I'm rendering some hair close up, with a fresnel-based material on them:
The majority of the render looks fine:
However, some areas have a scratchy-looking artifacts running perpendicular to the hair length:
What cases these artifacts?
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$\begingroup$ You might need to right-click and open the image in a new tab at full resolution to see the artifacts $\endgroup$ – ajwood Nov 21 '14 at 16:42
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I guess you are using curve segments as primitives for your hair. In this case Blender interpolates the segments between the simulated hair particles using cardinal splines. However it can only approximate the spline using linear, tube-like segments.
It looks like Blender does not close the gaps between the tubes properly. You can reduce the effect by increasing the Curve subdivisions in the Cycles Hair Rendering settings:
This will however hurt performance since more geometry is created.
Here is a comparison of Curve subdivisions set to 2 (top) versus Curve subdivisions set to 4 (bottom):
In the bottom image the white lines are gone, however you can still see that the curves are only approximated by straight tubular pieces (see the edge of the hair in the bottom left corner and the shading of the hairs in the bottom right corner).
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$\begingroup$ I was actually using line segments for the primitives. Switching to curve segments is an improvement $\endgroup$ – ajwood Nov 21 '14 at 18:39
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$\begingroup$ Ok, this is even worse of course since Blender does not even attempt to close the gaps then ;) $\endgroup$ – maddin45 Nov 21 '14 at 18:42
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$\begingroup$ The location where the artifacts show up threw me off.. There are hairs with more curvature than the offenders that look fine $\endgroup$ – ajwood Nov 21 '14 at 18:44
