Edit update: the email contents don’t match up with the actual post contents, so ignore this post. I’ll leave it for posterity, but it answers a question that is no longer being asked.
What you’ve just defined is the following:
- voxels with values 0-150 have opacity 1
- voxels with values 150-180 have linearly decreasing opacity from 1 to 0
- voxels with values 80-255 have opacity 0
If you want black (typically taken to be value 0), to be transparent, then set it to opacity 0.