/[gzz]/gzz/Documentation/Manuscripts/Irregu/irregu.tex
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revision 1.100 by tjl, Sat Nov 30 08:17:09 2002 UTC revision 1.101 by tjl, Sat Nov 30 08:34:57 2002 UTC
# Line 362  established and therefore this idea shou Line 362  established and therefore this idea shou
362  In order to create and maintain the illusion: ``we see a piece of the canvas'',  In order to create and maintain the illusion: ``we see a piece of the canvas'',
363  instead of ``we see the canvas through an irregularly shaped hole'',  instead of ``we see the canvas through an irregularly shaped hole'',
364  the motion must be carefully designed.  the motion must be carefully designed.
365  When the viewport moves on the canvas, it  The shape of the torn piece should not be translation-invariant
366  mustn't look like a (rectangular or irregular)  but should
367  "window" sliding on top of paper, but  change in an appropriate way with location and zooming.
 instead something like re-gluing and tearing away a different part of the paper.  
368    
369  To get the correct picture, imagine an animation where the first frame is a given  To get the correct picture, imagine an animation where the first frame is a given
370  torn piece of paper, the next frame is what would have happened if we had torn the paper  torn piece of paper, the next frame is what would have happened if we had torn the paper
# Line 378  As to the graphical appearance of the to Line 377  As to the graphical appearance of the to
377  there are two main reasons to go for nonphotorealistic rendering:  there are two main reasons to go for nonphotorealistic rendering:
378  firstly, to imply to the user that the viewport does not behave  firstly, to imply to the user that the viewport does not behave
379  exactly like a real paper; to make the overall view clearer.  exactly like a real paper; to make the overall view clearer.
380    Drawing only the silhouette edge\cite{saito90comprehensible} of the
381  Thus, instead of trying to draw a realistic torn piece of paper,  paper is therefore appropriate.
 we will only the silhouette edge\cite{saito90comprehensible}.  
382    
383  % When zooming, scaling the thickness of the silhouette edge  % When zooming, scaling the thickness of the silhouette edge
384  % with the scale of the paper is not good:  % with the scale of the paper is not good:
# Line 390  we will only the silhouette edge\cite{sa Line 388  we will only the silhouette edge\cite{sa
388  % (too photorealistic...), but neither is a constant width, which ...  % (too photorealistic...), but neither is a constant width, which ...
389  % Square root XXX refs: stroke scaling in pen drawings?  % Square root XXX refs: stroke scaling in pen drawings?
390    
391    If the usual shape of the viewport extends outside the edge of the underlying
392  Edge shapes: connected and scattered (and intermediates).  canvas, the metaphor of tearing demands that we only draw the
393  TOPOLOGY!  intersection of the two; see Fig~xxx.
   
 Finally, there is the question of what should happen when the viewport reaches the edge  
 of the canvas.  
 - distinguishing between edges of paper and the viewport useful  
     - edge of paper = line, edge of viewport = torn  
394    
395    
396    Finally, there are several design questions that are less important
397    and which are best left as options in the algorithms, to be tailored
398    to the particular application and taste.
399    For example, whether the edge should only ripple without altering
400    the topology or whether we should allow unconnected islands to
401    appear (scattered edge); whether adding something to the roughly
402    defined region to be viewed should always only move edges outwards
403    or whether they may move back inwards at some point (``ebbing'').
404    
405    
406  \subsection{Algorithm ``How?''}  \subsection{Algorithm ``How?''}

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