[Insight-developers] BUG in BinaryBallStructuringElement

[Bill Lorensen]

Dirk,

Is there anything in the literature that we can use to validate this?
Your logic seems correct to me.

Bill


On Wed, Oct 16, 2013 at 2:05 PM, Padfield, Dirk R (GE Global Research)
<padfield at research.ge.com> wrote:
> Hi All,
>
> I am writing to ask your advice about a bug I found in BinaryBallStructuringElement.
>
> For a while, I have been bothered by the fact that the BinaryBallStructuringElement return balls that are larger than the specified radius.  For example, when given a radius of 1, it returns the structuring element:
> 1    1    1
> 1    1    1
> 1    1    1
>
> But this structuring element has a radius that is more than 1!  If it truly had a radius of 1, it would be a cross shape in this case.
>
> When choosing a larger radius, the problem is more obvious.  Setting radius = 5 (leading to a structuring element size of 11x11) results in:
> 0    0    0    1    1    1    1    1    0    0    0
> 0    0    1    1    1    1    1    1    1    0    0
> 0    1    1    1    1    1    1    1    1    1    0
> 1    1    1    1    1    1    1    1    1    1    1
> 1    1    1    1    1    1    1    1    1    1    1
> 1    1    1    1    1    1    1    1    1    1    1
> 1    1    1    1    1    1    1    1    1    1    1
> 1    1    1    1    1    1    1    1    1    1    1
> 0    1    1    1    1    1    1    1    1    1    0
> 0    0    1    1    1    1    1    1    1    0    0
> 0    0    0    1    1    1    1    1    0    0    0
>
> This is clearly not an ellipse/circle with radius 5 because the interior ellipse/circle is touching each image border at five points rather than just one.  As it turns out, the code is actually defining a radius that is "X + 0.5", where X is the radius that is requested!
>
> The problem is in the specification of the ellipse axes on lines 70-76 of itkBinaryBallStructuringElement.hxx:
>   // Define and set the axes lengths for the ellipsoid
>   typename EllipsoidType::InputType axes;
>   for ( i = 0; i < VDimension; i++ )
>     {
>     axes[i] = this->GetSize(i);
>     }
>   spatialFunction->SetAxes(axes);
>
> In this case, "this->GetSize()" is equal to radius*2+1.  But, an ellipse/circle with radius X should have axes length 2X, not 2X+1!  In the implementation, the center of the ellipse is properly accounted for by setting it to "this->GetRadius+1", but the size of the ellipse is not correct!
>
> To correct this, we can make a simple change either
>     axes[i] = this->GetSize(i) - 1;
> or
>     axes[i] = this->GetRadius(i) * 2;
>
> The second is probably more intuitive.
>
> With this fix, we get for radius=1:
> 0    1    0
> 1    1    1
> 0    1    0
>
> and for radius=5:
> 0    0    0    0    0    1    0    0    0    0    0
> 0    0    1    1    1    1    1    1    1    0    0
> 0    1    1    1    1    1    1    1    1    1    0
> 0    1    1    1    1    1    1    1    1    1    0
> 0    1    1    1    1    1    1    1    1    1    0
> 1    1    1    1    1    1    1    1    1    1    1
> 0    1    1    1    1    1    1    1    1    1    0
> 0    1    1    1    1    1    1    1    1    1    0
> 0    1    1    1    1    1    1    1    1    1    0
> 0    0    1    1    1    1    1    1    1    0    0
> 0    0    0    0    0    1    0    0    0    0    0
>
> This is a true circle with radius 5!
>
> My questions are:
> 1) Is anyone else bothered by this bug?  I imagine that many users expect the corrected version and don't realize they are getting the incorrect one.
> 2) Do others agree with this fix?
> 3) Can we make this fix given the number of filters/applications that will change slightly as a result of this fix?
>
> Many thanks,
> Dirk
>
>
>
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