# Transform point using localToSceneTransform

Jim Graham james.graham at oracle.com
Wed Jul 25 18:19:42 PDT 2012

Hi Pavel,

You claim that we need to add new methods if we treat a point as if it
were a vector, but we would have to do that anyway to add methods that
take vector.

transform(Vector) - treats its arg as magnitudes
(and thus does a delta transform)

deltaTransform(Point) - delta transforms the arg
(which is what you want to do with magnitudes)

It's a matter of whether the semantics are in the method name or the
argument type.

Furthermore, an array of doubles does not have an "am I a set of points
or vectors" type attached to it so we do need to have 2 different method
names for the methods that take arrays of doubles, whether or not we
create a Vector class (whose sole purpose is to indicate that its values
which are identical in nature to the Point class are to be considered as
magnitudes).

We can either encapsulate this information in a new class and add lots
of methods for that new class, or we can encapsulate the information on
the naming and documentation of new methods which use old types - both
have equal expense in terms of method counts...

...jim

On 7/25/2012 1:49 PM, Pavel Safrata wrote:
> I think the main point is that even though both vector and point consist
> of three doubles, they are different kinds of animals. Point contains
> coordinates, Vector contains magnitudes/distances. We will probably want
> to provide methods for computing angle between vectors or vector length.
> Having angle between points or point length doesn't make any natural
> sense. If we provide those methods on Point3D, they will look weird, and
> javadoc will have to explain that in this case we treat the point as
> "relative distance vector" (taken from J2D that has to do that). Also
> transforming a point in space results in another point in space whereas
> transforming a vector is a different operation that results in a vector.
> With both guys represented as a single class we need to provide two
> different transform methods, one of them for treating a "point" as
> point, the second one for treating a "point" as vector (named
> deltaTransform). J2D does that (has a single "point" and needs to
> explain in javadoc how it is treated, but it doesn't have the
> convenience "vector algerbra" methods so the problems are smaller), J3D
> doesn't (distinguishes point and vector as different things).
>
> Unfortunately I haven't been involved in the past discussions so I
> probably don't have enough insight. I don't really want to question
> be great if somebody could sum up the reasons for the decision to
> represent both things by Point.
>
> Thanks,
> Pavel
>
> On 25.7.2012 17:54, Richard Bair wrote:
>>
>> Generally speaking I'd prefer to just use Point3D vs. adding a Vector,
>> Without having to read the whole thread, can you give a short summary
>> of the impact using Point3D has vs. a class named Vector3D?
>>
>> Richard
>>
>> On Jul 25, 2012, at 6:54 AM, Pavel Safrata wrote:
>>
>>> I think that if we define Vector*D as the class that represents
>>> magnitudes without position, and we use it for our magnitudes without
>>> position, we don't need any deltaTransforms, do we?
>>>
>>> Richard, what do you think about introducing Vector2D and Vector3D? I
>>> think the classes would be nice (to distinguish between specific
>>> points in space and magnitudes without position) but we would have to
>>> deprecate methods in Rotate that use Point3D as rotation axis and
>>> introduce different ones that use Vector3D, we would also need to
>>> deprecate the static axes defined in this class and define vector
>>> ones with different names..
>>>
>>> Thanks,
>>> Pavel
>>>
>>> On 25.7.2012 1:34, Pedro Duque Vieira wrote:
>>>> Hi Jim,
>>>>
>>>> Maybe we're talking about two different ways of performing the same
>>>> calculations, probably we come from different backgrounds.
>>>>
>>>> What I meant with Vector3d is a 4x1 matrix with the last element as 0
>>>> (homogeneous coordinates). We can then have methods on the Vector3d
>>>> class
>>>> to give us things we expect to retrieve from vectors: like angle,
>>>> magnitude, doing the cross product of two vectors, calculating the
>>>> angle
>>>> between two vectors.
>>>> As a reference one can look at the java3d implementation of Vector3d:
>>>> http://docs.oracle.com/cd/E17802_01/j2se/javase/technologies/desktop/java3d/forDevelopers/J3D_1_3_API/j3dapi/javax/media/j3d/Transform3D.html
>>>>
>>>>
>>>>
>>>> Thanks, best regards,
>>>>
>>>> On Wed, Jul 25, 2012 at 12:06 AM, Jim Graham
>>>> <james.graham at oracle.com>wrote:
>>>>
>>>>> Hi Pedro,
>>>>>
>>>>> delta transform is transform without translation. It is primarily
>>>>> used to
>>>>> transform magnitudes that have no position so you only modify them
>>>>> with the
>>>>> scale, shear, and rotation aspects of the matrix (i.e. everything
>>>>> but the
>>>>> translate components).
>>>>>
>>>>> For some reason I got the impression that Vector was only used to
>>>>> represent magnitude because people were bringing up delta
>>>>> transforms in the
>>>>> context of talking about it. If it is a 1x4 vector then it is just
>>>>> Point3D+perspective-w-element, [x,y,z,w] no? Point3D can
>>>>> participate in
>>>>> 4x4 transforms, supplying an implicit w=1 component, though it
>>>>> can't store
>>>>> the resulting w component if the matrix doesn't have 0,0,0,1 for
>>>>> the last 4
>>>>> elements (i.e. in perspective camera matrices)...
>>>>>
>>>>> ...jim
>>>>>
>>>>>
>>>>> On 7/24/2012 3:55 PM, Pedro Duque Vieira wrote:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>>
>>>>>> I've already commented on this in the matrix algebra thread, but I
>>>>>>> wanted to throw into this discussion the following point:
>>>>>>> I'm neutral on whether or not we need Vector - I'll leave that to
>>>>>>> the
>>>>>>> people with extensive 3D experience to comment. But, if
>>>>>>> transformation
>>>>>>> of vectors makes better sense to do a delta transform, and if we are
>>>>>>> adding deltaTransform() methods for other types (which I think we
>>>>>>> should
>>>>>>> do), then I think the "transform" method that does a "delta
>>>>>>> transform"
>>>>>>> on a Vector should actually be named "deltaTransform" for
>>>>>>> consistency,
>>>>>>> even if it is obvious that a vector should only be transformed that
>>>>>>> way...
>>>>>>>
>>>>>>>
>>>>>> I don't know what a delta transform is (never used it on my 3d work
>>>>>> background), so I cannot comment on that part.
>>>>>> All I can say is that I did use transform a vector by a given matrix
>>>>>> transform and it makes sense. It's basically a multiplication of a
>>>>>> 4x4
>>>>>> matrix transform with a 1x4 matrix, just like what happens when you
>>>>>> transform a point3d.
>>>>>> So I think:
>>>>>> - void transform(Vector3d)
>>>>>> Should be present on the transform3d class.
>>>>>>
>>>>>> Just my 2 cents,
>>>>>>
>>>>>>
>>>
>
>