Iray render is quite wonderful so far, but I'm having these strange, grid-like polka dots issue from some Iray outputs. Especially when its texture is plane color.
Like this. You can see grid dots from brown hair. 



There's nothing wrong with hair texture, it's just RGB (79, 45, 26) plane color image. It should be brown-ish gradient at least. I've struggled for a while to remove that, and when I tweaked hair texture's scale, it went like this. Dots became stripes.



Maybe there's something wrong with texture scale in Iray? Well the hair texture was 2048x2048 plane color tga file, so I tried 1x1 brown color tga file. And those dots were just gone.



I wonder if RL is aware of this issue. If then, how could I get clean render? Will the patch be coming soon?

What is the UV map looks like for the hair?

You may also try to replace your TGA file with PNG. 

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From the scaling you did, it may be a tiling issue. Could you delete the texture entirely, and just set the material itself to brown?

First, Thank you for your reply, guys. 

Well, the character's UV map looks like this, but I don't think uv is the problem because he's not the only one showing this issue. Actually I can see grid dots on many other characters too, even on complicated textures. They're just not much noticeable..


And I already tried 2048x2048 png file before, there was no difference. But I tried 512x512 png file just now, and look, grid dots got loose somehow?


And I emptied color map and set diffuse color brown. Of course it was clean.


So I think, as Rampa said, there's something wrong with texture tiling issue in Iray. Have anyone had same issue? I hope there's some solution, no more manual photoshop labor....

That is bizarre! I render a lot, but I've never seen such a strange application of texture in Iray. It would be interesting to see the problem locally.
Can you share some prop that you are having a problem rendering with?

I'm afraid I can't share character files, but I think you can test it with iClone's primitive object and random plane color image. I've tried a simple sphere in iClone before, and this is the result. 

Could you give your system specifications, because I can't repeat it. Assuming you have an Nvidia card, it could be a driver issue.

This is what I get for a sphere:



Like 4u2ges I have also done a lot with Iray, and I have never encountered this either.

We are looking at Preview renders now. Do you get the same when you export and use the standalone renderer?

My Iray settings are basic: 150 iterations and denoising at iteration 149. Lighting is default iClone lighting; I just added the sphere and colored it.

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Here's my system spec : i7-7700K, 32GB memory, GTX 1080 Ti 11GB(driver ver. 417.35), win 10, and both iClone and Iray plugin is lastest version.
I thought in the first place there's some kind of glitch in Iray plugin because those grid dots were also appeared on my colleague's PC. Her system specifications are pretty much same with me, and graphic card driver version is 416.34. Dots were always there if it's default iClone lighting or not. Render settings were Photoreal, GI bounce 8, itereation 500 with De-Noise 499.
.......And I didn't thought of Standalone Renderer until now. I tried saving MI scene option and loaded it from Standalone Renderer, and the result was clean! But the third render option, iClone Render(Image Sequence) still gave me the same dots. Iteration here is 150, De-Noise 149.

Iray preview render


Iray iClone Render(Image Sequence)


And Standalone Render with a saved MI scene file.


Does this mean I've got iClone visual setting wrong or something like that? Well I should update my graphic card driver and test some more options. 
And thank you animagic, for your kind answer. 

Finally found the cause of this problem. And it wasn't graphic driver or any other related to the modeling.

In the end of Preference tab there was an Iray option for texture compression, and it was High. I changed it to None, and Iray preview render is clean now. Didn't know about that, I only looked into Iray Render Settings window. I have no idea why it was set to high in the first place, but oh well.
And again thank you everyone for your reply.