# Identifying a surface as a cone

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• tricha122
In summary, the conversation discusses methods for identifying whether a surface is a cone using CAD software. One approach is to find the axis of the cone by projecting points onto a unit sphere and determining the intersection of normal lines. Another method involves using four data points to solve equations and find the cone's axis and vertex. Additional points can be used to test if the surface is a cone. Other suggestions include using the cross product of vectors, calculating the volume, and using the Hough transform algorithm.
tricha122
Hi, I’m using some CAD software trying to automate some surface identification, and I’m looking to find a way to identify whether a surface is a cone.

I will have access to vertices and normals at discrete points on the surface, but it will be expected that the number of these points will be different from cone to cone, and the cone orientation and position may be arbitrary.

I know that once I find the vertex of the cone I can project all the points I have to anunit sphere about the vertex. The normal to the circle that is transcribed projected on the sphere will give me the axis of the cone. But I need a way to determine the location of the vertex (which I may not have a point for)

Anyone have any suggestions? Thanks!

Assume we are given a set of at least three data, each one of which is the coordinates of a point on the surface and the components of the normal at that point. Each datum defines a line in 3D space that passes through the point on the surface in the direction of the normal. Call that the 'normal line' for that datum.

If the surface is a cone, there will exist a unique plane P through the origin of the coordinate system, such that the projections of the normal lines onto P all intersect at a single point Q. That point is the intersection of P with the axis of the cone.

The plane P is specified by two parameters, and the point Q where lines intersect is specified by a further two parameters. Hence there are four unknowns. Given four data we can write four equations, each one specifying that a different one of the four normal lines passes through Q. We can solve the equations to find the plane P and the point Q. From that we can obtain the equation of the cone's axis.

It remains to find the vertex of the cone. To do that, measure the distance of each datum from the cone axis. By linear extrapolation between data you will be able to find the point where the distance becomes zero. That point is the vertex.

Given the vertex, the axis and one datum, you can calculate the equation of the cone.
Use the first four points to solve the equations. That tells you what cone the surface must be if it IS a cone. Additional points after the fourth serve to test whether it IS a cone.

We can never be certain that it is a cone, because there could always be a small deformation in a non-sampled part of the surface. But if there is even one datum whose normal line does not have a projection on P that passes through Q, we can definitively say the surface is not a cone.

EDIT: It might be six points that are needed to complete the specification, ie to find the vertex and the shape. But only four equations should be needed to find P and Q.

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Hi there! One possible approach could be to use the cross product of two vectors formed by three non-collinear points on the surface. If the resulting vector is perpendicular to the surface, then it could indicate that it is a cone. You could also try using the formula for the volume of a cone and see if the calculated volume matches with the points you have. Additionally, you could use the Hough transform algorithm, which is commonly used in image processing for detecting shapes, to identify the cone. Good luck!

## 1. What is a cone?

A cone is a three-dimensional geometric shape that has a circular base and tapers to a point, resembling an ice cream cone or a party hat.

## 2. How can you identify a surface as a cone?

A surface can be identified as a cone if it has a circular base and its sides taper evenly to a single point, creating a curved surface.

## 3. What are the properties of a cone?

A cone has a circular base, a curved surface, and one vertex (or point). It also has a height, slant height, and radius, which are all important measurements in determining its size and shape.

## 4. What are some real-life examples of cones?

Some common examples of cones in everyday life include traffic cones, ice cream cones, party hats, and the shape of a volcano.

## 5. How is a cone different from other geometric shapes?

A cone is different from other geometric shapes because it has a curved surface and only one vertex, while other shapes like cylinders have flat surfaces and multiple vertices. Additionally, a cone has a unique tapering shape that sets it apart from other shapes.

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