[Vxl-maintainers] RE: component type of vcl_complex<T>?

[Ian S. <ian...@st...>]

There are several good reasons for treating complex<T> as a single scalar
rather than a two component pixel of component type T.

1. Mathematically, complex<T> is a scalar. We treat rgb<T> as a vector.
(think conversion to HSV. complex converted to (magnitude,argument) is still
a complex<T>)
2. Most functions that operate on multiplane images treat the two planes
independently. This is not correct behaviour for complex<T>. vil_convolve is
a good example.
3. What are the two components of a complex<T>? [re, im] is a rather
arbitrary separation, and can be dangerous
http://www.cs.berkeley.edu/~wkahan/JAVAhurt.pdf p 11.
4. AFAIK, there is no requirement in C++ that sizeof(complex<T>) =
2*sizeof(T), or that the data is stored as struct {T real; T imag;}. vil
assumes that a two component pixel can be split like this. (Of course every
real implmentation I have seen does look like this.)

Of course there are reasons for treating image<complex<T>> as a
two-component scalar and hence as a two plane image<T>.

1. Its what most (non-mathematical) programmers expect.
2. It reduces the number of types.

In partial answer to 2
We made complex<float> and complex<double> special cases, and there is no
reason to see this as a "slippery slope" to adding any more fundamental
pixel types.

Amtiha:
Are there cases where you are being forced to deal with image<complex<T> >,
you would not otherwise need to? I had thought the design was such that:

a. For templated functions, the code will deal with complex<T> just as
easily as it deals with rgb<T>. No extra code needed except in the rare case
where you need to specialisations (e.g. vil_convert)
b. For the non-templated vil_image_resource style functions - if you don;t
want to support complex<T> pixels, then don't bother.

If you have found this persuasive, by all means add any useful bits to the
vil/notes.html.

Ian.


> -----Original Message-----
> From: Amitha Perera [mailto:pe...@cs...]
> Sent: Wednesday, December 17, 2003 10:37 PM
> To: Ian Scott; Fred Wheeler
> Subject: component type of vcl_complex<T>?
>
>
> Ian,
>
> From the notes page of vil
>
> http://paine.wiau.man.ac.uk/pub/doc_vxl/core/vil/html/Design.html
>
> and from vil_pixel_format.h, the component type of vcl_complex<T> is
> vcl_complex<T>, and not T. Why?
>
> Fred and I discussed this a bit, and couldn't really think of what
> we'd lose by making the component type T. The advantage is that there
> would be fewer possible component types overall.
>
> Do you remember why that decision was made?
>
> Thanks,
> Amitha.
>