Function otsu_threshold 

pub fn otsu_threshold(data: &[f64], num_bins: usize) -> f64

Otsu’s method for automatic threshold selection

Finds the threshold that maximizes inter-class variance. Matches MATLAB’s graythresh: operates on all values including zeros, normalizes to [0,1] range, and returns threshold at bin edge.

Arguments

• data - Input data (e.g. flattened 3D image)
• num_bins - Number of histogram bins (typically 256)

Returns

The optimal threshold value