[ISSUE ] Emission sources competing

Hi guys,

i spotted an unusual behaviour with the emission shaders that i haven’t noticed before. I was trying to make an IES light shader following a tutorial from Chocofur, when i noticed a strange emission source phenomenon that could make a lot of problems in archviz. The emission shaders are virtually shutting each other down. Did someone ran into this before and should we report a bug? I found this problem on linux 2.71 / 2.75a / 2.77a versions. I would like to know if you can reproduce this issue on your computers, just to be sure i am not doing something wrong. The screencast videos can be downloaded from the link below. Sorry about the popup, just ignore it.

Emission shaders competing - Screencast

You might want to use another site to host your screencast:
My antivirus software reports that site as malware infected and does not allow access.

And your antivirus is probably right. Do you have some fast alternative (without registration)? I tried to attach three zip files to blenderartists but it refused them although they are under 15 MB…

for uploading blend files, pasteall.org

for videos… youtube?

One emission object blocks light from other object? Or just shutting off another object as if it is not in the scene?

Basically the addition of each next emission object ramps down the existing ones. The scaling of existing ones also ramps down the rest of them - not in numbers but in rendering. I was perplexed not to notice that before.

I made an IES light shader with emission nodes and introduced a light cube to the scene. The IES shaders immediately dimmed down. As i scaled the cube down, the strength of the rest of the emission shaders turned up. When i deleted the light cube, the IES turned to maximum.

I converted the IES to basic emission shaders and the phenomenon repeated. The IES lights even turn each other down depending on how many are there in the scene. The screencasts of the issue are attached below.


emission_002e.zip (1.55 MB)emission_003e.zip (1.58 MB)

Hi, i found a way to downgrade and zip the screencasts, they are attached in the previous post below the explanation.

Ok, video is fine.
However i can not reproduce this using 3 emissive cubes and screen mesh, similar to what was shown in video.
At this point, i beg your pardon, i’m suspecting i have not told all the details on purpose or that pc is to blame - there is no blend file attached one could open and see (or not) the same effect.

Is it possible you have some form of adaptive tone mapping going on? Any non default settings under: “Scene > Colour Management” ?

Thanks for the comments! I have checked the scene settings, it seems everything is normal. In the file that this composition was made the rendering clamp settings were 3.0 / 1.2 and i suspected that was the problem. But now i copied the objects to a totally fresh 2.77 file with no settings changed and the problem occurs again.

I attached the file - just turn the emission on the cube and scale it up and down. Also try deleting the cube. Then try with copying the wall lamps along the wall and deleting them, just like in the video. You should be able to reproduce everything.

I think the key to noticing the problem is a surface near the lamps on which the change of intensity is clear.


emission_competing.blend (514 KB)

The fact that the light intensity changes with the size of the cube is the intended behaviour:
The strength of emission shaders on meshes is measured in [SUP]Watts[/SUP]⁄[SUB]m[SUP]2[/SUP][/SUB]. So, the very same emission shader will look dimmer, the larger the emitting object is.

Not sure about the IES thing, though. Will try to do more testing.
This might be an issue of having too many objects using “Multiple Importance Sampling”: If everything is important, nothing is and Cycles will need much more samples to clear things up.

This might mean that the intensity of the lights will come back with more render samples.

This is, as Ikari Shinji says, MIS influence.
You could reduce IES material to plain emission probably; Cube can have Emission =1 and Color rgb=[0,0,0] (!) - if cube has MIS checked and it’s Glossy or Diffuse components are not hidden from scene small planes start to emit “something MIS-ish”.

Thanks guys, that helped me a lot! The competing emitter introduction is resolved by unchecking MIS on it. I can now have other emitters besides IES lights.

However, the issue of multiple emitters with IES shader dimming themselves out remains. When i am satisfied with IES configuration, multiplying it in the scene dims it out. I’ve tried unchecking clamp and it helps, but only to a point. And that gives a ‘firefly’ problem and needs much higher sample rate.

I guess this resolves the issue a bit, i will just have to be aware of IES intensity change when multiplying lights and unchecking MIS on non-IES emitters. Thanks guys, you’re great.