Using a gradient ramp to control hair thickness is fine, as you can see in the first image by comparing the render to the viewport.
But when I try to use vertex paint to control thickness, as you can see in the second image, the render does not look like the viewport.
I am using the latest build of Ornatrix (15953),
If you want to test this in the Max file, it is set up initially to use a gradient ramp. Then to test the issue with vertex paint:
1. Select 'geo_prt_featherL_002'
2. In the top modifier 'Ox Render Settings', clear the 'Existing 'Thickness Map' (right click on the map -> 'Clear').
3. Click on the 'Assign Map' button (green button), select 'Channel 4', then click on 'Assign'.
