Weird artifact on Iray renders
Karibou
Posts: 1,325
Look at the lower edge, where the gold and green meet. See the funny/blocky artifact in the Iray one? This is happening ONLY on my Iray renders, not in 3Delight. I've been troubleshooting this for hours and here's what I've discovered...
- It has nothing to do with sub-d or smoothing of the geometry. I've tried a dozen different perambulations of these and found no change.
- It isn't due to displacement or fancy shader "things." This is the base Iray uber shader with just diffuse and transparency. (Artifact looks just the same without transparency.) It looks the same if you have a regular DS shader rendered in Iray, too.
- It does not matter how high I set the Max samples, Max time, Render Quality, or Rendering Converged Rate, the artifact persists.
- It doesn't matter if I've lit the scene with an environment dome, emissive spheres, sun/sky only, or any combination thereof.
- It DOES have something to do with how Iray processes the border between the two colors. I had this happen on another texture, and by eliminating an "embossed" effect on the base texture, the artifact disappeared. Alas, this has no embossed effect, so that doesn't help me.
- It seems that Iray is filling entire polys that the color boundary crosses with one color.
Help???
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Comments
I can't see what you mean. Which is the Iray render?
I can see the little triangular dark spots above the gold trim.
I've come across this sort of thing in Iray on a few occasions for example Kens birds often seem to render with black squares in places, I haven't a clue what causes them I tend to just photoshop them away.
Top one.
Alas, I'm trying to develop texture sets compatible with Iray. Photoshopping them out isn't an option. :(
I can see it now that you have highlighted it. I don't know the solution. I'd suggest writing a bug report.
Alas, I'm trying to develop texture sets compatible with Iray. Photoshopping them out isn't an option. :(
With Kens birds I put it down to something to do with the transmaps and over lying feathers. Unfortunately I can't remember where else I've come across it but I have - not much help I know.
I've reported it as a bug. I'm really hoping it's NOT a bug, but rather something stupid that I've done (so it's easily fixed.)
Check your transparency maps...
In your favorite image editor, make sure they are NOT RGB images...greyscale/black and white...just not RGB.
They are. And it happens without transparency maps. :( But thank you for the thought!
I noticed a similar problem when rendering the Dred Pickup. At the time, I just wrote it off as an issue with IRAY's texture compression, but never fiddled with the thresholds in the advanced render settings as it wasn't important to me at the time.
An example of what I'm seeing is in the first image. You can see the blotchy red in the seats and the uneven edges of the yellow trim on the red paint. Both are supposed to be straight lines with shard edges.
The texture on the seats is 2048x2048 and the default Texture Compression High Threshold is 1024. While I haven't seen specific documentation on this, I assume that it means some, or perhaps all, textures greater than 1024 in size are compressed somehow. This likely involves scaling of some sort which can create artifacts like we're seeing.
Unfortunately, increasing the High Threshold to even 4096 didn't change the results. I also tried applying a level 2 subdivision to the pickup. The subdivision provided some nice results on the smoothness of the model when rendered, but didn't eliminate the blotchy red in the seats, nor fully clean up the yellow stripe on the paint.
I then exported the model at its base resolution and brought it into blender where I examined the UV maps and textures. The UV maps didn't look bad and are probably perfectly fine for a lot of render engines. However, the lines in the UV map for the seat trim were neither vertical, nor horizontal. They were skewed about 5 degrees off vertical.
You can see an example of the UVs of one of the seats in the second image. Each of those blocks is one pixel. The entire image covers fewer than 30x30 pixels of the original texture. The red line in the image was a maximum of 5 pixels wide and many of those pixels were anti-aliased by whatever image editing program they were created in. That just isn't enough detail given the UV maps and contrast between the red line and the gray.
In the specific case of the Dred Pickup, I believe one can't expect any better from IRAY. Your garment may have a similar issue.
As a test, I was able to clean up one of the trim lines in a seat by assigning it a new surface and then I simply copied the surface settings from the remainder of the seat, dropped the texture from the Base Color and set the Base Color to an appropriate shade of red that I sampled in GIMP from the original texture. Those results are in the last image. (I didn't do a complete job on it as it was just a test. My surface selection should have been better, but it was good enough to see if the problem could be fixed with that technique.)
I've seen something like this in LuxRender, which is also an unbiased renderer. How dense is the mesh in that draped cloth? The glitches I saw in Lux were associated with low-density mesh being hit by light at a sharp angle — apparently the edge of the object where the light rays should begin to cast shadows was being confused by the "chunky" mesh giving a stairstepped edge to some shadows. Could this be something similar?
I would think if it were that, it would vary with various levels of subdivision. I know what you are talking about in Lux, hair is something that will show it...and in 3Delight you'd be able to adjust the shadow bias to clean it up, some.
I think you hit the nail on the head with your texture observations -- it does correspond to texture scaling, especially over areas where the texture is stretched. In the process of developing textures, I've noticed that if I have a low-resolution texture -- even one with a solid fill -- especially over areas of very large polys, I'm seeing artifacts. Increasing the resolution of the texture has been the best fix I've discovered, which is hardly ideal when one is trying to cram a lot into one's GPU RAM. Still, that's about all you can do without reworking the geometry. This is an extreme example, but I've shown what Iray does with the morphing cloak when a simple 2-colored texture is applied over stretched polys. The second render is 3Delight, where no artifacts can be seen.
I've seen it too, on fingernails, no transparency.. glad it wasn't anything I was doing wrong, still very annoying.