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Arnold Grain Flickering

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()Hello folks!

I'm testing grain shading in Arnold 5 in Houdini 16.0. The simulation was bad just for the test purpose anyway. The top surface starts flickering during the rendering. And the rest area (side) is fine. 

When I apply a sprite to those grain particles there isn't any flicker during the playblast (see the playblast video). But when I use Arnold render to render those particles, the flicker starts to show up on top of the surface. So that might be a rendering problem (see the rendered video)...


Render: FirstGrainTest2_Flickering_H264.mov




Particle: 1,085,000

Shader: Arnold standard surface (no fancy tweaking just by default with custom ramp Cd)

Pixel Filter: Gaussian with size 2.25

Sample: 4/4/2/0/0/0/10 Noise seed: $F

Raydepth: 1/1/1/1/0/0/ Low light threshold:0.01

motion blur keys:8/8


What am I missing here? Thanks!

Note: I create a simplified scene with cache file attached. The reason is the original cache is simply too big to upload here (20GB, around 100 frames). The simulation cache is the same as the video one, I just crop out part of the grains. Good enough to show the problem anyway. The shader and light rig is the same as the video above, you will found not fancy stuff TBH... Maybe you need to find an HDR as well...



Pure file:Arnold_Grain_Demo.hip

Cache: Cache.7z







Edited by Marcocheng
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Hi There,


Hard to say by just looking at your video.

You have to see the Arnold light also have sampling.  


I would need to see the shader and the light to make some good tweaking,

If you can send me a ball with your shader I could make some test.

Your file is really kool and don´t expect you to put it up here.

So I would like to replicate this and see if I can help you out.

If you like.

just a simple grain and your shader setup.


Cheers Mangi



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Did a Simple setup with some grains etc...

render region flicker test. "ARNOLD"

will continue:2:00am Madrid Spain , 


video is really only a flicker test. 

for me it is pretty good: did not see any flicker in the full render.

whould be nice to test your shader


Cheers and good night



Edited by mangi
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4 hours ago, mangi said:

Hello Mangi,

I just upload a simplified scene file and cache in the top post. I crop out some of the particles to make the file more lightweight but the grain and light rig are definitely the same. Only one thing you need to do it yourself is to replace the dome texture. I have tested the scene render again and found the flickering area is basically focused on the direct light area (the grain inside the shadow area gonna be fine). I've try to fix the sample seed to 100 (fixed noise pattern) and remove the indirect specular blur but seems not working...

I've seen all two video you made and there isn't obvious direct lighting so that may be the reason you don't have the problem. Check my files or add a direction light like sun.

But thanks for spending time to test it anyway. Big thanks! Have a nice day





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I think the camera angle also matter to the flicking issue. You will get less flickering while grain facing the camera. But when the grain is larger facing angle to the camera you will definitely get more flicking. (random dots flicking) To my instinct, it seems like a pixel filtering issue...



First: Grain facing the camera

Last: grain isn't directly facing the camera (larger facing angle).


Edit: More test today. I render several AOVs out for debugging and increase the directional light sample from 1 to 4. It doesn't help, and the flickering appears in the direct diffuse layer.


Edited by Marcocheng
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Hi There.

Buff.... I did several render testes...

I finally saw something that is making it seem as though there is "FLICKER" 

To be able to make a accurate result this is what I came across.

Here are some snap shots to explain.

Points are desappearing from one frame to the next and that makes it seem like a flicker from render.

We first must see this and then take it back into render okay...

I also saw this right now in the video you sent:

If you don´t mind for these test if you could send me a very small scene without the sim with your set up.

Not the exported catch files. This way I can make some other test.. etc...

A Sphere with just a small amout of point count. With your Attribute creates etc. and any texture etc from the arnold shader.

There are many ways to teak Arnold So if I am missing any component the result will make a Huge Difference.

Any way One step at I time .


The import thing is also finding the most light config , 

And a faster Arnold render. "ASS EXPORTED" 

Thats is why I am asking for your information....


Maybe when you write the Catch files out this made some issue:

because If you see the Video "No Catch.mov" no points disappear.

But the video "First render.mov" was with catch exported like you did, And points disappear.















disappear .mp4


Disappered_ your quicktime.jpg

Edited by mangi
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Just for self confirmation:

points in the dop sim have the same point count per frame:

Just made this test to make sure I was not having a strange idea of what issue  was going on.

Not sure but was really tricking to figure this one out.

je. question was it flickering or not. ???? that will be tested......







Edited by mangi
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18 hours ago, mangi said:

Just for self confirmation:

points in the dop sim have the same point count per frame:

Just made this test to make sure I was not having a strange idea of what issue  was going on.

Not sure but was really tricking to figure this one out.

je. question was it flickering or not. ???? that will be tested......








Hi, I crop out some of the particles by using a group node (by a geo) and a delete node to reduce cache size (that's why some of the particles seem to disappear when they reach the bound), the cache cost 16GB of space, let me upload the source setup HIP and you cook all the node. Sorry for the confusion



Edited by Marcocheng
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Excuse my lateness on this reply.

"COMPUTER BLOW UP" that window "BSoD" Blue screen.

Anyway back to the testing.

I made another scene, yours was from version 16,
I am on 17 "HOUDINI" you mentioned this I know.


The Arnold Standard Shader in your file is depreciate,
You can use it but, there is a new Standar Shader.
They keep that shader so you can load your earlier project.

It was really hard to do test with your sim etc.... 1000000 plus points, older shaders etc.. sim size etc.. etc...

About the flickering in your Arnold Setup.

Skydome light and Distant light.

This test, I used solid color white: in the skydome. I did not have your HDRI file, so some that I used would not really serve the real situaltion, ok

you have to match the resolution of the HDRI with the render option size. In the Arnold panel.

I made this test similar to your setup to make a conclusion, 

The Specular is causing these flickering.

What is the Solution ??? 

I was pumping up The Rays And Samples of the Lights. Got it quite steady, "BUT"
It will reduce this, however the reneder Time gets really High. Out of production TIMES. 

The Bose will say NOWAY, just joking...

So I decided just to see where the flickering was originating from.

That is up to you to decide how the cost of time to make this render work.

I will make some more simple test etc this week ....


If you need to make specular pass in render and then compo , may go good. I will experiment like this....


But, This is quite normal for Arnold , due to the fact that it uses brute force Render, 

Will, randomly shoot rays in the scene and make the render, So when it shoots a random ray into the sky dome and then comes back, it will produce these flickerings.

Becuase the ray will hit a different part of the skydome and  different grain, 

The grain are all over and with variation colors. So for me it is working how it should.

Well this is the explanation that my bose gave me when they where developing this render for 3D applications. quite some time ago.

Don´t quote me on this okay.

This is why you get faster renders. 

There is a denoise output but I have not experimented with this yet , 

But in compo postpro , there are noise reducing fliters that maybe a kool way to get fast renders and remove this specular flickering, you will have some changes.

Nuke has kool stuff, but I guess this is not the purpose of this test.



These two renders have the same settings, the one that has no flicker has no Specular Rays.
Just to see where they are coming from OKAY.


Compare. the image on the right has a black line on top is "Specular"

              the image on the left has "no Specular


And sorry for this delay.

PS My new clean installation of My PC is Steady...
I had to whip out the WHole DRIVE from scratch...





Specular mov.mov

Edited by mangi
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Hello Mangi,

Wow, that's a very detailed test. Thanks for spending time on this first of all.


I've actually tested the render with AOV output (diffuse + specular pass), the flickering seems to happen in the diffuse ray as well (while the specular pass is black when I switch off specular attribute in Arnold shader)... I'm not 100% sure the flickering was caused by the specular...


And yes I'm a v16 user... I don't know if they upgrade the standard shader in v17 (both are arnold 5 right?) Please to have more info with that!


Seems to disable specular and increase light sample might help...




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Kool Marco,

I am not really sure if in HTOA 16 has the both Standard Shaders. 

But the both versions on arnold are the same in the both houdini 16 & 17.

Just look and see if it is like this: "STANDARD"


Still testing, was able to get GI SKYDOME & Direct LIGHT Spec and Diffuse , with no flicker and really low samples.

But the test is with just a few GRAINS. 

I did notice as you make things smaller it will produce the flicker..

Arnold RENDER POINTS and has a Size .


Kool Marco  




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