Talk:Subdivision surface
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[edit] Concerns about article quality
This whole articale is full of wrong and 50/50 right statements. The use of the word "arbitrary" just makes my toneails curl.
Furthermore it mixes statements that apply solely to Catmull-Clark scheme into an articale about the general subject matter.
This needs a compelet re-edit.
MoritzMoeller
[edit] Temporarily removed statements
I have removed the following and placed it here for now. --Ben Houston 01:54, 21 August 2006 (UTC)
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- B-spline relationship
- B-spline curves are refinable: their control point sequence can be refined and the iteration process converges to the actual curve. This is a useless property for curves, but its generalization to surfaces yields subdivision surfaces.
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- Refinement process
- Interpolation inserts new points while original ones remain undisturbed. Refinement inserts new points and moves old ones in each step of subdivision.
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- Extraordinary points
- The Catmull-Clark refinement scheme is a generalization of bi-cubic uniform B-splines. Any portion of the surface that is equivalent to a 4x4 grid of control points represents a bi-cubic uniform B-spline patch. Surface refinement is easy in those areas where control points valence is equal to four. Defining a subdivision surface at vertices with valence other than four was historically difficult; such points are called extraordinary points. Similarly, extraordinary points in the Doo-Sabin scheme have a valence other than three.
- Most schemes don't produce extraordinary vertices during subdivision.
[edit] Negative aspects of Sub D?
It would be great if the entry mentioned some of the negative aspects of using sub-d. (For instance, isn't it difficult to get specular highlights to remain even over a surface? Also, isn't it harder to get the same level of detail and precision as opposed to NURBs?) --Navstar 18:29, 20 October 2006 (UTC)
No, these 'negative aspects' are problems of the modeling application, not subdivision surfaces per se. Moritz
[edit] too much nonsense
- Calling subdivision surfaces "recursive in nature" is not generally true, as they are basically iterative interpolations. The subdivision algorithm might be implemented as a recursion anyhow, but this does not make too much sense as there is no single mathematical criteria in the concept itself except counting. The main criteria for the number of recursions would be "does the surface look nice enough", which is of course not a mathemtical but a question of opinion and taste.
- The limiting subdivision surface can never be achieved as this would require an infinite number of iterations thus requiring infinite time. As every iteration multiplies e.g. the number of quads by the factor 4, memory consumption soon exceeds available physical memory in a given computer system. This is one of several mayor disadvantages compared to NURBS. Subdivision Surfaces are therefor mainly used for rendering purposes in computer animation and visualisation packages, but not for CAD, CAM or CAE.
- I have not heard about controlpoints on a mesh surface yet, which consist of vertices.
From my point of view there is too much nonsense in this article yet so that i dont feel like contributing the above to the article itself. I do agree with MoritzMoeller that it requires complete rewriting. Bubbleshooting 20:49, 24 January 2007 (UTC)
- If the word "recursive" displeases you, why didn't you change it? It appears precisely once in the article; this would not have been anything close to a full rewrite.
- I don't see the need to point out that the limit cannot be achieved. An understanding of limits (which one would gain by following the link) clearly shows that it cannot be attained. Plus, the article points out very clearly that in practical use, only a number of iterations are used.
- Once again, if you don't like the term "control points" being used where it is, feel free to change it. To me, as someone who has worked with subdivision surfaces before, it's perfectly fine to call them control points, as vertices at each iteration do control where new (and possibly old) vertices will be. But if that could be confusing, change the term to something else. So long as the meaning is clear, there shouldn't be a problem.
- Moritz's comments are outdated. The article has been through some significant changes over time. And nothing you've pointed out would reasonably be considered "nonsense". And even if it did, it would require changing maybe 20 words, not a full rewrite. Korval 03:06, 25 January 2007 (UTC)
I cannot see that Moritz's comments do not apply any more. I concentrated in my comment on some examples which where obviously and at first glance wrong statements. It would not change the quality of the article as a whole to change only 20 words. The whole paragraph Comparison to NURBS surfaces is obviously written by someone who knows little to nothing about NURBS, therefor its pointless to change some words only. The same applies if a whole paragraph is written by someone who does not know how to distinguish between iteration and recursion. And adding what is obviously opinion of the author only without any argument does not augment the article quality either.
And you are again adding nonsense regarding your comment on limits. A limit is in general something which can definitely be achieved, and not the opposite as you are implying.
And a controlpoint is a well defined and well understood concept in computer graphics. Someone using it for something very different does not add to his reputation, nor to the quality of the article.
So i still do see the need for a complete rewrite by someone who knows about the basics mentioned above as well as about the subject of the article. Bubbleshooting 10:54, 26 January 2007 (UTC)
- See, now you're just making stuff up to be argumentative. If you change the word "recursive" to "iterative", you will have fixed the problem you cite, because the paragraph describes the facts of the process of subdivision, which you claim fits the definition of an iterative process. If that is true, then calling it an iterative process is all that is needed, because the actual description of said process is correct. It's merely the declaration that the process is recursive that is inaccurate.
- As to the "paragraph" (more accurately, a set of lists) about Subdivision vs. NURBS, why don't you fix it if you know so much about NURBS? That's the whole point of Wikipedia; if you see something wrong and you know what the right thing is, do something about it.
- As to control points, I have implemented subdivision surfaces. Referring to the vertices of a particular subdivision level as "control points" is what we do. Since your expertise is clearly not in the realm of subdivision surfaces, perhaps you should consider accepting that this is just how we talk about our particular algorithm. Korval 02:47, 27 January 2007 (UTC)
Knowing when there is too much salt in the soup does not necessarily require to be able to cook. So i followed your advise and removed the nonsense statements about comparison to NURBS. The author might have compared some software packages, but obviously did not know about the basics.
There is still a mess left about the main principles of subdivision surfaces compared to some specifics like catmull clark subdivision, which is equally messed up on a page of its own here on WP. Bubbleshooting 23:21, 7 February 2007 (UTC)
