[Insight-developers] TransformIndexToPhysicalPoint/TransformPhysicalPointToIndex seem slow

[Bill Lorensen]

Are you building a Debug or Release build? There can be a huge
difference in speed between the two.

Bill


On Tue, Jun 2, 2009 at 6:07 PM, kent williams
<norman-k-williams at uiowa.edu> wrote:
> I'll state in advance I haven't tried a release build, which I'm sure would
> help somewhat, but...
>
> I'm trying to blend two images that potentially have different origins,
> spacing and dimensions.  So I am using this idiom:
>
>    ImageType::Pointer curImage; // set elsewhere...
>    itk::ImageRegionIteratorWithIndex<ImageType>
>      curIt(accumulator,accumulator->GetLargestPossibleRegion());
>    ImageType::PointType accPoint;
>
>    for(;!curIt.IsAtEnd(); ++curIt)
>      {
>      ImageType::IndexType index(curIt.GetIndex());
>      accumulator->TransformIndexToPhysicalPoint(index,accPoint);
>      ImageType::IndexType curIndex;
>      if(curImage->TransformPhysicalPointToIndex(accPoint,curIndex))
>        {
>        ImageType::PixelType curPixel(curImage->GetPixel(curIndex));
>        curIt.Set(Blend(curIt.Value(),curPixel));
>        }
>      }
>    }
>
> This sort of Index->Point->Index transformation seems unavoidable, unless
> I'm missing something, but it's dog slow compared to (for example) stepping
> through two images with a regular ImageRegionIterator for both images.  And
> this code doesn't even address proper interpolation of the voxels from the
> second image.
>
> It would probably be quicker to resample the second image to the first
> image's spacing, then figure out the overlapping regions and use regular
> RegionIterators, but that seems like overkill.  Certainly I could detect
> cases where both images have the same spacing and do the simpler traversal
> too.
>
> But is there a way to make this sort of idiom faster?
>
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