Isotropic Closing function produces smaller binary object than the input

[NNrico]

Hello,
I want to submit a consideration regarding the correctness of the function IsotropicClosing.
In brief: it does not behave as one expects.

I tried the function IsotropicClosing on a binary image consisting of a single active point in the centre.
The output is a complete black binary image, instead of an image with a single point at the centre. I consider this an issue because when applying the morphological closing of an image it should return an image greater or equal than the input. This does not happen and you can check it by running the following code in Python.

import diplib as dip
import numpy as np
arr = np.zeros((10,10))
arr[5,5] = 1
arr = arr>=1

#10x10 binary image with only pixel(5,5) active
image = dip.Image(arr)
dip.viewer.Show(image,title='inputimage')
image_closed = dip.IsotropicClosing(image,3)
dip.viewer.Show(image_closed,title='image closed')

By inspecting the source code the function IsotropicClosing calls (as expected) a IsotropicDilation followed by a IsotropicErosion but their parameter is not the same the one of IsotropicDilation i guess is rounded below (Lesser(...)) and the the one of IsotropicErosion is rounded up (Greater(...)). Here's a snipped of the C++ source code, version 3.5.2.

void IsotropicDilation(
      Image const& in,
      Image& out,
      dfloat distance
) {
   DIP_THROW_IF( !in.IsForged(), E::IMAGE_NOT_FORGED );
   DIP_THROW_IF( !in.IsScalar(), E::IMAGE_NOT_SCALAR );
   DIP_THROW_IF( in.DataType() != DT_BIN, E::DATA_TYPE_NOT_SUPPORTED );
   DIP_STACK_TRACE_THIS( Lesser( EuclideanDistanceTransform( !( in.QuickCopy() ), "object", "square" ), distance * distance, out ));
   // Note: QuickCopy() discards the pixel sizes. This allows `distance` to be in pixels rather than a physical distance.
}

void IsotropicErosion(
      Image const& in,
      Image& out,
      dfloat distance
) {
   DIP_THROW_IF( !in.IsForged(), E::IMAGE_NOT_FORGED );
   DIP_THROW_IF( !in.IsScalar(), E::IMAGE_NOT_SCALAR );
   DIP_THROW_IF( in.DataType() != DT_BIN, E::DATA_TYPE_NOT_SUPPORTED );
   DIP_STACK_TRACE_THIS( Greater( EuclideanDistanceTransform( in.QuickCopy(), "object", "square" ), distance * distance, out ));
   // See note above in IsotropicDilation().
}

I am afraid there is not a quick fix solution to this but it is definetly an incorrect behaviour since the structure element should be the same for both erosion and dilation.

[crisluengo]

Thank you for reporting this bug!

I've fixed it here: 903c5b0