Red pixel errors in renders

Hi there

I seem to remember somebody else having this issue before, but I cannot figure out what is causing it.

I have a scene lit with an HDRI using Textured Environment on an environment light - this in Lightwave 2020.0.2

On VPR preview, after some time refining the image, bright red pixels or "fireflies" start appearing on rendered surfaces. It also shows up in the final render.

The weird thing is the same secen opened and rendered on my Macbook doesnt do it - its only happening on my Windows machine which I use for rendering.

Its definitley to do with the HDRI image or the light itself, when I switch that light off the problem goes away. Also... its not the surface I dont think, because re-serfacing the object doesnt get rid of the problem.

Any ideas?
 

ActionBob

Member
Hi there

I seem to remember somebody else having this issue before, but I cannot figure out what is causing it.

I have a scene lit with an HDRI using Textured Environment on an environment light - this in Lightwave 2020.0.2

On VPR preview, after some time refining the image, bright red pixels or "fireflies" start appearing on rendered surfaces. It also shows up in the final render.

The weird thing is the same secen opened and rendered on my Macbook doesnt do it - its only happening on my Windows machine which I use for rendering.

Its definitley to do with the HDRI image or the light itself, when I switch that light off the problem goes away. Also... its not the surface I dont think, because re-serfacing the object doesnt get rid of the problem.

Any ideas?
Haven't been on here in a long time, but i think i can help with your issue. since tou are using HDRI imagery as a lighting solution, i recommend utilizing two copies of it. one the high res versions for reflections, etc, but lighting should utilize a lower res and blurred version of this. the high res hdri image provably has some very "hit" values which are producing the hot spots. using a lower res, blurry version with produce the lighting you want and should improve render times as well.

Cheers.

Adrian
 
Thanks Adrian... thats an interesting theory. I'll definitely test it out.

What has helped in the meantime and almost eliminated the red pixels, was lowering the light samples to 1 and the MIS samples down to 300. The default settings were 2 for light samples and 512 for MIS samples. I had to fiddle with the numbers quite carefully as gowing too low on MIS destroyed the lighting entirely.

there is still the odd red pixel sparkling every now and then but its much better.

I wander if what Ive done with the numbers is similar to using a lower res image?
 
Im trying to attach the HDRI but the forum wont let me attach it - even when zipped. Im using the "Attach files" button on this forum interface. Its only 5MB.

Is there another way you are supposed to share files?
 
Haven't been on here in a long time, but i think i can help with your issue. since tou are using HDRI imagery as a lighting solution, i recommend utilizing two copies of it. one the high res versions for reflections, etc, but lighting should utilize a lower res and blurred version of this. the high res hdri image provably has some very "hit" values which are producing the hot spots. using a lower res, blurry version with produce the lighting you want and should improve render times as well.

Cheers.

Adrian
Hi Adrian

I tried the technique you suggested. The blurred version of the HDRI changed the look of the lighting too much, but did cut down on the amount of red pixels.

Then I tried a scaled down / lower res version of the image and it made no difference.

Whats interesting is Ive been using this image for weeks now in other scenes with no problems... it just been this one scene where its happening.

For now Im going to have to live with it and paint out the red sparkles by hand, my deadline is looming.

thanks again for your help and suggestions
 

ActionBob

Member
Could be LW. Double check your geometry on the objects being affected by the "fireflies." Or try to increase your raytrace bounced just a tad to see if it helps - but not too much as the render times increase... There could be some non-planers causing issues or doubled geo. For the lower res HDRI imagery, blur, but not too much.

Posting the scene might of helped others, but honestly, I haven't used my LW in a couple years and have not upgraded to latest versions. So not sure I would be able to open.

Hope you get it figured out.

-Adrian
 

raymondtrace

Founding member
...The weird thing is the same secen opened and rendered on my Macbook doesnt do it - its only happening on my Windows machine which I use for rendering...
Do the two systems have displays of different bit depths? Are you copying the image rendered on one platform to the other to be certain it is not a system color management issue?

...Im using the "Attach files" button on this forum interface. Its only 5MB.

Is there another way you are supposed to share files?
The forum attachment limit is very low, <1MB. File sharing can be done through other services like Google Drive, Box, DropBox, etc...with a link posted here to the forum.
 

Sensei

TrueArt Support
In old times (pre LW 2018 new renderer) people were using "Limit Dynamic Range", AFAIK, to avoid pixels with too high value.. This option is still available in the Processing tab.
Zoom-in and check what is true HDRI value of such "red pixel".. Maybe it exceeds 1,0,0 (or in the case of white 1,1,1) very much.. ?

Its definitley to do with the HDRI image or the light itself, when I switch that light off the problem goes away.
Proper examination of the issue is the key for fruitful fix..
Enabling HDRI might just cause appearance of the issue, while issue is just hidden.
Replace HDRI by white 1,1,1 environment. Replace HDRI by sky etc. etc.
Don't take anything for granted until true issue & final fix is found.

Any ideas?
Do you have UV maps? Maps have enough UV borders?

On VPR preview, after some time refining the image, bright red pixels or "fireflies" start appearing on rendered surfaces. It also shows up in the final render.
..you should attach screen-shots.. and/or attach example scene..
Examine RGBA value of these pixels in floating point mode, before they are truncated to integers 24/32 bits.
 
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Thanks for the help everyone.

@Sensei - Your step by step variable approach has revealed that its definitely an error caused by the HDRI image Im using. When I switch it out for another image the problem goes away. I guess I should have tried that right in the beginning but I needed the specific lighting look from this particular image.

Its just a bit surprising because Ive used this same HDRI image in many other scenes with no problems. There must just be something about this specific scene that reveals the problem.

Unfortunatly I can't attach the scene or rendered images as the job is under NDA.

@raymondtrace - regarding your query on the different display monitors. I dont think it was that, the images show up with the red pixels on my Windows machine and then also display that way when copied across to my Mac.

@ActionBob, thanks for the suggestions. I checked those things and they were all fine before realizing it was the HDRI image itself creating the problem.

Thanks again everyone
 
@Sensei - the "Limit Dynamic Range" feature you mentioned... I cant seem to find this in LW 2020, does it perhaps go by a different name now?

Would this be corrcly applied in the Precessing tab under Image Editor?
 
@Sensei - the "Limit Dynamic Range" feature you mentioned... I cant seem to find this in LW 2020, does it perhaps go by a different name now?

Would this be corrcly applied in the Precessing tab under Image Editor?

In LW 2020 "Limit Dynamic Range" is in the Effects/Backdrop window (Ctrl F5) in the Processing tab.

Since we don't really know where the red pixels are coming from, you should also try Despike (which is there to minimize fireflies).
It is also possible to reduce only the dynamic range of reflections (Reflection Limit). Both options (and more) are set in the Render Settings in the Render Tab.

Most of these errors come from rough surfaces and very bright light sources (mostly in HDR images).
Very strange, however, is the difference between Windows and Mac render.

ciao
Thomas
 

slacer

Member
Did you check your buffers for those spots?
 

Attachments

  • buffers_test.PNG
    buffers_test.PNG
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@slacer... sorry for the delay. Ive just had a look at that now.

The red pixels are present in:

- Specular Direct
- Specular Shading
- Diffuse Direct
- Diffuse Shading

and NOT present in:
- Specular Indirect
- Diffuse Indirect
- Shadow
 
The red pixels are present in:

- Specular Direct
- Specular Shading
- Diffuse Direct
- Diffuse Shading

and NOT present in:
- Specular Indirect
- Diffuse Indirect
- Shadow

This is normal when you light with an environment light and an HDR image. The reflection of the HDR image is in the Specular Direct buffer not in Specular Indirect.

I have seen you use only 1 sample for light samples.
If you have a rough surface that seems too less to me. (even if it reduced the red pixels). Keep in mind: these samples multiplied by your camera samples are causing possible noise in the reflection of an environment light. Of course, it can also be an issue in the HDR image itself. Do you remember where you got it? Is it freely available on the internet? Then we could download it from there.

Does your surface use a normal map or/and bump map? How rough is it?

Sorry for the many questions, but it's hard without the scene file.

ciao
Thomas
 
@Sensei - I tried decreasing the ray recursion limit. No effect on the red pixels. Also the surface in question is being lit directly by the HDRI... I dont think there is any bounce affecting it.

@Thomas Leitner - regarding the light samples, I set that lower as an experiment and it actually helped clear things up a bit strangely - there were less red pixels. But under normal circumstances I always find myself cranking that number up to help get rid of noise.

The surface itself has a normal map, spec map, roughness map and colour map. But nothing unusual or out of the ordinary. Ive tried disabling each of those maps and it has zero effect on the red dots. Ive even tried applying a whole new surface to the object and it doenst get rid of the red pixels.

I mentioned a few posts back that I definitely think its the HDRI image causing the problems.

I can actually share it, here is a Wetransfer link: https://we.tl/t-11CtC8pkHm

The HDRI map was generated out of Lightwave and then some post tweaking in After Effects. Disclaimer - I know its not a great HDRI map BUT... it has worked perfectly without causing this issue in about 80 other scenes. I'll see if I can get the issue to happen in a scene with a simpler object that I can share.

Thanks again for the help everyone. Ive sinced fixed the renders for this shot in After Effects and moved on to others. But it would be great to understand this completely.

Im betting its definitely my error with the HDRI map I created.
 
Right... Ive managed to whip together the scene with a simplified stand-in object that is producing the same red pixels.

Hopefully this attachment of the scene works....

The HDRI is too big to be included and will need to be grabbed from the Wetransfer link above

Screenshots of Mac vs Windows scene to follow soon.
 

Attachments

  • red pixel test scene copy.zip
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