The default hair interpolation in Houdini behaves in a suboptimal way for grooming long hair. Specifically, the guides are not only influencing the shape of the hair as we would expect but also the local density of the generated hair (Image 1). This creates a clumping-like effect even without any hairclump nodes in the chain. Note that in the shown example I am already using an attribute map to drive the density.
The “influence radius/decay” parameters in Houdini do not exactly resolve this issue as they are essentially hiding the clumping effect by increasing the radius of them. You can still see noticeable segmenting near the roots and tips (Image 2). Increasing the radius parameter beyond the set limit of 0.2 begins to inhibit the guide's ability to shape the generated hair and delivers unpredictable results (Image 3).
This kind of interpolation does not happen in Ornatrix or Xgen as far as I know. Attached is a similar setup in Xgen with a basic density map (Image 4). The interpolated curves are only following the guide shapes and not the local density.* Done this way, long hair grooms need few interpolating guides to control flow while also giving enough hair density near the tip to style with modifiers.
So am I missing something here? Is there a way to completely turn off local density interpolation of the guide hairs and instead let them only interpolate shape?
*(at least to a non-noticeable amount; Xgen does still only grow hair in large general regions where there is a guide to drive the shape but that isn’t a concern at the scales shown here)
With Grow Unguided Hair enabled, the influence radius will not affect the density. This will also show where the guide influence radius does not fully overlap with the density map.
Houdini's linear guide interpolation is not suitable for use with such sparse guides. I would expect there to be dozens if not hundreds of guides to cover the area shown and have the generated hair follow the guides appropriately.
