[Insight-developers] fast image pixel access method

[Bradley Lowekamp]

One more thought.

If you are generating the list of sample points once and then using them multiple times to sample there are perhaps other interesting things which could be done.

CPU likes to access memory in a linear fashion. Depending on your density, and the size of the neighborhood etc.. If you access your random points in a sequential fashion you may see benefits. That is 1) generate a sequential set of random points 2) sample the points in order. Step 2 should may see some benefits... Now under what conditions does it out weight the first? 

Brad

On May 13, 2013, at 11:49 AM, "Gao, Yi" <gaoyi.cn at gmail.com> wrote:

> Hi Brad,
> 
> Thank you for the suggestion!
> 
> Sorry in my previous code I had the mistake for randomly picking position for the center point. Now I changed it to be in the inner region.
> index[0] = rg.lrand32(1, size[0] - 2);
> index[1] = rg.lrand32(1, size[1] - 2);
> index[2] = rg.lrand32(1, size[2] - 2);
> 
> And I also turn off the boundary condition by:
> it.NeedToUseBoundaryConditionOff();
> 
> Now with the same parameters, nbhditerator is around 4 seconds (vs direct access 5.5 seconds)!
> 
> I will modify my code to take advantage of this! 
> 
> Thank you!
> 
> Best,
> yi
> 
> 
> 
> On Mon, May 13, 2013 at 11:34 AM, Bradley Lowekamp <blowekamp at mail.nih.gov> wrote:
> Hello,
> 
> On May 13, 2013, at 11:03 AM, "Gao, Yi" <gaoyi.cn at gmail.com> wrote:
> > When using NeighborhoodIterator, i'm not sure if the boundary checking is automatically turned on. I guess if that is by default on, then there is not much speed gain with it.
> 
> 
> Because the region you specify to the Iterator is on the boundary it will turn on the boundary checking. You can either turn off the condition:
> 
> http://www.itk.org/Doxygen/html/classitk_1_1ConstNeighborhoodIterator.html#a438cb0146b802b04a771a2461952cff8
> 
> or you can change the region size you initialize the Iterator with.
> 
> You can just use the operator<< to check what the iterator is doing.
> 
> Also consider using a large type for the accumulation type, such as itk::NumericTraits<T>::AccumulatorType;
> 
> And consider using std::accumulate(it.Begin(), it.End(), 0 )
> 
> Brad
> 

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