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Beginners question on subdivisions

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#1

Hi all,

Everything I have read regarding subdivisions tells me that each time a model is subdivided the available polygons are divided by a factor of 4, however, I am not seeing uniform divisions if the polycount is to be believed.

For example, selecting a basic cube3d, making it editable and converting it to polymesh, Zbrush tells me it is 512 polys. So, basic maths tells me dividing those poly’s by four should give me a total of 2048 polys but instead I get 1984 polys. Further subdivisions give me 7936 and 31744 respectively.

So I thought, ok lets try with something else…a circle3d at 112 polys. Subdivision 1 gives me 432 polys not the expected 448. Further divisions give me 1728 and 6912.

Why is the first subdivision always (seemingly) incorrect?

#2

ZBrush counts points (vertices), not polygons, and polygons share points.

#3

If you hover the cursor over the tool icon, you can see the polygon count in addition to the vertex/point count. This polygon count should support your math where the points wont (for the reasons Zber mentioned).

As a side note, it might also be worth mentioning that subdivision creates n-polygons per n-sides. So if the first level contains any tri polygons, those particular polygons will only get subdivided into 3 quad polygons on the first subdivision rather than 4.

#4

Thanks for the replies guys.

The maths don’t stack up whether you are counting by vertices or faces. Cryrid’s explanation about the tri’s is interesting but not the cause of the issue.

When I looked at the basic cube in Polyfame mode it does indeed have tri’s - 32 per pole but they don’t change, no matter how many times you subdivide. In fact not all the faces subdivide - it’s just those polys along the edges of the cube that do…so the book I am learning from is telling me a bit of a porky when it says “subdivision divides each polygon by four to give more resolution to work with”.

I am fine with that, I just thought there was something wrong with Zbrush that it wasn’t increasing the poly’s the way I thought it was supposed to given the books text.

#5

When I looked at the basic cube in Polyfame mode it does indeed have tri’s - 32 per pole but they don’t change, no matter how many times you subdivide. In fact not all the faces subdivide

They subdivide and change alright, you just wont see this change drawn on the polyframe display. By default zbrush doesn’t show you the wireframe for all subdivision levels, as the wires would get so small that it would make the polyframe impossible to read. If you temporarily delete lower subdivision levels then you’ll see that the triangles have in fact been turned into 3 quads each (which then turn into four quads each the next time they are subdivided, and so on).

Subdivision affects all the faces on a mesh. The only time it doesn’t is when you have part of the model masked. This will locally subdivide just part of the mesh, but wont create subdivision levels since it is altering the topology to make such a localized change possible.

#6

Cool, thanks for the explanation Cryrid, much appreciated.