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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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:icondrowelfmorwen:
DrowElfMorwen Featured By Owner Aug 31, 2018  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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:iconhentaibee:
hentaibee Featured By Owner Jun 9, 2018
Works great  in octane as well 😀
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:iconemarukk:
emarukk Featured By Owner Feb 26, 2018  Hobbyist Digital Artist
I was just wondering how to do this :-D - testing soon.
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:iconsimplysherbert:
simplysherbert Featured By Owner Feb 4, 2018  Hobbyist Digital Artist
WOW!
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:iconalexhellmann:
AlexHellmann Featured By Owner Nov 19, 2017
Thank you kindly for sharing.
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:iconkungh:
Kungh Featured By Owner Edited May 11, 2017
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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:iconsickleyield:
SickleYield Featured By Owner May 11, 2017  Professional Digital Artist
Now this is an interesting idea, I'll have to try it!
Reply
:iconkungh:
Kungh Featured By Owner May 12, 2017
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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:icondestinysgarden:
DestinysGarden Featured By Owner May 13, 2017
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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:iconsickleyield:
SickleYield Featured By Owner May 12, 2017  Professional Digital Artist
Great, thanks for sharing that!
Reply
:iconfilara:
Filara Featured By Owner May 2, 2017  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!!!
Reply
:iconsickleyield:
SickleYield Featured By Owner May 2, 2017  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).
Reply
:iconfilara:
Filara Featured By Owner May 2, 2017  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 :(
Reply
:iconthedude561:
thedude561 Featured By Owner Oct 26, 2016
This is something that is crucial for Iray. Thanks
Reply
:iconcalasade:
calasade Featured By Owner Oct 1, 2016  Professional Digital Artist
MAN! Can't BELIEVE I didn't think of that. Great tip!
Reply
:iconthedude561:
thedude561 Featured By Owner Jul 5, 2016
Great info, Thanks again.
Reply
:iconpleblu:
Pleblu Featured By Owner Jun 7, 2016  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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:iconsickleyield:
SickleYield Featured By Owner Jun 7, 2016  Professional Digital Artist
Well that's good at least:)
Reply
:iconpleblu:
Pleblu Featured By Owner Jun 5, 2016  Professional General Artist
Very interesting! I also use mesh lights over everything else. This changes everything....
Reply
:iconsickleyield:
SickleYield Featured By Owner Jun 5, 2016  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.
Reply
:icontracywong:
TracyWong Featured By Owner Jun 5, 2016  Professional Digital Artist
This is fantastic! Thank you so much! :D
Reply
:iconavmorgan:
avmorgan Featured By Owner Edited Mar 8, 2016  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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:iconsickleyield:
SickleYield Featured By Owner Mar 8, 2016  Professional Digital Artist
And it won't if you ues refraction.
Reply
:iconfkdesign:
fkdesign Featured By Owner Feb 12, 2016
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????
Reply
:iconsickleyield:
SickleYield Featured By Owner Feb 12, 2016  Professional Digital Artist
It's the cutout opacity in the surfaces tab.
Reply
:iconmacsavers:
macsavers Featured By Owner Dec 21, 2015  Hobbyist Digital Artist
Whoa!
Reply
:iconcherubit:
Cherubit Featured By Owner Oct 29, 2015  Professional Digital Artist
You have the best tutorials :love: Gonna read everything when I'm on vacation :heart:
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:iconsickleyield:
SickleYield Featured By Owner Oct 29, 2015  Professional Digital Artist
Awesome, happy rendering. :)
Reply
:iconkcrileygyer:
KCRileyGyer Featured By Owner Sep 13, 2015  Hobbyist Digital Artist
Thank you, it's great not having to make sure the mesh light is out of camera shot now. Awesome tip. I always look forward to your Iray tips and tutorials.
Reply
:icondeividman:
Deividman Featured By Owner Aug 25, 2015  Professional Digital Artist
Pure gold info, Sickle, thanks a lot!
Reply
:icondamnedcomic:
DamnedComic Featured By Owner Edited Aug 20, 2015
Has anyone seen any HDRs that are not at ground level?

I've been looking for a night cityscape HDR that I would use for a high rise interior scene.

Been having a heck of a time finding one.

Thanks!
Reply
:iconghoyer2014:
ghoyer2014 Featured By Owner Jan 7, 2016
A good hires jpg used as a "Background" can also light your scene.  Here is a technique I use.

1)  Load your jpg as a "Background" type in the Environment tab.
2)  Load the same jpg as an Environment Map in the Render Settings tab.  This will provide the IBL for your scene but not the image.  Turn Draw Dome off.
3)  Adjust Tone Mapping to your liking.  I find that turning Gamma down as well as the F/stop can really add more punch to a background image.

Here is an example of how good this can look.  The model is the Loft Apartment, available at Renderosity.  www.renderosity.com/mod/bcs/lo…

drive.google.com/file/d/0B6QHg…
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:iconsickleyield:
SickleYield Featured By Owner Aug 20, 2015  Professional Digital Artist
I've seen couple of freebies from other sites that were just a night sky, but they're rare.
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:icondamnedcomic:
DamnedComic Featured By Owner Aug 20, 2015
Yes I'm finding this to be the case.
Reply
:iconniceshoez:
Niceshoez Featured By Owner Aug 16, 2015  Professional General Artist
Great as usual oh conveyor of 3d knowledge!
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:iconsickleyield:
SickleYield Featured By Owner Aug 17, 2015  Professional Digital Artist
Thanks. :) DT is really an expert!
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:iconniceshoez:
Niceshoez Featured By Owner Aug 17, 2015  Professional General Artist
I went a a little further and put it down to .0001 because I was doing a closeup in a cramped elevator and the sphere meshlight left a slight shimmer at .001 when close up. the one decimal place over seemed to help. no shimmer or very very faint.
Awesome! thank you and DT again!
Reply
:iconsickleyield:
SickleYield Featured By Owner Aug 17, 2015  Professional Digital Artist
:)
Reply
:icondamnedcomic:
DamnedComic Featured By Owner Aug 20, 2015
I was going to write in to say I did the same.
Seems to work just fine.
Also if the mesh is behind the camera you can get away with lower settings as long as nothing uber reflective is in front of the camera.
Great tip, just the same.

Thanks for all you share!
Reply
:iconsickleyield:
SickleYield Featured By Owner Aug 20, 2015  Professional Digital Artist
You're welcome! :)
Reply
:iconspyrorue:
SpyroRue Featured By Owner Aug 16, 2015   Digital Artist
Ooo Nice one! Thanks so much for sharing this :) Very useful
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:iconsickleyield:
SickleYield Featured By Owner Aug 17, 2015  Professional Digital Artist
You're welcome!  It was a light bulb moment when he said that on the forum.
Reply
:iconspyrorue:
SpyroRue Featured By Owner Aug 18, 2015   Digital Artist
Yes indeed! One of my first questions when starting on Iray I had was "Can I have an invisible Emitter?" Tried and tried... Never thought an Almost 0 value, but not 0 alone would do it!
Reply
:iconsickleyield:
SickleYield Featured By Owner Aug 18, 2015  Professional Digital Artist
Me neither! Whaddaya know :)
Reply
:iconmavrosh:
Mavrosh Featured By Owner Aug 16, 2015  Professional General Artist
Works like a charme, also combined with mapped lighting. Great info, thank you!
Reply
:iconsickleyield:
SickleYield Featured By Owner Aug 17, 2015  Professional Digital Artist
Awesome, I'm glad it's working. :)
Reply
:icondamnedcomic:
DamnedComic Featured By Owner Aug 16, 2015
This is mega huge for me and has bee the number one reason I have not used mesh lights in many circumstances.
Reply
:icongazukull:
gazukull Featured By Owner Aug 16, 2015  Hobbyist Digital Artist
BOOM!  Perfect!  Thank you....
Reply
:icongazukull:
gazukull Featured By Owner Aug 16, 2015  Hobbyist Digital Artist
I mean, this was the one feature I was missing from Octane render.  Besides being a bit slower / no displacement (is there?), IT'S AWESOME :)
Reply
:iconkittenwylde:
kittenwylde Featured By Owner Aug 16, 2015  Hobbyist Digital Artist
Awesome! Thanks for the tip.
Reply
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