vector_to_opacity - dyn not working as expected or not intuitive

[cboulay]

docstring:

visbrain/visbrain/utils/color.py

Lines 656 to 657 in b2214ea

	dyn : tuple | (0., 1.)
	Minimum and maximum of the transparency levels.

relevant code:

visbrain/visbrain/utils/color.py

Lines 685 to 686 in b2214ea

	xtr_min = max(dyn[0] * _data.min() / clim[0], 0.) if clim[0] != 0. else 0.
	xtr_max = min(dyn[1] * _data.max() / clim[1], 1.) if clim[1] != 0. else 0.

Let's say we want the opacity to range from 0.5 to 1.0: dyn=(0.5, 1.0)

If clim[0] is a small number, smaller than _data.min()/2, then xtr_min will always be > 1.0. Then the subsequent call to alpha = normalize(_data, xtr_min, xtr_max) will always return 1.0 for every value.

I think the intended result can be achieved (at least for the 'ascending' case) with something a bit simpler:
alpha = normalize(np.clip(_data, clim[0], clim[1]), dyn[0], dyn[1])