# Re: [geometry] Points and Vectors Proposal

```Hi.

[Note: there is a problem with the quoted part in your
message.]

On Tue, 24 Apr 2018 01:31:43 +0000, Matt Juntunen wrote:
```
```Hi Gilles,

The hierarchy would be wrong from a conceptual POV: A vector can be
described by Cartesian coordinates, but it should be possible to
introduce new coordinate systems (polar in 2D, spherical in 3D) ...
```
```
```
```This approach doesn't limit the coordinate system at all. We can
still make implementations of Point<EuclideanXD> and
Vector<EuclideanXD> based on other coordinate systems. I think it'll
actually be easier in this structure, since the details of the system
are explicitly defined in a single base class. For example, to create
polar vectors and points, we would create a PolarCoordinate2D base
class and PolarPoint2D and PolarVector2D subclasses.
```
```
What you propose (in the branch) is:
public class Point3D extends Cartesian3D

Then if we'd implement spherical coordinates, we'd have (similarly):
public class Point3D extends Spherical3D

Obviously, that will not work; so I may be missing what you
are trying to achieve...

```
```...algorithms that use vectors would/should still work (transparently
doing the appropriate conversions if necessary).
```
```
```
```This is a general issue with the current code, separate from the
changes I'm proposing here. I'm not introducing a new issue.
```
```
What is the general issue?  That the code assumes Cartesian
coordinates?
My understanding is that your proposal exposes an "implementation
detail" (a set choice of the coordinate system).

```
```I understand (and agree with) the performance goal, but let's
be careful to not define an API that does not correspond to
the underlying concepts.
```
```
```
```Agreed. One vote in favor of having these utility methods is that I
used some of them while transitioning the other geometry classes for
my proof-of-concept. For example,
o.a.c.geometry.euclidean.threed.Plane uses the
Vector3D.dotProduct(Cartesian3D, Cartesian3D) method to compute the
origin offset directly from the origin point and plane normal.
```
```
I think that two issues are compounded here; one is the static
"utility" functions (whether they are more performant then
"pure" OO methods should be demonstrated, with benchmarks),
the other is the OO hierarchy, which should make sense from a
subject domain (i.e. geometry) POV.  Here I was again referring
to the fact that e.g. a vector in Euclidean 3D-space is equally
well represented by
(x, y, z)
or
(r, theta, phi)
or
(r, theta, z)
or
...

Perhaps a "Cartesian3D" instance should be returned by an
accessor, rather than be the parent (?).

```
```What will happen when we introduce
Spherical3D(r, theta, phi)
alongside
Cartesian3D(x, y, z)
?
They should be able to get along just fine. They would each have
subclasses that perform point and vector operations using their
respective coordinate systems. The only issue would be trying to mix
them, which as I mentioned above, is an existing issue with the
current code base. However, I think having the coordinate systems
encapsulated in base classes is a good first step in solving this.
```
```
[See above.]

```
```If "Cartesian3D" _implements_ "Point3D" and "Vector3D", it
would still work (i.e. refactor so that "Point3D" becomes
an interface and does not assume that the coordinates are
Cartesian).
```
```
```
```I'm not quite sure what you're picturing for the Point3D interface
here. Even so, if Cartesian3D implemented both interfaces, the
compiler wouldn't be any help in catching simple programming errors.
```
```
IMO, a design is not primarily aimed at detecting programming
errors but should help the user avoid them. ;-)

Regards,
Gilles

```
```
Thanks,
Matt
________________________________
From: Gilles <gilles@xxxxxxxxxxxxxxxxxxxxx>
Sent: Monday, April 23, 2018 4:32 AM
To: dev@xxxxxxxxxxxxxxxxxx
Subject: Re: [geometry] Points and Vectors Proposal

Hi.

On Mon, 23 Apr 2018 05:36:09 +0000, Matt Juntunen wrote:
```
```Hi all,

I'd like to propose an update to the Euclidean Point/Vector classes
```
in the geometry project. We currently have a single CartesianXD class
```per dimension (eg, Cartesian2D) that implements both the Point and
Vector interfaces. This is similar to the previous commons-math
version where we had VectorXD classes that were also both Points and
Vectors. The change to the current version was through discussion on
MATH-1284 (https://issues.apache.org/jira/browse/MATH-1284). My
proposal is to flip the current inheritance hierarchy so that the
CartesianXD classes become the base classes for separate PointXD and
VectorXD classes.
```
```
The hierarchy would be wrong from a conceptual POV: A vector can be
described by Cartesian coordinates, but it should be possible to
introduce new coordinate systems (polar in 2D, spherical in 3D) and
algorithms that use vectors would/should still work (transparently
doing the appropriate conversions if necessary).

```
```PointXD classes only implement the Point interface
and VectorXD classes only implement Vector. The Cartesian base
classes
contain the actual x, y, z coordinate values along with a few other
```
common methods (such as getSpace()). For performance and convenience,
```we can create static methods in the VectorXD classes that accept the
Cartesian base class instances, so that users can perform common
```
vector operations using either type. For example, if you have a giant
```list of Points, these static methods would allow you to compute dot
products without needing to convert the Point instances to Vectors
first.
```
```
I understand (and agree with) the performance goal, but let's
be careful to not define an API that does not correspond to
the underlying concepts.

What will happen when we introduce
Spherical3D(r, theta, phi)
alongside
Cartesian3D(x, y, z)
?

```
```I've partially implemented this in a branch so you can get a better
idea of what I'm picturing:
https://github.com/darkma773r/commons-geometry/tree/point-vector.
The
commons-geometry-core and commons-geometry-euclidean sub-modules
contain the changes.

[https://avatars1.githubusercontent.com/u/3809623?s=400&v=4]<https://github.com/darkma773r/commons-geometry/tree/point-vector>

darkma773r/commons-geometry<https://github.com/darkma773r/commons-geometry/tree/point-vector>
commons-geometry - Apache Commons Geometry
github.com

```
The main benefit I see from this approach is code clarity. The intent
```of the code seems much clearer to me when the names of the types
```
exactly match what they represent mathematically. For example, one of
```the constructors for the Plane class currently looks like this:

public Plane(final Cartesian3D p, final Cartesian3D normal, final
double tolerance)

With my proposed changes, it would look like this:

public Plane(final Point3D p, final Vector3D normal, final double
tolerance)

The code is easier to read and the compiler will also help prevent
algorithm errors.
```
```
That API is better indeed.

If "Cartesian3D" _implements_ "Point3D" and "Vector3D", it
would still work (i.e. refactor so that "Point3D" becomes
an interface and does not assume that the coordinates are
Cartesian).

Best regards,
Gilles

```
```
Thoughts?

Regards,
Matt Juntunen
```
```

```
```

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