Volume rendering doesn't show when image intensity values are small

[aylward]

If the intensity values in an image only span a small range near zero, the volume rendering appears blank/empty. However, 2D slices display as expected.

Example dataset is at : https://drive.google.com/file/d/1HeoPACKr_a-sYmEf1d2PGoCJotLrUb-y/view?usp=sharing

[jourdain]

default edge gradient is too small for that dataset. Bumping its value allow the volume to be seen.