Mind-Blowing Mesh Light Tip For Iray!
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Dimension Theory is an expert on all things light and HDRI-related, and I use his products regularly.  The other day he said something extremely useful on the forum:

You can actually use mesh lights without them being visible in your scene.  Set the cutout opacity to .001, then set the mesh light settings as normal.  They will not appear in the scene or in reflections.

This is life-changing info for me, because I use mesh lights a lot; they really look the best on human skin in most circumstances so I consider them mandatory in most scenes.  I can confirm that this works because I've tested it this weekend.

Test it yourself, use it, pass it on.  Just don't credit me with the information.  I never would have figured it out if he hadn't mentioned it in a post.
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Comments
DrowElfMorwen's avatar
DrowElfMorwen|Hobbyist Digital Artist
Thank you for this!!! :o Seriously I was having such major problems with positioning how I wanted the lights because they're not invisible... well, I guess now they are. HA!! :la:
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hentaibee's avatar
Works great  in octane as well 馃榾
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emarukk's avatar
emarukk|Hobbyist Digital Artist
I was just wondering how to do this :-D - testing soon.
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simplysherbert's avatar
simplysherbert|Hobbyist Digital Artist
WOW!
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AlexHellmann's avatar
Thank you kindly for sharing.
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Oh, it will appear in the scene or in reflections but it will be hard to notice. :D For really invisible mesh lights, put a black (rgb 0.0.0) texture map (Im using 200x200 texture but even smaller than that should work) in the cutout opacity texture image slot, while leaving cutout opacity value at 1.0 and that should turn mesh invisible but still emiting light. 
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SickleYield's avatar
SickleYield|Professional Digital Artist
Now this is an interesting idea, I'll have to try it!
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I just made a 1x1 black layer in layered image editor of the cutout opacity texture image slot and red ball is invisible while still casting light, so it works that way too.

Test1 by Kungh   Test2 by Kungh   
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DestinysGarden's avatar
OMG Kungh, you get my hero of the day award for that trick. I was trying to turn the opacity down to .001, but as others noted previously, the emitter is still (faintly) visible, especially if it is in direct line of sight from the camera. Putting a black map in the cutout opacity does the trick though. Thanks!
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SickleYield's avatar
SickleYield|Professional Digital Artist
Great, thanks for sharing that!
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Filara's avatar
Filara|Professional Digital Artist
SHUT. UP. WHY AM I ONLY STUMBLING ON THIS NOW?!?!?!?! I'm a lover of mesh lights and I ALWAYS have that problem! GAH! I can't believe I'll never have that problem again! Thank you SickleYield! I LOVE YOU!!!
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SickleYield's avatar
SickleYield|Professional Digital Artist
:) 

Oh, does it work again now?  At one point they had updated it to where it didn't work any more (the mesh light wouldn't cast light while invisible).
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Filara's avatar
Filara|Professional Digital Artist
Don't say that! I'll have to test it out. Otherwise I'll have to go back to fiddling around till I get it right :(
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thedude561's avatar
This is something that is crucial for Iray. Thanks
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calasade's avatar
calasade|Professional Digital Artist
MAN! Can't BELIEVE I didn't think of that. Great tip!
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thedude561's avatar
Great info, Thanks again.
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Pleblu's avatar
Pleblu|Professional General Artist
I just tried it, and while it's not completely invisible, if your scene is busy enough someone might not notice.

Lighthing by Pleblu
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SickleYield's avatar
SickleYield|Professional Digital Artist
Well that's good at least:)
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Pleblu's avatar
Pleblu|Professional General Artist
Very interesting! I also use mesh lights over everything else. This changes everything....
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SickleYield's avatar
SickleYield|Professional Digital Artist
This worked in the version where I suggested it.  I'm not sure it does now. :(  When I retested it recently it seems not to.  I think they changed the behavior of emission and opacity between versions.
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TracyWong's avatar
TracyWong|Professional Digital Artist
This is fantastic! Thank you so much! :D
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avmorgan's avatar
avmorganEdited |Professional Digital Artist
hmmm... good to know. I was applying a refraction weight of 1 with a refraction index of 1 to produce an invisible mesh. I think it's also necessary to dial off all the glossy inputs as well... That said, being clear did not seem to prevent the mesh from emitting light with the luminance cranked up.
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SickleYield's avatar
SickleYield|Professional Digital Artist
And it won't if you ues refraction.
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fkdesign's avatar
I am so sorry but seems I am new and do not get it... where to change it? I changed every place where says opacity in the light I wnated to not see in the render but it still shows up... is a rectangle one... can you help, please????
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anonymous's avatar
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