The Opacity gradient curve is set graphically.

The intensity range is located on the abscissa axis while the Opacity percentage is represented on the ordinate axis.

All the voxels having an intensity value located inside the curve, give them a contribute proportionally to the Opacity percentage set. The rest of the voxels, located outside the curve, are not visualised.

Below you can see two examples of different levels of opacity on the green channel. High opacity level on the left, low opacity level on the right.