On Sep 29, 10:05 am, Paolo <pgri...@gmail.com> wrote:
> On Sep 29, 11:55 am, Axel M <axe...@gmail.com> wrote:
>
>
>
>> On 29 Sep., 17:45, Paulo Penteado <pp.pente...@gmail.com> wrote:
>
>>> On Sep 29, 12:24 pm, Axel M <axe...@gmail.com> wrote:
>
>>>> Great, I did not know about this construction, and honestly I do not
>>>> understand how it works (is there any documentation about it?).
>>>> Anyways, I tried it, and unfortunately I saw that it needed ~20%
>>>> longer (the complete function, not the rebin only). So, it is not
>>>> faster.. but it is great though.
>
>>> It is replicating a structure of a single field which contains the
>>> array input ({temp:input}), then selecting only a single field (the
>>> first, 0) of the resulting structure array. Documentation for this
>>> would be on creation and use of structures.
>
>> Ok, I got it. Thanks! Then probably it is the memory allocation for
>> the array of structures which takes so long... it would be great if
>> the ITT people would develop a _fast_ vector replicate, I fear
>> rebinning is not the best option.
>
>> In any case, based on the answers, I assume that my problem is rather
>> on the matrix multiplication part, so I can probably do nothing for
>> that.
>
>> Thanks a lot
>
> well considering your original problem - you need to apply
> a linear transformation to N vectors v_i=(x_i,y_i,z_i),
> for i going from 0 to a large N, right?
>
> I would just explicitely compute the transformed vectors
>
> z_i=(xx_i,yy_i,zz_i)
>
> by just writing out in the program the computation for every
> component,
> i.e.
>
> xx=x*c1+y*c2+z*c3+c4
> and same for yy,zz with appropriate constant coefficients c1,c2,c3,c4
> (that are the same for all i).
>
> But then maybe i misunderstood the problem...
>
> Ciao,
> Paolo
Yeah, I think you are right.
Another way to see it:
FUNCTION vc2rc, v0,v1,v2,v3,vc
xform = [[v1],[v2],[v3]]
n = <number of points in vc>
for i=0, n-1
temp = vc[*,i]
temp = temp # xform + v0
vc[*,i] = temp
end
END
This assumes that you can change vc itself and that v0 is a 3-vector.
In this case, there is only one copy of the point array, as it is
being transformed in place. In other schemes, there may have been as
many as three or four copies. If it is not OK to change vc, then this
function would have to make a vr array of the same shape as vc and
return it. But it is still the best solution as far as memory goes.
Yeah, the for loop is going to be slow, but a test will tell if it is
faster than other approaches. If the program causes paging to disk
with the original approach, then the for loop may be faster. If speed
is really, really important, then the above can be implemented in a C
DLM.
And yes, the three lines with "temp" can be collapsed into one, but
IDL will make small temps anyway here and so a single line may not be
much faster. I left it as three lines for clarity.
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