%!
% 
% Date: Thu, 14 Jul 88 23:04:37 EDT
% To: NeWS-makers@brillig.umd.edu
% Subject: Bounded Canvases
% From: dennis@boulder.colorado.edu
% 
% Below is a modified 'dragcanvas' from 'util.ps'.
% It forces dragged canvases to remain completely within
% their parent canvases.  In particular, it will force
% dragged windows to remain completely visible on the screen.
% 
% To use it, put something like
% 	(newdragcanvas.ps) run
% into your user.ps file, where newdragcanvas.ps contains the following code.
% 
% --------------------------------------------------

% Modify dragcanvas so that canvas will never leave the
% parent canvas screen.
%
% ----------------------------------------------------------------------
% |                                                                    |
% | (x,y+h)----------(x+w,y+h)        (x',y'+h)----------(x'+w,y'+h)   |
% |   |        (x0,y0) |   ------>     |          (x0',y0')   |        |
% |   |                |               |                      |        |
% | (x,y)------------(x+w,y)          (x',y')------------(x'+w,y)      |
% |                                                                    |
% (0,0)-----------------------------------------------------------------
%
% Assume that the canvas is initially at (x,y) with respect to the
% parent canvas.
% Assume that the cursor is at (x0,y0) with respect to the parent canvas,
% which means that it is at (x0-x,y0-y) w.r.t. canvas.
%
% Assume that we drag the cursor to (x0',y0') with respect to the parent
% canvas.  This implies that the canvas should be dragged to the new
% coordinates (x',y').
%
% Notice that this implies that
% 1. dx = (x'-x) = (x0'-x0)
% 2. dy = (y'-y) = (y0'-y0)
% Solving for x' and y' gives
% 3. x' = x + dx = x + (x0' - x0)
% 4. y' = y + dy = y + (y0' - y0)
%
% The following code is supposed to enforce the constraint that
% the canvas always stays on the parent canvas;
% i.e., it never leaves the visible
% part of the screen.  This is intended to mimic the action of windows
% under sunwindows.
%
% DragFrame operates by using the current cursor position (w.r.t parent
% canvas) to determine the location of the window.
% Note that cursor is relative to parent rather than the canvas because
% getanimated gives such absolute coordinates during the drag.
% We also note that the canvas is moved using 'movecanvas'.
% In this situation, the movement delta vector is such that
% it must be given the new coordinates of the canvas
% w.r.t the parent canvas, so it must be given (x',y')
% as its arguments.
% We choose to enforce our constraint by calculating the minimum and
% maximum values for (x',y')
% that will ensure that the window never leaves the screen.
%
% Let PCW be the width of the parent canvas and
% let PCH be the height of the parent canvas.
% Let w be the width of the canvas and h be its height.
% Let xmax be the x' such that the right edge of the canvas
% would be dragged to the right edge of the parent canvas.
% Let xmin be the x' such that the left edge of the canvas
% would be dragged to the left edge of the parent canvas.
% Similarly for ymax and ymin.
%
% When moving left, the minimum x' value is, of course, 0.
% When moving right, notice that the maximum value for x' is PCW - W
% Similar observations hold for the y coordinate.
%
% From these observations, we obtain the equations:
% 5. xmin = 0
% 6. ymin = 0
% 7. xmax = (PCW - w)
% 8. ymax = (PCH - h)
%
% We calculate and save these values and use them to bound
% the 'movecanvas' coordinates.
%
% In the following code, the original x and y coordinates
% are kept as follows:
% 	xinit = x
% 	yinit = y
%
%

systemdict begin

/dragcanvas { % dragframe? (currentcanvas) => -
30 dict begin
    gsave
    %[1 0 0 1 0 0] setmatrix % Device dependant!
    initmatrix % Fixed for X11/NeWS. -Don
    /Canvas currentcanvas def
    /dragframe? exch def
    /ParentCanvas Canvas parentcanvas def

    % Get and save width and height of Canvas
    Canvas setcanvas clippath pathbbox /height exch def /width exch def

    % Calculate xmin and ymin
    /xmin 0 def
    /ymin 0 def

    gsave
	% Calculate xmax and ymax
	ParentCanvas setcanvas clippath pathbbox % 0 0 PCW PCH
	height sub /ymax exch def
	width  sub /xmax exch def

	% Get coordinates of Canvas w.r.t its Parent Canvas.
	Canvas ParentCanvas setcanvas getcanvaslocation
        /yinit exch def /xinit exch def
    grestore

    % remember the initial location of the cursor w.r.t parent canvas
    currentcursorlocation % realtive to canvas
    yinit add /yo exch def xinit add /xo exch def % relative to parent and save

    % When not dragging frame, give this routine to getanimated

    /absmovecanvas { % x y => -  where x,y cursor relative to parent canvas
		     % that is, they are more-or-less absolute coordinates
        gsave Canvas setcanvas
%	[1 0 0 1 0 0] setmatrix % Device dependant! -Don
	% Convert the incoming parent-relative cursor coordinates
	% to (x',y') using equations 3 and 4.
	yo sub yinit add exch
	xo sub xinit add exch
	% Apply bounds to these coordinates
	ymin max ymax min exch
	xmin max xmax min exch
	movecanvas
	grestore
    } def


    ParentCanvas createoverlay setcanvas
    0 0 dragframe? {{
	% this routine will be repeatedly called by getanimated
	% to move the canvas
	% the stack on entry will be x y of current cursor location
	% relative to parentcanvas.
	% Convert the incoming parent-relative cursor coordinates
	% to (x',y') using equations 3 and 4.
	yo sub yinit add exch
	xo sub xinit add exch
	% Apply bounds to these coordinates
	ymin max ymax min exch
	xmin max xmax min exch
	% Now, draw a bounding box from these coordinates
        moveto 0 height rlineto width 0 rlineto 0 height neg rlineto
        closepath
    }} {{
	% this routine will be repeatedly called by getanimated
	% to move the canvas
	% the stack on entry will be x y of current cursor location
	% relative to currentcanvas.
        absmovecanvas
        newpath
    }} ifelse getanimated waitprocess aload pop
    dragframe? {absmovecanvas} {pop pop} ifelse
    grestore
end
} def

end % systemdict