View Full Version : Skin artifacts with transparency.

Mr Rid
05-24-2014, 02:49 AM
I keep running into issues with Skin nodes with transparent surfaces near them, and I can find no setting that will correct it.

Here, the forehead skin renders luminous behind a transparent hair surface, instead of shadowed. A similar thing happens to Skin polys sharing points with transparent polys beside them.

When I apply Skin to an eyeball behind a transparent cornea, the face surface around the eye gets weird with these red flecks and black splotches.


I have another scene with a mesh with Skin applied but no transparency, that insists on rendering a little white spot in the tight crease of a mesh no matter what I try. The mesh is perfectly constructed, and subdiv, trigons, double/single-sided, or any lighting or texturing change will not make the spot go away.

Anyone else dealing with these Skin issues?

05-24-2014, 11:31 AM
I think I've seen this before, where VPR will render the Skin Material correctly and F9 will not.

Try this: under the Surface Editor, look for the setting for Exclude from VStack--I don't remember if it needs to be off or on, so try both.

I've also seen an issue where the skin renders artifacts when it penetrates another object surface, so you'll want to avoid that.

Hope this helps.


05-24-2014, 11:35 AM
Oh, if you're using texture maps, try disabling Pixel Blending where you see the problem--sometimes this feature can cause problems at the edges of a texture, especially masks. I think it's because Pixel Blending is essentially blurring the image and it may pull in color from the outside of edges (the BG color maybe?).


05-24-2014, 12:24 PM
Really looks like that hair geometry is intersecting the head. Gonna be an issue I think.
Take a look at this thread: http://forums.newtek.com/showthread.php?137774-SSS-Glow-at-Intersection-of-2-Objects

As far as the white spots go....
Are you using an HDRI? It could have a super bright high end. Try enabling Limit Dynamic Range (the check box under Image Processing has more options than in Render Settings). Sometimes I like the look I get with simply raising the Gamma in the image editor instead. There are other, more involved, "clamping" methods too.
Take a look at this: http://forums.newtek.com/showthread.php?138274-Glitter-in-skin-shader

05-25-2014, 07:49 AM
yeah I don't think it's transparency but rather the sss rays getting diverted because of the intersecting polygons.

05-25-2014, 08:06 AM
unfortunately the hair would have to be in a separate layer and then you can check-box "unseen by rays", that will solve the glowing issue. I wish we could do a lot of this stuff on a per-material basis instead of a per-layer.

Mr Rid
05-28-2014, 01:09 AM
Thanks for the replies!

I am trying to get Sigma2 to work, per Chris Jones suggestion in JoePoe's link. It eliminates the interpenetrating poly problems, but so far it is a trade off as I am not getting backlit translucency or moist skin to look as good as what I get with Simple Skin. But one shader looks better in certain lighting than the other.

But I think that if a surface is completely transparent with no spec, reflect or refract, then it shouldnt be 'deflecting' rays or showing up in any way, let alone the weird red fleck/black splotch thing I am getting with surrounding Simple Skin polys. And if an interpenetrating poly is black, then it seems like it should make the Skin dark, not luminous. Some transparent polys close in front of the Skin are also refracting too bright.

I am not using HDRIs, but a key light is 1200%. I cant figure out the white spot thing. V-stack and pixel blending are not the issue. I dont see anything "illegal" about the polys or deforms in the problem area.

Rendering with Simple Skin or Sigma2 is also regularly causing a particular scene I am working with to crash LW every so many frames, or freeze my entire workstation, and it even blue screened once. I disable the nodes and it renders fine. I dont know WTH is going on.

05-28-2014, 11:47 AM
Anything to do with light/Shading samples? (RenderTAB) Default is set to 1

05-28-2014, 03:57 PM
I had the same problems, though off the top of my head, I cant remember exactly what I did to fix it.
I will take a look tonight when I get home.

Increasing RPE was a big thing, and clamping really bright spots - though this was primarily for IBL.

As far as I can recall the whiter than white transparency problems (as seen around the eyes in the thread posted by JoePoe)
came from SimpleSkin when the subdermal distance was greater than the thickness of the polys. Are you using a subdermal distance map?
For me, with SimpleSkin its the subdermal distance map that makes/breaks the shader.

EDIT: after re-looking at the pics you posted, I would definitely look at the subdermal distance (I think that is where the red is coming from)

05-28-2014, 04:18 PM
this shows the evolution of the skin shader I used. You can see early on the extremely bright areas. I am fairly certain I painted these out using subdermal distance. If I remember correctly I darkened the overly bright areas (brought the subdermal colour closer to the surface)
I will double check when I get home....