Setting transparency of a 3D object according to specific values of a 3D matrix

I have a 3D matrix visualized using the vol3d function (link below). I want to set the transparency of the visualized 3D object accoridng to specified values. For example values < 100 have transparency 0.8 and so on. So far I am only capable of changing the transparency of the whole object by defining the transparency of the alphamap. Code example:

h = vol3d('CData', multiDimMatrix);
alphamap(.5 .* alphamap);

https://se.mathworks.com/matlabcentral/fileexchange/22940-vol3d-v2

Best Regards