I would suggest you look at the documentation

  https://www.nitrc.org/plugins/mwiki/inde...

First of all, it really helps visualization if you load an unthresholded image as an overlay, and subsequently apply a thresholding value (the minmax range). When you threshold an image, you artificially set all non-significant voxels to be zero, and since the data is smooth the surviving clusters are surrounded by neighbors of zeros instead of near-threshold values. This causes artificially dark interpolation.

The other option is to use nearest neighbor interpolation, which will cause jagged images:

gl.overlayloadsmooth(0)

The scripting/templates/jagged menu item illustrates this issue.