[Insight-developers] precision truncation of 'delta' in ResampleImageFilter

[Johnson, Hans J]

My changes were almost certainly cosmetic.  I don't recognize this code.

Hans


On 3/21/11 3:28 PM, "Michael Stauffer" <mstauff at verizon.net> wrote:

>Hi,
>
>I'm cleaning up ResampleImageFilter, removing redundant and unnecessary
>code. I have a question about lines 637-661, copied below. 'git blame'
>shows the code was originally from Dirk Padfield, and most recently
>edited by David Cole and Hans Johnson.
>
>This code truncates values to a set precision to avoid accumulated
>rouding errors while repeatedly adding a small delta. The comments here
>suggest it should be truncating the 'delta' variable, but it's operating
>on 'inputIndex'. As best I can understand, it should be operating on
>'delta' and not 'inputIndex'. As it stands, 'delta' doesn't get truncated
>to the desired precision at all. 'inputIndex' is initialized before this
>code in order to calculate delta, then it's set again in the subsequent
>while loop, to what looks like the same value in the first loop
>iteration, so this precision adjustment here actually doesn't do anything.
>
>The code is repeated again later in the subsequent while loop, where it
>operates on 'inputIndex', which looks correct.
>
>Modules/Filtering/ImageGrid/include/itkResampleImageFilter.txx
>Lines 637-661:
>
>  // Delta is precise to many decimal points, but this precision
>  // involves some error in the last bits.  This error can accumulate
>  // as the delta values are added.
>  // Sometimes, when the accumulated delta should be inside of the
>  // image, it will be slightly
>  // greater than the largest index in the image, like 255.00000000002
>  // for a image of size 256.  This can cause an empty column to show up
>  // at the right side of the image. If we instead
>  // truncate this delta value to some precision, this solves the problem.
>  // Therefore, the following routine uses a
>  // precisionConstant that specifies the number of relevant bits,
>  // and the value is truncated to this precision.
>  for ( unsigned int i = 0; i < ImageDimension; ++i )
>    {
>    IndexValueType roundedInputIndex = (IndexValueType)( inputIndex[i] );
>    if ( inputIndex[i] < 0.0 && inputIndex[i] !=
>(double)roundedInputIndex )
>      {
>      --roundedInputIndex;
>      }
>    double inputIndexFrac = inputIndex[i] - roundedInputIndex;
>    double newInputIndexFrac = (IndexValueType)( precisionConstant
>                                       * inputIndexFrac )
>                               / precisionConstant;
>    inputIndex[i] = roundedInputIndex + newInputIndexFrac;
>    }



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