View Full Version : Hair Rendering Artifacts

03 March 2007, 07:43 PM
Hi all,

I'm working on a character for a short film (see below) and it's going quite well, BUT I've been getting strange rendering artifacts that I can't get to go away. So far I've ignored them, but I've gotten a shot I can't get to work right and I figured I'd post to see if anyone recognized the cause since it takes about 4-8 hours to test each sample solution.

Look at the lip in this sample movie to see what I'm talking about.

-It appears not to be shadow related as turning shadows off doesn't solve it.
-Changing the hair shader between hair render / geometry / muh (shown above) doesn't remove the artifact just changes it. The hairs appear as black hairs in hair renderer.
-changing rendering methods doens't solve it
-not geometry approximation
-disconnecting the texture map causes the artifacts to change, but not go away. I've had the problems in hair scenes with no textures at all.
-tried changing the scene clipping settings.
-changed the bsp and hair bsp settings with no luck!

The artifacts appear as dark, shadowy, rectangular shapes under the hair and then suddenly move midway through the shot. I wouldn't care if they didn't move! lol

Any thoughts would be greatly appreciated.

03 March 2007, 08:07 PM
Firstly I don't know why it happens, but I would like to suggest a workaround, like seperating you hair to different parts (by extract polygon), isolate the affected area and apply individual hair. For morphing, driven the extract polygons by deformer cage, see if it can helps.

03 March 2007, 09:20 PM
Thanks Dickma,

That's actually a very good idea if I can manage to isolate a problem to a specific area (like the lip here,) but unfortunately the artifacts flash all over the hair surfaces. Some of the worst problems are in the ears where the error pops around seemingly randomly even with very slight movements, almost like it's a z-depth issue of some sort. In fact, one of the first things I tried was increasing the ray trace limits because it looked so much like the problems you get when rays don't trace deeply enough. I've tried a number of different hair breakdowns and divisions with no luck.


03 March 2007, 01:21 AM
Is the angle between hair and skin very very shallow?

03 March 2007, 06:25 PM
Is the angle between hair and skin very very shallow?

Is hair generated from a sub-D model? and as Mic_Ma said, the space between the polygon and hair guide is very close? May be because the hair comb along the polygons so that is lack of space.

I think when you are morphing your character, the hair guides may be sink under the skin so the rendering may be changed.

And personally I think, if you generated hair guide from a sub-D model, mostly the distribution of the hair is not good as the hairguide generated with polygon approximation with 0. Weird clumps will generate at the vertexes with very small polygons. so that is why I suggest the extraction of polygons, generate the hair with the extracted polygons with no sub-D, and driven that with deformer cage.

04 April 2007, 10:19 PM

Thanks for the tips. I wanted to check that out extensively before I posted anything as it certainly seemed like it might be an explanation. Unfortunately, though, that's not it either! I tried changing the hairs to emit at level 0 and making them all stand on end, but I still get the artifacts! I've also observed them a bit more in general. They're always rectilinear to the frame edges and symmetrical regardless of the geometry. So I get parallel vertical lines flashing in and out and left to right mirrored down the center of the frame on my rabbit's ears even though the rabbit is not symmetrical. It's very odd. Hiding other hair elements or objects doesn't make them go away either.


04 April 2007, 01:45 AM
So I get parallel vertical lines flashing in and out and left to right mirrored down the center of the frame on my rabbit's ears even though the rabbit is not symmetrical. It's very odd.

Haven't worked much with hairs but I did a lot of game particle effects these last few weeks, and this description reminds me much of the artifacts you get with camera facing sprites. I wonder if your hair artifacts have anything to do with that?

04 April 2007, 02:46 AM
Yes, it does sound an awful lot like that. That's a good thought. It really looks as though the hair is being divided into planes sorted by depth and then some of them are rendering black instead of shaded. It turns up as soft and dark in this instance because I've got very transparent hairs, but if I turn that off and render with the geometry shader, I get hard, solid black hairs in the exact same pattern and locations.

At this point, I'm rendering anyway and just ignoring the artifacts as I can't get them to go away and the deadline is Thursday. It's usually not a tremendous problem in shots with a static camera, but in the shots with a moving camera, the artifacts can get so severe that the shots are all but unwatchable.

04 April 2007, 02:43 AM
here's another couple of example movies to show what I'm talking about: (
(this one shows the errors very clearly. watch the ears.) (
(no flickering yet in this render, but look at the large, dark black line moving across the body. This corresponds to NO geometry whatsoever in the scene and appears even with shadows completely disabled.)
(this is an entirely different scene producing similar artifacts. It don't even use any texture maps. In this scene, artifacts don't appear to be rectilinear to frame boundaries. However, they also don't match any geometry!)

I'm thinking that I may as well go ahead and report this to softimage as a bug unless anyone has any further ideas.

04 April 2007, 12:09 AM
Hey colin,

The only experience I've had alike yours is when I tried Final Gathering for the rendering of hair, back in the good old days of XSI 3.5, I think. :D
We were never able to find out why did FG cause such strange artifacts, and switching to muHair didn't help either. I do believe that a report was sent to Softimage by one of my teachers though, but I don't quite recall if they got back to us or not.

We solved it by not using FG in our final render.

I've not had that problem ever since, but we've not had to do much stuff with hair until now.

Sorry about not being able to help more.

- A.


After some consideration, you might want to lower the FG radius's values. It might help, if you are indeed using FG, that is. XD

11 November 2007, 08:01 PM
I'm having this same problem with my project right now, has anyone found a solution? It looks like my entire character (a dog) is sparkling as it moves.

11 November 2007, 09:02 PM
unfortunately, I never solved it. This problem was with my thesis film and I simply ended up having to hand it in with the problem. ouch. Fortunately, I wasn't going to VFX school, so the VFX were just a bonus anyhow. It definitely seemed to be related somehow to depth from the camera as far as I can tell.

12 December 2007, 09:06 AM
Maybe this is the fix?

My problem was a noobish aliasing problem, but your problem might be fixed by this.

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