Center of a irregular hexahedra given the vertices

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Discussion Overview

The discussion revolves around the computation of the center of irregular convex hexahedras, specifically addressing the definition and calculation methods for determining the "center" of such shapes. Participants explore different interpretations of "center," including geometric and volume-based considerations, within the context of a 3-D structured mesh grid.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that the center of a hexahedron can be computed as the sum of the indices divided by the number of indices, questioning if this holds true for all polygons and polyhedra.
  • Another participant challenges the definition of "center" in irregular shapes, prompting a discussion on what is meant by "geometrical/spatial center."
  • A participant expresses uncertainty about whether they are referring to a volume-based center, noting that averaging the vertices may not yield the center of volume.
  • There is a mention of the center of mass having a clear definition, suggesting that the term "center" is too vague without further specification.
  • One participant raises the concept of the centroid of a triangle and its relation to the centroid of a hexagon, indicating that the centroid can be calculated as a weighted average of the centroids of constituent triangles.
  • A later post reiterates the initial question about the validity of averaging the indices to find the center, providing an example involving a cube transformed into a pyramid shape to illustrate the discrepancy between average coordinates and center of mass.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the definition of "center" or the appropriate method for calculating it in the context of irregular hexahedras. Multiple competing views regarding the interpretation and calculation methods remain present.

Contextual Notes

Participants express uncertainty regarding the assumptions behind the definitions of "center" and the implications of using different averaging methods. The discussion highlights the need for clarity in terminology and the potential limitations of various approaches.

pyroknife
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I am trying to compute the center of irregular convex hexahedras (6 faced volume).

I have the 8 indices that define the hexahedra. I was reading that the center is simply the sum of the indices divided by the number of indices and this is true for all polygons and polyhedras. Is this true?
 
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How do you define "center" in an irregular shape?
 
I am asking myself that same question. I believe I am referring to the geometrical/spatial center, but I am not positive.

Basically I have a 3-D structured mesh grid, with cells. And I am trying to find the "center" of that cell.
I believe it is a volume-based center?

Hmmmm, if it is a volume based center, then an averaging of the vertices would not yield the center of volume.
 
What do you mean by "geometrical/spatial center"?

There is a center of mass with a clear definition, for example. There might be other well-defined points, but you have to decide what you are actually looking for. "The center" is too unclear.
 
I will look into this and figure out what it is that I actually want.What "type of center" does an averaging of all the nodal points provide?
 
I'm not sure if that point has any special meaning.
 
The "centroid" of a triangle has coordinates equal to the average of the coordinates of the vertices. Other figures, such as a hexagon, can be divided into triangles. The coordinates of the centroid of such a figure is the weighted average of the coordinates of the centroids of the triangles, weighted by area.
 
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pyroknife said:
I am trying to compute the center of irregular convex hexahedras (6 faced volume).

I have the 8 indices that define the hexahedra. I was reading that the center is simply the sum of the indices divided by the number of indices and this is true for all polygons and polyhedras. Is this true?

Start with a cube. Take four of the corners and bring them close together so that the cube becomes something close to a four sided pyramid in shape. The average of the eight corner z coordinates is one 1/2 half of the height, but the center of mass is only 1/4 the height.
 
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