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 |
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: |
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?''} |