[vtk-developers] vtkDataArrayTemplate

[John Biddiscombe]

Benjamin

OK I see the problem

Solution
1) Rewrite vtkPointLocator to not use GetPoint()/GetTuple()
2) there is no 2)

Point locator could be quite easily changed. all you need do is
1) At the start, get the Points array, it can be either float or double. 
find out which and store the points data pointer
2) Modify the main loop to be templated over type float/double and 
remove GetPoint, replace with data[n] etc...

try and see if it runs any faster. It's been a while since I looked at 
the PointLocator code, so it might be a bit more messy than that. 
(Actually, I'm sure it's more messy as there are many GetXXX functions 
all implemented individually as I recall).

Sorry that I don't have a better solution. Perhaps you could have a play 
with KdTree, it is a point locator too and might be implemented more 
efficiently. Many of these classes (vtkPointLocator) date back to the 
origins of vtk and then double precision wasn't even used for points as 
I recall!

What algorithm are you implementing?

JB
> Hi John
>
> Thanks for the reply. I've got a few comments because there still are 
> some issues with this
>
> - One performance bottleneck is the vtkPointLocator. It can't use the 
> GetPointer method because it operates on vtkPoints which wraps around 
> arrays and it doesn't seem to offer that interface. Why is there a 
> vtkPoints interface anyway (Roughly 50% of the time spent in 
> GetClosestNPoints goes to GetPoint and because of this, the code isn't 
> even race free)
> - Any GetPoint Method currently wraps around GetTuple. And they're 
> used really all over the place
> - GetPointer is not component sensitive - so retreiving tuple i 
> results in arr->GetPointer(i*arr->GetNumberOfComponents()). An 
> tuple-sensitive arr[i] operator would be a lot nicer no?
>
> Cheers
> Benjamin
>
> John Biddiscombe wrote:
>> Benjamin
>>
>> The GetTuple interface is only intended if you really don't know what 
>> type of data you are expecting. You should use the GetPointer cast to 
>> float/double etc - this gives you raw access to the array in native 
>> form. Only use the GetTuple in emergencies.
>>
>> vtkDataArrayTemplate is already remplated over types for the bulk of 
>> work in creating the class definitions for each type. You should look 
>> at some of the imageing filters for examples of how to get a scalar 
>> pointer then execute on whatever type you need.
>>
>> I'm keeping this reply brief as timeis too short. Hopefully someone 
>> else will give you further details if you need them.
>>
>> JB
>>> Hi
>>>
>>> I noticed that my code ran very slow so I did some profiling and I 
>>> noticed to my horror, that about 50% of all time is spent inside the 
>>> GetTuple (and it's wrappers). Looking at the code I noticed that for 
>>> every access a copy of a tuple is generated which is pretty insane 
>>> when all you want is a distance to another point...
>>> I do realize that in vtk, you never know whether you work with 
>>> floats or doubles or ints etc, but I'm sure there are other ways 
>>> that are not this hard on performance
>>>
>>> Are there plans on improving this? I've looked at a few ideas and my 
>>> interest was caught by Eigen2, a very fast Math Library making 
>>> extensive use of templates challenging even the mkl in terms of 
>>> speed. They make heavy use of template expressions so there are no 
>>> intermediate copies at any point. I'd guess something like this 
>>> would be very much doable for vtk
>>>
>>> Even though it would be quite some work, I imagine all of vtk would 
>>> gain significantly from this - in terms of genericity, design and 
>>> performance
>>>
>>> Any comments?
>>>
>>> Cheers
>>> Benjamin
>>> _______________________________________________
>>> vtk-developers mailing list
>>> vtk-developers at vtk.org
>>> http://www.vtk.org/mailman/listinfo/vtk-developers
>>
>>
>


-- 
John Biddiscombe,                            email:biddisco @ cscs.ch
http://www.cscs.ch/
CSCS, Swiss National Supercomputing Centre  | Tel:  +41 (91) 610.82.07
Via Cantonale, 6928 Manno, Switzerland      | Fax:  +41 (91) 610.82.82