Mental ray reflection pass has white dots

#1

Hi

I’m trying to composite a slightly high resolution still (4k x 2.5k) in Nuke but I’m having trouble reproducing the beauty as I’m getting some noise and fireflies in highlighted areas of my reflection pass. My master beauty exr looks clean and crisp with little noise and no evidence of these artifacts. I’m rendering linearly out of Maya 2015 using mental ray with RGBA 32bit(float) frame buffer. I have also changed my render passes’ local settings to 32bit. I have tried different Unified Sampling settings (even as high as Quality = 8, min/max samples = 1/800).

To light the scene I’m using IBL with a HDR file attached that I created in HDR Light Studio. I have read a number of threads which mention similar problems to mine except that the fireflies appeared in the beauty and not just in an individual pass. It looks like the hdr lighting is the culprit in most of these cases and a suggested solution is to clamp the light values in the hdr file. I want to have as much exposure control as possible in Nuke so clamping isn’t really an option for me.

I should also mention that all my shaders are mia material x passes.

Has anyone else come across this issue where the noise only happens in a particular render pass (the reflection pass in my case) but not in the master beauty, and did you resolve it? Why does mental ray clean up the master beauty but not the reflection pass? :banghead:

Master Beauty:

Reflection Pass:

#2

I’m really having trouble finding a workable solution here. Does anyone suggest using a clamped hdri for the reflection pass and the original hdri for all the other passes? I’m also using a carpaint_phen_x shader so maybe I should plug the clamped hdri into the specular pass as well?

Any advice would be greatly appreciated.

Thanks!

#3

I’ve had this issue before related to mia_material advanced->specularbalance attribute value.
This attribute scales the specular component of the shader. The mia_material produces extremely high values for specular highlights. In my case reducing the value removed some bad white dots. However this only effected the beauty pass not the raster passes, similar to what you describe.

#4

Hi

Since the problem seems to be only in reflection, have you considered to use a ray_switcher and feed a slightly modified HDR into the reflection ?

/risto

#5

What is the brightest value of your HDR image?
What are the values of reflection glossiness and glossy samples in the mia_material?

#6

The brightest value is around 26. I set a max clamp value of 1 in Nuke and it has cleared things up a bit but the reflection pass is still unusable.

The material shown in the attached images has a glossiness value of .500 and samples are 64. I tried bumping the samples up to 256 a few days ago but this had no effect, although I haven’t tried this value with the clamped HDR.

Thanks a lot for the help!

#7

If the values goes up to 26 then it does not matter how much specular samples you have. It will flicker anyway. Either you tone down your HDR or then you find some kinda trickery with the ray_switcher.

/risto

#8

Thanks for the tip about the ray_switcher. I’ve seen it mentioned before but have no idea how to use it. I’m going to check out the Maya Monday ray_switcher tutorial on the autodesk blog.

I’ll let you know if it works

#9

There is a simpler soloution. just add a camera lense shader like mia_exposure_simple or mia_exposure_photographic, and you should be fine.

#10

Do you use PS or Nuke to blur the HDR? I have PS CS6 which allows me to blur but it doesn’t let me duplicate the background layer. To do this I have to change to 16bit channels which clamps everything. I want to offset the copied layer before adding the blur so that the edges remain seamless.

I am quite new to compositing so I’m not an expert in Nuke. The blur node doesn’t seem to give the same level of blurring as PS.

#11

I have two mib lookup spheres plugged into the appropriate slots of an mip rayswitch node which is then plugged into my render camera’s environment slot.

How do you turn off primary visibility of the rayswitch environment? I just want a black background in my render. I’m rendering the back plate on a separate render layer.

EDIT: I discovered the mip_rayswitch_environment node which has a background slot and an environment slot. You have to plug your mip_rayswitch into the environment slot and leave the background slot set to black and then plug this new node into the camera environment slot. Who said MR was complicated? :wink:

#12

Lens shaders only effect the beauty pass so if you want to continue compositing with raster passes then this will not solve this issue. Im assuming this is the classic firefly issue which can occur in most any renderer. The ray-switch solution is getting complicated, perhaps you should consider some changes to your HDR as suggested by ristopuukko. How about tone-mapping your HDR image with Nuke’s softclip node or PS’s lens filter. With tone-mapping you can reduce the extreme high end of your HDR without effecting the base exposure range.