Talk:Catmull–Clark subdivision surface

>> The new mesh will consist only of quadrilaterals, which won't in general be flat.

Could someone clarify this?

Does this sentence mean:

"The new mesh will consist only of quadrilaterals. Each quadrilateral won't in general be flat."

or

"The new mesh will consist only of quadrilaterals. The mesh won't in general be flat."

It means that the quadrilateral won't in general be flat. --Fredrik Orderud 20:40, 4 September 2007 (UTC)

isn't the image wrong? A cube shouldn't become a sphere with catmull-clark! —Preceding unsigned comment added by 85.225.115.25 (talk) 19:15, 30 December 2007 (UTC)

No, Cubes do indeed become spheres when subdivided using Catmull-Clark. Perhaps you're thinking of regular subdivision or are expecting some form of chamfered cube. 71.171.252.81 (talk) 06:25, 6 February 2008 (UTC)

They do NOT become spheres. There is literally no way to create a sphere using a Catmull-Clark surface (meaning x² + y² + z² = r²) -- see NURBS Control Points for a brief mention of this problem. A Catmull-Clark surface generates a series of connected cubic, uniform, polynomial B-Splines, and thus cannot represent non-trivial conic sections. They can approximate spheres (i.e. they "look like" spheres as far as your eye can tell), but no matter how dense your control cage is, you will never generate a sphere. There are more complex subdivision schemes that can create true spheres, but they lose a lot of the elegance of a Catmull-Clark surface, and so are more rarely used. Ahelps (talk) 19:09, 23 December 2008 (UTC)

>> and take the average R of all n edge midpoints for edges touching P, where each edge midpoint is the average of its two endpoint vertices

Can someone clarify this? Does this mean the endpoint vertices from the original edge or the new edge? Broodle (talk) 02:47, 28 April 2008 (UTC)

This formulation is indeed misleading, most likely even wrong. I learned about this scheme in a lecture at university, and our materials say that R is the average of the 'edge points' of the edges incident to the original point P, i.e. the edge points that you computed in the step before. 84.226.52.126 (talk) 20:56, 22 August 2009 (UTC)

Remove section on software using Catmull-Clark

I propose deleting the section "Software using Catmull–Clark subdivision surfaces". It is not especially interesting for an encyclopedia, and it is highly unlikely that this list will ever become all-inclusive. For the moment it is just getting long enough to become a visual distraction to the encyclopedic content surrounding it. --Berland (talk) 09:30, 14 August 2008 (UTC)

Recursive evaluation section

I find this section rather difficult to understand. A few graphical masks would be of much more help than "face point"/"edge point"/... (btw: are you sure it's correct or do I still not understand it? :) Is an edge point really the average of all neighbouring face points? (I thought the points were only dependent on the original pts?) —Preceding unsigned comment added by 91.19.90.230 (talk) 13:49, 21 October 2008 (UTC)