- From: Dr. Olaf Hoffmann <Dr.O.Hoffmann@gmx.de>
- Date: Mon, 23 Apr 2012 12:10:43 +0100
- To: www-style@w3.org, public-fx@w3.org

Aryeh Gregor: ... > > Correct. This isn't in the spec now because it's not clear what to do > with negative values of p'. If p' is positive, then it's clear we > want (x'/p', y'/p'). And if p' is nonpositive for all four corners of > a box, we want the box not to render. But it's not at all obvious > what to do if some corners of a box have p' > 0 and some have it <= 0. > > I've proposed a solution at > <https://www.w3.org/Bugs/Public/show_bug.cgi?id=15605#c9>. Perhaps > you could give some feedback on it. > This is the old problem with either objects behind the eye of the observer or within the eye(-plane) of the observer. In reality you can't see what is behind you back. And if an object sticks into your eye you have other problems than projection ;o) Anyway, maybe already more useful to say "we want (x'/|p'|, y'/|p'|) for p' not 0". Mathematically there is no big problem with objects behind the observer - nevertheless you might not want to render it. But this may depend on the taste of the author of the shape, that has to be displayed. A ususal approach is to leave the choice to this author (this is shared care - the author of the applications should now best, what matters, the viewer just has to render the stuff) - maybe yet another property, not sure, if this is useful for many authors. Options could be 'simple' - if parts of the object is behind the observer, don't render it at all 'around' - render objects behind the observer as well (needs a check, if it causes problems, if parts of the object are behind and parts are in front) 'cut' - cut objects close to the eye-plane and display only what is in front of the observer (needs presumably an additional length-value to decide, at which distance from the observer to cut). Objects in the eye-plane - a specific problem of the central projection, in reality nothing specific happens, if an object is 100m on the left or right of you, only if it sticks directly into your eye. A spherical projection (don't know, if this is the correct english translation, it is more realistic compared to eyes, you can see similar effects on super-wide-angle lenses and especially fish-eyes) avoids this practically except for one point. But because this method does not project lines on lines or points, you may not like it, because it is more difficult to draw - I have done approximations of this including animation with SVG and PHP, hand made 3D projections, large files, but looks nice, see for example here: http://upload.wikimedia.org/wikibooks/de/f/f8/SVGzeichenreihenfolge06.svg ;o) If you want to go on with central projection - what I assume - I think at least you have to exclude or care about parts of objects around the eye-plane to be rendered. Another more simple approach of shared care with central projection is to leave everything to the author - just note in the draft, that the author has to take care, that everything to be rendered is in front of the observer and that all objects with parts not in front of the observer will not be displayed at all. And maybe you can add a note, that future versions may add another property as mentioned above, if it turns out, that it is required/desirable to have more access to this issue for the author. With this approach, you can solve more difficult problems later, if required, and you can go on to implement the simple and maybe practically more useful aspects faster. But if you try to project infinite to finite values somehow, to get something interesting/relevant, I think you need a continuous function/distortion to get something acceptable - this is comparable again to something that super wide angle photo lenses do - you will always have to accept some kind of distortion, if you want to detect viewing angles on a flat area, which are normally not accessible for this approach. Maybe experts from Canon, Leica, Nikon, Sigma, Tamron, Tokina, Zeiss etc can help to get a nice function to solve the problem to render everything somehow without too much distortion - but this is even more complex than the spherical projection. But maybe there are simple enough solutions to smooth the function 1/z somehow around z=0 with limited numerical efforts. Obviously one can replace it with something like b/(z*z+a)^1/2 or b*exp(-z*z/a) around 0 and adjust a and b to get a finite continuous differentiable replacement of 1/|z| and |z| small compared to characteristic sizes of the rendered objects. Still this approach will cause non-trivial distortions of objects, having parts around z=0 - maybe still too difficult for viewers, that have to render (X)HTML and SVG documents fast online. > > My assumption is, that this is a quite simple function, with most of the > > not completely trivial things you need already stored in p'. > > It's mathematically pretty simple. If w > 0, (x, y, z, w) is mapped > to (x/w, y/w, z/w). If w < 0, (x, y, z, w) is mapped to nothing. The > point (0, 0, 0, 0) is also mapped to nothing. And for (x, y, z) not > the origin, the point (x, y, z, 0) is mapped to the point at infinity > in the direction of (x, y, z). But how to translate this into > something that can actually be rendered on the screen is not so > obvious, which is why the spec doesn't yet say anything about it -- we > don't have text that's agreed upon. Obviously there has to be some normative text finally, else the feature is not testable ... Meanwhile one could add this hint about 'work in progress/missing text' in an info-box to the draft to inform readers, that they can't understand, what is not decided yet ;o) Olaf

Received on Monday, 23 April 2012 11:11:14 UTC