function hScrollpanel = imscrollpanel(varargin)
%IMSCROLLPANEL Scroll panel for interactive image navigation.
%   HPANEL = IMSCROLLPANEL(HPARENT, HIMAGE) creates a scroll panel
%   containing the target image (the image to be navigated). HIMAGE
%   is a handle to the target HIMAGE.  HPARENT is the handle to the
%   figure or uipanel object that will contain the new scroll panel.
%   HPANEL is the handle to the scroll panel, which is a uipanel object.
%
%   A scroll panel makes an image scrollable. If the size or magnification
%   makes an image too large to display in a figure on the screen, the
%   scroll panel displays a portion of the image at 100% magnification
%   (one screen pixel represents one image pixel). The scroll panel adds
%   horizontal and vertical scroll bars to enable navigation around the
%   image.
%
%   IMSCROLLPANEL changes the object hierarchy of the target image. Instead
%   of the familiar figure->axes->image object hierarchy, IMSCROLLPANEL
%   inserts several uipanel and uicontrol objects between the figure and 
%   the axes object. 
%
%   API Function Syntaxes
%   ---------------------
%   A scroll panel contains a structure of function handles,
%   called an API. You can use the functions in this API to manipulate
%   the scroll panel. To retrieve this structure, use the IPTGETAPI
%   function, as in the following.
%
%       api = iptgetapi(HPANEL)
%
%   Functions in the API, listed in the order they appear in the structure,
%   include:
%
%   setMagnification
%
%       Sets the magnification of the target image in units of 
%       screen pixels per image pixel.
%
%           api.setMagnification(new_mag) 
%
%       where new_mag is a scalar magnification factor.
%
%   getMagnification
%
%      Returns the current magnification factor of the target image
%      in units of screen pixels per image pixel.
%
%           mag = api.getMagnification()
%
%      Multiply mag by 100 to convert to percentage. For example,
%      if mag=2, the magnification is 200%.
%
%   setMagnificationAndCenter 
%
%       Changes the magnification and makes the point cx,cy in the
%       target image appear in the center of the scroll panel. This
%       operation is equivalent to a simultaneous zoom and recenter.
%
%           api.setMagnificationAndCenter(mag,cx,cy) 
%
%   findFitMag                
%
%       Returns the magnification factor that would make the target 
%       image just fit in the scroll panel.
%
%           mag = api.findFitMag() 
%
%   setVisibleLocation        
%
%       Moves the target image so that the specified location is
%       visible. Scrollbars update.
%
%           api.setVisibleLocation(xmin,ymin) 
%           api.setVisibleLocation([xmin ymin]) 
%
%   getVisibleLocation
%   
%       Returns the location of the currently visible portion of the
%       target image.
%
%           loc = api.getVisibleLocation()
%
%       where loc is a vector [xmin ymin].
%
%   getVisibleImageRect
%   
%       Returns the current visible portion of the image.
%
%           r = api.getVisibleImageRect()
%
%       where r is a rectangle [xmin ymin width height].
%
%   addNewMagnificationCallback
%
%       Adds the function handle FCN to the list of new-magnification callback
%       functions.
%
%           id = api.addNewMagnificationCallback(fcn)
%
%       Whenever the scroll panel magnification changes, each function in
%       the list is called with the syntax:
%
%           fcn(mag)
% 
%       where mag is a scalar magnification factor. 
%
%       The return value, id, is used only with
%       removeNewMagnificationCallback.
%
%   removeNewMagnificationCallback
%
%       Removes the corresponding function from the new-magnification callback
%       list.
%
%           api.removeNewMagnificationCallback(id)
%
%       where id is the identifier returned by
%       api.addNewMagnificationCallback.
%
%   addNewLocationCallback
%
%       Adds the function handle FCN to the list of new-location callback
%       functions.
%
%           id = api.addNewLocationCallback(fcn)
%
%       Whenever the scroll panel location changes, each function in
%       the list is called with the syntax:
%
%           fcn(loc) 
%
%       where loc is [xmin ymin].
%
%       The return value, id, is used only with
%       removeNewLocationCallback.
%
%   removeNewLocationCallback
%
%       Removes the corresponding function from the new-location callback
%       list.
%
%           api.removeNewLocationCallback(id)
%
%       where id is the identifier returned by
%       api.addNewLocationCallback.
%
%   Notes
%   -----
%   Scrollbar navigation as provided by IMSCROLLPANEL is incompatible with the
%   default MATLAB figure navigation buttons (pan, zoom in, zoom out). The
%   corresponding menu items and toolbar buttons should be removed in a custom
%   GUI that includes a scrollable uipanel created by IMSCROLLPANEL.
%
%   When you run IMSCROLLPANEL, it appears to take over the entire figure
%   because by default HPANEL has 'Units' set to 'normalized' and 'Position'
%   set to [0 0 1 1]. If you want to see other children of HPARENT while
%   using your new scroll panel, you must manually set the 'Position' property
%   of HPANEL.
%
%   Example
%   -------
%
%       % Create a scroll panel
%       hFig = figure('Toolbar','none',...
%                     'Menubar','none');
%       hIm = imshow('saturn.png');
%       hSP = imscrollpanel(hFig,hIm);
%       set(hSP,'Units','normalized',...
%               'Position',[0 .1 1 .9])
%       
%       % Add a magnification box and an overview tool
%       hMagBox = immagbox(hFig,hIm); 
%       pos = get(hMagBox,'Position');
%       set(hMagBox,'Position',[0 0 pos(3) pos(4)])
%       imoverview(hIm) 
%
%       % Get the scroll panel API to programmatically control the view
%       api = iptgetapi(hSP);
%
%       % Get the current magnification and position
%       mag = api.getMagnification() 
%       r = api.getVisibleImageRect()
%
%       % View the top left corner of the image
%       api.setVisibleLocation(0.5,0.5)
%
%       % Change the magnification to the value that just fits
%       api.setMagnification(api.findFitMag())  
%
%       % Zoom in to 1600% on the dark spot
%       api.setMagnificationAndCenter(16,306,800)  
%
%   See also IMMAGBOX, IMOVERVIEW, IMOVERVIEWPANEL, IMTOOL, IPTGETAPI.

%   Copyright 1993-2004 The MathWorks, Inc.  
%   $Revision: 1.1.6.1 $  $Date: 2004/11/19 17:36:48 $

   iptchecknargin(2, 2, nargin, mfilename);
   parent = varargin{1};
   hIm = varargin{2};

   iptcheckhandle(parent,{'figure','uipanel','uicontainer'},mfilename,'HPARENT',1)
   iptcheckhandle(hIm,{'image'},mfilename,'HIMAGE',2)
   
   hAx = ancestor(hIm,'Axes');
   
   % Check that hIm is only image child of axes
   axKids = get(hAx,'Children');
   axImKids = findobj(axKids,'flat','Type','image');
   if ~isequal(get(hIm,'parent'),hAx) || ~isscalar(axImKids)
       eid =  sprintf('Images:%s:axDoesNotContainOneImage',mfilename);
       error(eid,'Expected axes to contain a single image object HIMAGE.')
   end

   % Check that axes is a child of parent
   if ~isequal(get(hAx,'parent'),parent)
       eid =  sprintf('Images:%s:axNotChildOfParent',mfilename);
       error(eid,'Expected axes containing HIMAGE to be a child of HPARENT.')
   end

   kids = get(parent,'Children');
   axKids = findobj(kids,'flat','Type','axes');
   
   img = get(hIm,'cdata');
   imageWidth  = size(img,2);
   imageHeight = size(img,1); 
   
   % Force imageWidth and imageHeight to be nonzero in case either is
   % zero. Also fake the XData and YData as needed. - geck 221748
   if imageWidth==0
       imageWidth  = 1;
       set(hIm,'XData',[1 1])
   end
   if imageHeight==0
       imageHeight  = 1;
       set(hIm,'YData',[1 1])
   end
   
   % Check if XData or YData are non-default, warn and reset
   [isXDataDefault,isYDataDefault] = isDefaultXYData(hIm);
   if ~isXDataDefault || ~isYDataDefault
       wid =  sprintf('Images:%s:nonDefaultXDataOrYData',mfilename);
       msg1 = sprintf('IMSCROLLPANEL currently requires default XData and YData.\n'); 
       
       if ~isXDataDefault
           set(hIm,'XData',[1 imageWidth])
           msgXData = sprintf('HIM has non-default XData, resetting to [%d %d].\n',...
                              1,imageWidth);
       else
           msgXData = '';
       end

       if ~isYDataDefault
           set(hIm,'YData',[1 imageHeight])
           msgYData = sprintf('HIM has non-default YData, resetting to [%d %d].',...
                              1,imageHeight);
       else
           msgYData = '';
       end
       
       warning(wid,'%s%s%s',msg1,msgXData,msgYData)
   end
   
   sbw = 13; % scroll bar width
   
   % Magnification as ratio of screen per image pixel.
   screenPerImagePixel = 1; 

   maxMag = 1024; % arbritrary big choice, 1024 screen pixels for one
                  % image pixel seems more than anyone could want
   
   % hScrollpanel contains:
   %    hSliderHor
   %    hSliderVer
   %    hFrame
   %    hScrollable
   hScrollpanel= uipanel(...
       'BorderType','none',...
       'parent', parent,...
       'Units','normalized',...
       'Position',[0 0 1 1]);
      
   sliderColor = [.9 .9 .9];
   
   hSliderHor = uicontrol(...
       'Style','slider',...       
       'Parent',hScrollpanel,...
       'Value', 0.5,...
       'BackgroundColor',sliderColor);
   
   hSliderVer = uicontrol(...
       'Style','slider',...       
       'Parent',hScrollpanel,...
       'Value', 0.5,...
       'BackgroundColor',sliderColor);
   
   if isJavaFigure
       % Must use these ActionEvents to get continuous events fired as slider
       % thumb is dragged. Regular callbacks on sliders give only one event
       % when the thumb is released.
       hSliderHorListener = handle.listener(hSliderHor,...
                                            'ActionEvent',@scrollHorizontal);
       hSliderVerListener = handle.listener(hSliderVer,...
                                            'ActionEvent',@scrollVertical);
       setappdata(hScrollpanel,'sliderListeners',...
           [hSliderHorListener hSliderVerListener]);
   else
       % Unfortunately, the ActionEvent route is only available with Java
       % Figures, so platforms without Java Figure support get discrete
       % events only when the mouse is released from dragging the slider
       % thumb.
       set(hSliderHor,'callback',@scrollHorizontal)
       set(hSliderVer,'callback',@scrollVertical)       
   end
   
   hFrame = uicontrol(...
       'Style','frame',...       
       'Parent',hScrollpanel);
   
   % hScrollpanel contains:
   %    hAx
   hScrollable = uipanel(...
       'BorderType','none',...
       'parent', hScrollpanel,...
       'units', 'pixels',...
       'Position',[1 1 getOnScreenImW getOnScreenImH]);
   
   setChildColorToMatchParent(hScrollable,parent);
   
   % Position axes just above horizontal scrollbar to make coordinate
   % calculations simpler.
   set(hAx,...
       'Parent',hScrollable,...
       'Units','pixels',...
       'Position',[1 1 getOnScreenImW getOnScreenImH],... 
       'TickDir', 'out', ...
       'XGrid', 'off', ...
       'YGrid', 'off', ...
       'XLim',[.5 (imageWidth  + .5)],... % in case someone set xlim,ylim
       'YLim',[.5 (imageHeight + .5)],...       
       'Visible','off');
   
   viewport = []; % gets set by call to updatePositions

   updatePositions
   
   % newMagnificationCallbackFunctions is used by sendNewMagnification() to
   % notify interested parties whenever the magnification changes.
   newMagnificationCallbackFunctions = makeList;
   
   % Pattern for set associated with callbacks that get called as a
   % result of the set.
   insideSetMagnification = false;

   % newLcationCallbackFunctions is used by sendNewLocation() to
   % notify interested parties whenever the location changes.
   newLocationCallbackFunctions = makeList;

   % Pattern for set associated with callbacks that get called as a
   % result of the set.
   insideSetVisibleLocation = false;
   
   % Initialize so scrollbars are in sync with location of image
   cx = 0.5 + imageWidth/2; % Must add 0.5 for default spatial coords
   cy = 0.5 + imageHeight/2;
   setMagnificationAndCenter(screenPerImagePixel,cx,cy)
   
   set(hScrollpanel,'ResizeFcn',@resizeView);   
   
   api.setMagnification                = @setMagnification;
   api.getMagnification                = @getMagnification;
   api.setMagnificationAndCenter       = @setMagnificationAndCenter;   
   api.findFitMag                      = @findFitMag;
   api.setVisibleLocation              = @setVisibleLocation;
   api.getVisibleLocation              = @getVisibleLocation;   
   api.getVisibleImageRect             = @getVisibleImageRect;
   api.addNewMagnificationCallback     = @addNewMagnificationCallback;
   api.removeNewMagnificationCallback  = @removeNewMagnificationCallback;
   api.addNewLocationCallback          = @addNewLocationCallback;
   api.removeNewLocationCallback       = @removeNewLocationCallback;

   % undocumented interface, may change in the future
   api.setImageButtonDownFcn           = @setImageButtonDownFcn;
   api.findMagnification               = @findMagnification;
   api.turnOffScrollpanel              = @turnOffScrollpanel;   
   
   setappdata(hScrollpanel,'API',api);

   %----------------------------------
   function setImageButtonDownFcn(h)
   
     set(hIm,'ButtonDownFcn',h)
     
   end   

   %----------------------------------------
   function id = addNewMagnificationCallback(fun)
     id = newMagnificationCallbackFunctions.appendItem(fun);
   end

   %-------------------------------------
   function removeNewMagnificationCallback(id)
     newMagnificationCallbackFunctions.removeItem(id);
   end

   %----------------------------------------
   function id = addNewLocationCallback(fun)
     id = newLocationCallbackFunctions.appendItem(fun);
   end

   %-------------------------------------
   function removeNewLocationCallback(id)
     newLocationCallbackFunctions.removeItem(id);
   end

   %----------------------------------
   function turnOffScrollpanel()
   % This function turns off the scrollpanel. First, it sets the axes parent
   % to the current parent of the scrollpanel, then it deletes the handle
   % to the scrollpanel. This is needed by clients who want to reuse the
   % handles to an image and/or axes object.
   
     spParent = parent;
     if ishandle(spParent)
        set(hAx,'Parent',spParent)
        delete(hScrollpanel)
     end
     
   end   
   
   %----------------------------------
   function ratio = getMagnification()

      if ishandle(parent)
          ratio = screenPerImagePixel;
      else
          ratio = [];
      end

   end

   %-------------------------------
   function setMagnification(ratio)

     % Pattern to break recursion
     if insideSetMagnification
         return
     else
         insideSetMagnification = true;
     end     

     if ishandle(parent)      
         
         minMag = 1/min(imageWidth,imageHeight); % always show at least one
                                                 % pixel in each dimension
   
         ratio = max(min(ratio,maxMag),minMag);
         
         % Get current center so we can hold it.
         r = getVisibleImageRect;
         cx = r(1) + r(3)/2;
         cy = r(2) + r(4)/2;     
         [xmin,ymin] = findXminYmin(cx,cy,ratio);

         screenPerImagePixel = ratio;      
         setVisibleLocation(xmin,ymin) % Set location to hold center cx,cy
         resizeView
         sendNewMagnification
      end

      % Pattern to break recursion
      insideSetMagnification = false;
      
   end
   
   %--------------------------------------
   function sendNewMagnification(varargin)

     list = newMagnificationCallbackFunctions.getList();
     for k = 1:numel(list)
       fun = list{k};
       fun(screenPerImagePixel);
     end

   end
   
   %----------------------------------   
   function fitMag = findFitMag

      fitMag = findMagnification(imageWidth, imageHeight);

      %TODO: return getMagComboBox().findMinAllowableMag(fitMag);

   end

   %----------------------------------   
   function loc = getVisibleLocation

     pos = getVisibleImageRect();
     loc = pos(1:2);
   
   end
   
   %----------------------------------   
   function pos = getVisibleImageRect
      
      % xmin, ymin should be edge of pixel with row=1, col=1.
      % The units are user units as defined by XData and YData
      
      hScrollablePos = get(hScrollable,'Position');

      xdata = get(hIm,'XData');
      ydata = get(hIm,'YData');      
          
      if isFullImageWShowing
          [xmin,width] = getMinAndDim(xdata,imageWidth);
      else
          dxOnePixel = getDeltaOnePixel(xdata,imageWidth);
          xmin = -dxOnePixel * (hScrollablePos(1)-1)/screenPerImagePixel + ...
                 xdata(1) - dxOnePixel/2;
          width = dxOnePixel * viewport(1) / screenPerImagePixel;
      
      end
      
      if isFullImageHShowing
          ydata = get(hIm,'YData');          
          [ymin,height] = getMinAndDim(ydata,imageHeight);
          
      else
          dyOnePixel = getDeltaOnePixel(ydata,imageHeight);

          % account for scrollbar if showing
          hScrollablePosY = hScrollablePos(2);
          if isSliderHorShowing
              hScrollablePosY = hScrollablePosY - sbw; % shift Y-origin by sbw
          end
          
          maxYInScreenPixels = getOnScreenImH - viewport(2);
          ymin = dyOnePixel*(maxYInScreenPixels + (hScrollablePosY-1) )/screenPerImagePixel + ...
                 ydata(1) - dyOnePixel/2;
                    
          height = dyOnePixel * viewport(2) / screenPerImagePixel;
      
      end
      
      pos = [xmin ymin width height];
      
      %-------------------------------------------
      function [dimMin,dim] = getMinAndDim(dimData,imDim)

          delta = dimData(2) - dimData(1);
          deltaOnePixel = getDeltaOnePixel(dimData,imDim);          
          dimMin = dimData(1) - deltaOnePixel/2;
          dim = delta + deltaOnePixel;

      end
          
   end

   %----------------------------------   
   function setVisibleLocation(varargin)

     % Pattern to break recursion
     if insideSetVisibleLocation
         return
     else
         insideSetVisibleLocation = true;
     end     
   
     iptchecknargin(1, 2, nargin, sprintf('%s/setVisibleLocation',mfilename));
    
     switch nargin
       case 1
         loc = varargin{1};
         xIm = loc(1);
         yIm = loc(2);
       case 2
         xIm = varargin{1};
         yIm = varargin{2};                           
     end     
   
   % xIm, yIm is the location of the minimum image coordinates in the
   % corner of the viewport. 
   %
   % If 'YDir' is 'reverse' this will be the top left corner. If 'YDir' is
   % 'normal' this will be the bottom left corner.
   
      pos = get(hScrollable,'Position');
   
      if isFullImageWShowing
          % Do nothing, ignore x
      else
          xdata = get(hIm,'XData');
          dxOnePixel = getDeltaOnePixel(xdata,imageWidth);
          
          pos(1) = -(xIm - xdata(1) + dxOnePixel/2)*screenPerImagePixel / ...
                   dxOnePixel + 1;
            
          numerator = 1 - pos(1);
          xSlider = numerator/(getOnScreenImW - viewport(1));
          xSlider = min(max(xSlider,0),1);
          set(hSliderHor,'Value',xSlider)
      end
      
      if isFullImageHShowing
          % Do nothing, ignore y
      else
          ydata = get(hIm,'YData');
          dyOnePixel = getDeltaOnePixel(ydata,imageHeight);

          maxYInScreenPixels = getOnScreenImH - viewport(2);
          pos(2) = (yIm - ydata(1) + dyOnePixel/2)*screenPerImagePixel / ...
                   dyOnePixel - maxYInScreenPixels + 1;
          if isSliderHorShowing
              pos(2) = pos(2) + sbw; % shift Y-origin by sbw
          end
          
          numerator = pos(2) - 1 + maxYInScreenPixels;
          if isSliderHorShowing
              numerator = numerator - sbw;
          end
          ySlider = 1 - numerator/(getOnScreenImH - viewport(2));
          ySlider = min(max(ySlider,0),1);          
          set(hSliderVer,'Value',ySlider)
      end

      set(hScrollable,'Position',pos)
      
      sendNewLocation
     
      % Pattern to break recursion
      insideSetVisibleLocation = false;      
      
   end

   %--------------------------------------
   function sendNewLocation(varargin)

     list = newLocationCallbackFunctions.getList();
     for k = 1:numel(list)
       fun = list{k};
       fun(getVisibleLocation);
     end

   end
   
   %----------------------------------   
   function setMagnificationAndCenter(s,cx,cy)
   
   % cx and cy are center of new viewport
   % s is requested mag in screenPerImagePixels
   
     if (s <= maxMag)  
         % Find xmin,ymin that correspond to holding this cx, cy in the center.
         screenPerImagePixel = s;
         [xmin,ymin] = findXminYmin(cx,cy,s);

         setMagnification(s) 
         setVisibleLocation(xmin,ymin)  
     end
     
   end

   %----------------------------------   
   function [xmin,ymin] = findXminYmin(desired_cx,desired_cy,s)
   
   % desired_cx, desired_cy is the requested center in image coordinates.
   
     imW = imageWidth;
     imH = imageHeight;
    
     vpW = viewport(1);
     vpH = viewport(2);
    
     visImW = findVisImageDim(imW,vpW);
     visImH = findVisImageDim(imH,vpH); 
    
     onScreenImW = imW*s;
     onScreenImH = imH*s;
     
     xmin = findValidImTL(desired_cx,imW,onScreenImW,vpW,visImW);
     ymin = findValidImTL(desired_cy,imH,onScreenImH,vpH,visImH);
        
       %-------------------------------------------
       function dim = findVisImageDim(imDim, vpDim)

         visibleImDim = vpDim/s;
         dim = min(visibleImDim,imDim);

       end

       %-------------------------------------------------
       function TL = findValidImTL(desiredVisImCenter,...
                                   imDim,...
                                   onScreenImDim,...
                                   vpDim,...
                                   visImDim)
       
         if (onScreenImDim <= vpDim) % TL is TL of image
             TL = 0;
             
         else % constrain TL so you cannot scroll past the edge of image

             desiredVisImTL = desiredVisImCenter - vpDim/s/2.; 
             
             validVisImTL = clip(desiredVisImTL,imDim,visImDim);
             TL = validVisImTL;
         end

         %----------------------------------   
         function out = clip(in, imDim, visImDim)
         % clip so coordinate stays inside image

           out = max(in,0.);
           out = min(out, imDim - visImDim);

         end

      end

       
   end
   
   %----------------------------------   
   function mag = findMagnification(w,h)

      % Calculate screenPerImagePixel so image region 
      % with width w, height h fits in scrollpanel with no scrollbars showing.
      set(hScrollpanel, 'units', 'pixels');
      spPos = get(hScrollpanel, 'position');
      spWidth  = spPos(3);
      spHeight = spPos(4);
      set(hScrollpanel,'units','normalized')
      
      xMag = spWidth / w;
      yMag = spHeight / h;
      mag = min(xMag,yMag);

   end
   
   %----------------------------------
   function vp = getViewport()
      
      vp = viewport;

   end

   %----------------------------------
   function scrollHorizontal(varargin)

      hScrollablePos = get(hScrollable, 'Position');
      xPos = findPos(viewport(1),getOnScreenImW,hSliderHor);
      set(hScrollable,'Position', [xPos hScrollablePos(2:4)]);      

      sendNewLocation % Tell any interested parties that the location changed

   end

   %-----------------------------------
   function scrollVertical(varargin)
   
      hScrollablePos = get(hScrollable, 'Position');      
      yPos = findYPos;
      set(hScrollable,'Position',[hScrollablePos(1) yPos hScrollablePos(3:4)]);
      
      sendNewLocation % Tell any interested parties that the location changed

   end
   
   %----------------------------------  
   function resizeView(varargin)
        
      % Temporarily disable ResizeFcn to avoid recursion
      actualResizeFcn = get(hScrollpanel,'ResizeFcn');
      set(hScrollpanel,'ResizeFcn','')

      hScrollablePos = get(hScrollable, 'Position');
      updateViewport % Need to do this to get positions correctly
      
      xPos = findPos(viewport(1),getOnScreenImW,hSliderHor);
      yPos = findYPos;
      
      set(hScrollable,'Position', [xPos yPos hScrollablePos(3:4)]);
      
      updatePositions % call this last to make sure position of hAx and
                      % hScrollable are right even if full image visible
   
      % Restore ResizeFcn
      set(hScrollpanel,'ResizeFcn',actualResizeFcn)      

   end

   %----------------------------------     
   function [pos] = findYPos

      pos = findPos(viewport(2),getOnScreenImH,hSliderVer);
      
      if isSliderHorShowing
          pos = pos + sbw;
      end
   
   end

   %----------------------------------       
   function isShowing = isSliderHorShowing
   
         isShowing = strcmp('on',get(hSliderHor,'Visible'));
         
   end

   %----------------------------------       
   function updateViewport
   
      spPos = getpixelposition(hScrollpanel); 
      spWidth  = spPos(3);
      spHeight = spPos(4);

      set(hSliderVer, 'units', 'pixels');
      set(hSliderHor, 'units', 'pixels');
      set(hFrame, 'units', 'pixels');
      
      onScreenImW = getOnScreenImW;
      onScreenImH = getOnScreenImH;
      
      showSliderHor = (onScreenImW > spWidth) && (spWidth - sbw > 0);
      showSliderVer = (onScreenImH > spHeight) && (spHeight - sbw > 0);
      
      if showSliderHor
          set(hSliderHor, 'position', [1 1 (spWidth-sbw) sbw]);
          set(hSliderHor,'visible','on')          
          viewport(2) = (spHeight - sbw);
      else
          set(hSliderHor,'visible','off')                
          viewport(2) = spHeight;
      end
      
      if showSliderVer
          set(hSliderVer, 'position', [(spWidth-sbw)+1 sbw+1 sbw (spHeight-sbw)]);
          set(hSliderVer,'visible','on')          
          viewport(1) = (spWidth  - sbw);
      else
          set(hSliderVer,'visible','off')          
          viewport(1) = spWidth;
      end

      % Slider and frame positions depend on which sliders are showing
      if showSliderVer && ~showSliderHor
          set(hSliderVer, 'position', [(spWidth-sbw)+1 1 sbw spHeight]);          
          set(hFrame,'Visible','off')          
          
      elseif ~showSliderVer && showSliderHor
          set(hSliderHor, 'position', [1 1 spWidth sbw]);
          set(hFrame,'Visible','off')          
      
      elseif showSliderVer && showSliderHor          
          set(hFrame,'position', [(spWidth-sbw)+1 1 sbw sbw]);
          set(hFrame,'Visible','on')          
      
      else
          set(hFrame,'Visible','off')          
      
      end          

      set(hSliderVer, 'units', 'normalized');
      set(hSliderHor, 'units', 'normalized');
      set(hFrame, 'units', 'normalized');

      % Only update sliders if they are showing
      if showSliderHor
          updateSlider(viewport(1), onScreenImW, hSliderHor)
      end
      
      if showSliderVer
          updateSlider(viewport(2), onScreenImH, hSliderVer)
      end
      
   end
      
   %----------------------------------       
   function updatePositions

      onScreenImW = getOnScreenImW;
      onScreenImH = getOnScreenImH;
      
      updateViewport      
      adjustPositions

      %-----------------------
      function adjustPositions
   
         % Make adjustments in case image fits fully inside viewport ("Full View")
         hScrollablePos = get(hScrollable,'position');
         [axX,scrollableX,scrollableW] = ...
             getAdjustedAxPos(viewport(1), hScrollablePos(1), onScreenImW);
         [axY,scrollableY,scrollableH] = ...
             getAdjustedAxPos(viewport(2), hScrollablePos(2), onScreenImH);
                  
         set(hScrollable,'position',[scrollableX scrollableY scrollableW scrollableH]); 
         
         if isSliderHorShowing && isFullImageHShowing
             axY = axY + sbw;
         end
         
         % Also make sure axes dimensions match onScreenDims
         set(hAx,'position',[axX axY onScreenImW onScreenImH])         
         
      end
      
   end


   %----------------------------------     
   function isShowing = isFullImageWShowing
   
      isShowing = isFullImageShowing(viewport(1),getOnScreenImW);

   end

   %----------------------------------     
   function isShowing = isFullImageHShowing
   
      isShowing = isFullImageShowing(viewport(2),getOnScreenImH);

   end
   
   %----------------------------------     
   function onScreenImW = getOnScreenImW

     onScreenImW = imageWidth * screenPerImagePixel;
   
   end
   
   %----------------------------------     
   function onScreenImH = getOnScreenImH

     onScreenImH = imageHeight * screenPerImagePixel;
   
   end

end % imscrollpanel

%--------------------------------------------------
function updateSlider(vpDim,onScreenImDim,hSlider)

   if isFullImageShowing(vpDim,onScreenImDim)
       maxStep = inf;
       minStep = 0.01;
   else
       f = vpDim/onScreenImDim;
       maxStep = 1/(1/f - 1);
       minStep = min(1, maxStep/10); % must be between 0 and 1.
   end     
   
   set(hSlider,'SliderStep',[minStep maxStep]);

end

%--------------------------------------------------
function [axPos,scrollablePos,scrollableDim] = ...
    getAdjustedAxPos(vpDim,scrollablePos,onScreenImDim)

   if isFullImageShowing(vpDim,onScreenImDim)
       axPos = (vpDim - onScreenImDim)/2 + 1;
       scrollablePos = 1;
       scrollableDim = vpDim;
   else
       axPos = 1;
       scrollableDim = onScreenImDim;       
   end
      
end

%----------------------------------     
function [pos] = findPos(vpDim,onScreenImDim,hSlider)

   % Find position of hScrollable with respect to viewport
   if isFullImageShowing(vpDim,onScreenImDim)
       pos = 1;
   else
       pos = (vpDim-onScreenImDim) * get(hSlider, 'value') + 1;
   end
   
end

%----------------------------------     
function isFull = isFullImageShowing(vpDim,onScreenImDim)

   if (vpDim >= onScreenImDim)
       isFull = true;
   else
       isFull = false;
   end

end


%-------------------------------------------
function deltaOnePixel = getDeltaOnePixel(dimData,imDim)

    delta = dimData(2) - dimData(1);
    if (imDim ~= 1)
        deltaOnePixel = delta/(imDim-1);
    else
        deltaOnePixel = 1;
    end

end