from vedo import *
from vedo.pyplot import histogram, plot

cmap = 'nipy_spectral'
alpha = np.array([0, 0, 0.05, 0.2, 0.8, 1])

vol = Volume(dataurl+"embryo.slc")
vol.cmap(cmap).alpha(alpha).add_scalarbar3d(c='white')
xvals = np.linspace(*vol.scalar_range(), len(alpha))

fig = histogram(vol, logscale=True, c=cmap, ac='white')
fig+= plot(xvals, alpha*max(fig.frequencies), '--ow', like=fig).z(1)

show([
      (vol, Axes(vol, c='w'), f"Original Volume\ncolor map: {cmap}"),
      (fig.clone2d("center",1.2), "Voxel scalar histogram\nand opacity transfer function")
     ],
     N=2, sharecam=False, bg=(82,87,110), zoom=1.1,
).close()