[question] confused by extract_mesh.ipynb, the dataset from img seems not be used?

hi,
i read the code in extract_mesh.ipynb and noticed the var `dataset` in line 14 of cell 2 is not used in code below to export the colorless mesh. In my perspective, the var `sigma` which feeds to marching cube and then extract the mesh is created by linspace data from numpy. So the img data is actually not used in mesh exportation? pls give me some explain for this question, thanks

@kwea123

Read extract_mesh.ipynb, especially line 14 of cell 2 and the later mesh-export steps. Trace how dataset, sigma, the numpy linspace data, and marching cubes are used, then document whether the image data contributes to the exported colorless mesh.

Written by the indexing model from the issue text.