Area lights brightens in reflection different if background is behind light

Check this out:

The part of the area light which is not covered by the background mesh is brighter in the lower right image?!

In the top line I pasted in a strip from the lower line showing that originally the area light is dimmer
when it is rendered alone. But when the other two area lights left and right are on the mid light gets brighter.

Any idea whats going on?

It almost looks like a bug to me, one might try to say it’s just transparency in the reflection, but it shouldn’t really be there if the light is bright enough.

Then again, I’m not sure if this is the expected behavior because of how you’re not actually using geometry to emit the light and that you’re just emitting rays from an area in space, perhaps someone with more knowledge on arealight implementations in other engines can explain.

When you say area light do you mean area light or an emission material mesh? It would be nice t have the exact file that you are using to check…

Here is the file:


Area-mesh-bug.blend (125 KB)

Hi CeKuhnen. The way I read it it is not a bug at all.

Area lamps (actually all lamps, since mesh lights are not lamps) cast no shadows.

That means that anything behind any lamp will be seen by an object. Therefore two lamps stacked will contribute twice as much light, and a diffuse object behind an area lamp will contribute light to the object being lit UP TO FULL WHITE.
A mesh light however is opaque (unless you make it not so) therefore what is directly behind it does not contribute anything through it.

The only time they will look the same is when the area lamp is cranked up to a brightness that makes it a moot point as to whether anything behind it throws any light through it - once it’s blown out it’s white, it can’t get any more white, right. For the mesh light though, as you reduce intensity it gets grayer, but still is just as opaque, so no matter how brightly shining or reflecting the object is behind it, it will not contribute.

I am afraid I am not being clear here at all…:spin:

Here’s a picture of your setup with 4 area lamps layered and staggered and with the background and lamps colored and turned down to make it very hard for the lit surface to ever go to white. Now you can see that the area lamps clearly illuminate through each other, and the diffuse surface behind them all can be seen.

And now the same lights, white, and cranked up. If you replicate this you can see, as you turn the brightness up, how areas that have four lights overlapping go to white first then three light areas then two etc. If you do the same with mesh lights you will see that no matter how low you start you will not see the edges of the lights.

The moral: lamps are not lights.