    gets called again the ClipRect list will be rebuilt.

    SEE ALSO
    BeginUpdate EndUpdate,
    graphics/layers.h, graphics/clip.h, graphics/regions.h


layers.library/LockLayer                               layers.library/LockLayer

    NAME
    LockLayer -- Lock layer to make changes to ClipRects.

    SYNOPSIS
    LockLayer( dummy, l )
           a0     a1

    LONG dummy;
    struct Layer *l;

    FUNCTION
    Make this layer unavailable for other tasks to use.
    If another task is already using this layer then wait for
    it to complete and then reserve the layer for your own use.
    (this function does the same thing as graphics.library/LockLayerRom)

    Note: if you wish to lock MORE THAN ONE layer at a time, you
        must call LockLayerInfo() before locking those layers and
        then call UnlockLayerInfo() when you have finished. This
        is to prevent system "deadlocks".

    Further Note: while you hold the lock on a layer, Intuition will block
        on operations such as windowsizing, dragging, menus, and depth
        arranging windows in this layer's screen.  It is recommended that
        YOU do not make Intuition function calls while the layer is locked.

    INPUTS
    dummy - unused
    l - pointer to a layer

    BUGS

    SEE ALSO
    UnlockLayer, LockLayerInfo, UnlockLayerInfo,
    graphics.library/LockLayerRom, graphics/layers.h, graphics/clip.h


layers.library/LockLayerInfo                       layers.library/LockLayerInfo

    NAME
    LockLayerInfo -- Lock the LayerInfo structure.

    SYNOPSIS
    LockLayerInfo( li )
               a0

    struct Layer_Info *li;

    FUNCTION
    Before doing an operation that requires the LayerInfo
    structure, make sure that no other task is also using the
    LayerInfo structure.  LockLayerInfo() returns when the
    LayerInfo belongs to this task.  There should be
    an UnlockLayerInfo for every LockLayerInfo.

    Note: All layer routines presently LockLayerInfo() when they
    start up and UnlockLayerInfo() as they exit.  Programmers
    will need to use these Lock/Unlock routines if they wish
    to do something with the LayerStructure that is not
    supported by the layer library.

    INPUTS
    li - pointer to Layer_Info structure

    BUGS

    SEE ALSO
    UnlockLayerInfo, graphics/layers.h


layers.library/LockLayers                             layers.library/LockLayers

    NAME
    LockLayers -- lock all layers from graphics output.

    SYNOPSIS
    LockLayers( li )
            a0

    struct Layer_Info *li;

    FUNCTION
    First calls LockLayerInfo()
    Make all layers in this layer list locked.

    INPUTS
    li - pointer to Layer_Info structure

    BUGS

    SEE ALSO
    LockLayer, LockLayerInfo, graphics/layers.h


layers.library/MoveLayer                               layers.library/MoveLayer

    NAME
    MoveLayer -- Move layer to new position in BitMap.

    SYNOPSIS
    result = MoveLayer( dummy, l, dx, dy )
    d0            a0     a1 d0  d1

    BOOLEAN result;
    LONG dummmy;
    struct Layer *l;
    LONG dx,dy;

    FUNCTION
    Move this layer to new position in shared BitMap.
    If any refresh layers become revealed, collect damage and
    set REFRESH bit in layer Flags.

    INPUTS
    dummy - unused
    l - pointer to a nonbackdrop layer
    dx - delta to add to current x position
    dy - delta to add to current y position

    RETURNS
    result - TRUE if operation successful
         FALSE if failed (out of memory)

    BUGS
    May not handle (dx,dy) which attempts to move the layer ouside the
    layer's RastPort->BitMap bounds .

    SEE ALSO
    graphics/layers.h, graphics/clip.h


layers.library/MoveLayerInFrontOf             layers.library/MoveLayerInFrontOf

    NAME
    MoveLayerInFrontOf-- Put layer in front of another layer.

    SYNOPSIS
    result = MoveLayerInFrontOf( layertomove, targetlayer )
                     a0           a1
    BOOLEAN result;
    struct Layer *layertomove;
    struct Layer *targetlayer;

    FUNCTION
    Move this layer in front of target layer, swapping bits
    in and out of the display with other layers.
    If this is a refresh layer then collect damage list and
    set the LAYERREFRESH bit in layer->Flags if redraw required.

    Note: this operation may generate refresh events in other layers
       associated with this layer's Layer_Info structure.

    INPUTS
    layertomove - pointer to layer which should be moved
    targetlayer - pointer to target layer in front of which to move layer

    RESULTS
    result = TRUE    if operation successful
         FALSE   if operation unsuccessful (probably out of memory)

    BUGS

    SEE ALSO
    graphics/layers.h


layers.library/NewLayerInfo                         layers.library/NewLayerInfo

    NAME
    NewLayerInfo -- Allocate and Initialize full Layer_Info structure.

    SYNOPSIS
    result = NewLayerInfo()
    d0

    struct Layer_Info *result;

    FUNCTION
    Allocate memory required for full Layer_Info structure.
    Initialize Layer_Info structure in preparation to use
    other layer operations on this list of layers.
    Make the Layer_Info unlocked (open).

    INPUTS
    None

RESULT
    result- pointer to Layer_Info structure if successful
        NULL if not enough memory

BUGS

SEE ALSO
    graphics/layers.h


layers.library/ScrollLayer                           layers.library/ScrollLayer

    NAME
    ScrollLayer -- Scroll around in a superbitmap, translate coordinates
                      in non-superbitmap layer.

    SYNOPSIS
    ScrollLayer( dummy, l, dx, dy )
             a0     a1 d0  d1

    LONG dummy;
    struct Layer *l;
    LONG dx,dy;

    FUNCTION
    For a SuperBitMap Layer:
    Update the SuperBitMap from the layer display, then copy bits
    between Layer and SuperBitMap to reposition layer over different
    portion of SuperBitMap.
    For nonSuperBitMap layers, all (x,y) pairs are adjusted by
    the scroll(x,y) value in the layer.  To cause (0,0) to actually
    be drawn at (3,10) use ScrollLayer(-3,-10). This can be useful
    along with InstallClipRegion to simulate Intuition GZZWindows
    without the overhead of an extra layer.

    INPUTS
    dummy - unused
    l - pointer to a layer
    dx - delta to add to current x scroll value
    dy - delta to add to current y scroll value

    BUGS
    May not handle (dx,dy) which attempts to move the layer ouside the
    layer's SuperBitMap bounds.

    SEE ALSO
    graphics/layers.h


layers.library/SizeLayer                               layers.library/SizeLayer

    NAME
    SizeLayer -- Change the size of this nonbackdrop layer.

    SYNOPSIS
    result = SizeLayer( dummy, l, dx, dy )
    d0            a0     a1 d0  d1

    BOOLEAN result;
    LONG dummy;
    struct Layer *l;
    LONG dx, dy;

    FUNCTION
    Change the size of this layer by (dx,dy). The lower right hand
    corner is extended to make room for the larger layer.
    If there is SuperBitMap for this layer then copy pixels into
    or out of the layer depending on whether the layer increases or
    decreases in size.  Collect damage list for those layers that may
    need to be refreshed if damage occurred.

    INPUTS
    dummy - unused
    l - pointer to a nonbackdrop layer
    dx - delta to add to current x size
    dy - delta to add to current y size

    RESULTS
    result - TRUE if operation successful
         FALSE if failed (out of memory)

    BUGS

    SEE ALSO
    graphics/layers.h, graphics/clip.h


layers.library/SwapBitsRastPortClipRect

    NAME
    SwapBitsRastPortClipRect -- Swap bits between common bitmap
                    and obscured ClipRect

    SYNOPSIS
    SwapBitsRastPortClipRect( rp, cr )
                  a0  a1

    struct RastPort *rp;
    struct ClipRect *cr;

    FUNCTION
    Support routine useful for those that need to do some
    operations not done by the layer library.  Allows programmer
    to swap the contents of a small BitMap with a subsection of
    the display. This is accomplished without using extra memory.
    The bits in the display RastPort are exchanged with the
    bits in the ClipRect's BitMap.

    Note: the ClipRect structures which the layer library allocates are
    actually a little bigger than those described in the graphics/clip.h
    include file.  So be warned that it is not a good idea to have
    instances of cliprects in your code.

    INPUTS
       rp - pointer to rastport
       cr - pointer to cliprect to swap bits with

    BUGS

    SEE ALSO
    graphics/clip.h, graphics/rastport.h, graphics/clip.h


layers.library/ThinLayerInfo                       layers.library/ThinLayerInfo

    NAME
    ThinLayerInfo -- convert 1.1 LayerInfo to 1.0 LayerInfo.
    OBSOLETE OBSOLETE OBSOLETE OBSOLETE OBSOLETE

    SYNOPSIS
    OBSOLETE OBSOLETE OBSOLETE OBSOLETE OBSOLETE
    ThinLayerInfo(li)
              a0

    struct Layer_Info *li;
    OBSOLETE OBSOLETE OBSOLETE OBSOLETE OBSOLETE

    FUNCTION
    return the extra memory needed that was allocated with
    FattenLayerInfo. This is must be done prior to freeing
    the Layer_Info structure itself. V1.1 software should be
    using DisposeLayerInfo.

    INPUTS
    li - pointer to LayerInfo structure

    BUGS

    SEE ALSO
    DisposeLayerInfo, FattenLayerInfo, graphics/layers.h


layers.library/UnlockLayer                           layers.library/UnlockLayer

    NAME
    UnlockLayer -- Unlock layer and allow graphics routines to use it.

    SYNOPSIS
    UnlockLayer( l )
             a0

    struct Layer *l;

    FUNCTION
    When finished changing the ClipRects or whatever you were
    doing with this layer you must call UnlockLayer() to allow
    other tasks to proceed with graphic output to the layer.

    INPUTS
    l - pointer to a layer

    BUGS

    SEE ALSO
    graphics/layers.h, graphics/clip.h


layers.library/UnlockLayerInfo                   layers.library/UnlockLayerInfo

    NAME
    UnlockLayerInfo -- Unlock the LayerInfo structure.

    SYNOPSIS
    UnlockLayerInfo( li )
             a0

    struct Layer_Info *li;

    FUNCTION
    After the operation is complete that required a LockLayerInfo,
    unlock the LayerInfo structure so that  other tasks may
    affect the layers.

    INPUTS
    li - pointer to the Layer_Info structure

     BUGS

     SEE ALSO
    LockLayerInfo, graphics/layers.h


layers.library/UnlockLayers                         layers.library/UnlockLayers

    NAME
    UnlockLayers -- Unlock all layers from graphics output.
            Restart graphics output to layers that have been waiting

    SYNOPSIS
    UnlockLayers( li )
              a0

    struct Layer_Info *li;

    FUNCTION
    Make all layers in this layer list unlocked.
    Then call UnlockLayerInfo

    INPUTS
    li - pointer to the Layer_Info structure

    BUGS

    SEE ALSO
    LockLayers, UnlockLayer, graphics/layers.h


layers.library/UpfrontLayer                         layers.library/UpfrontLayer

    NAME
    UpfrontLayer -- Put layer in front of all other layers.

    SYNOPSIS
    result = UpfrontLayer( dummy, l )
    do               a0  a1

    BOOLEAN result;
    LONG dummy;
    struct Layer *l;

    FUNCTION
    Move this layer to the most upfront position swapping bits
    in and out of the display with other layers.
    If this is a refresh layer then collect damage list and
    set the LAYERREFRESH bit in layer->Flags if redraw required.
    By clearing the BACKDROP bit in the layers Flags you may
    bring a Backdrop layer up to the front of all other layers.

    Note: this operation may generate refresh events in other layers
       associated with this layer's Layer_Info structure.

    INPUTS
    dummy - unused
    l - pointer to a nonbackdrop layer

    RESULTS
    result - TRUE    if operation successful
         FALSE    if operation unsuccessful (probably out of memory)

    BUGS

    SEE ALSO
    graphics/layers.h


layers.library/WhichLayer                             layers.library/WhichLayer

    NAME
    WhichLayer -- Which Layer is this point in?

    SYNOPSIS
    layer = WhichLayer( li, x, y )
    d0            a0  d0 d1

    FUNCTION
    Starting at the topmost layer check to see if this point (x,y)
        occurs in this layer.  If it does return the pointer to this
        layer. Return NULL if there is no layer at this point.

    INPUTS
    li  = pointer to LayerInfo structure
    (x,y)    = coordinate in the BitMap

    RESULTS
    layer - pointer to the topmost layer that this point is in
        NULL if this point is not in a layer

    SEE ALSO
    graphics/layers.h