How to make a rectangular light in mental ray?

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Old 08 August 2011   #1
How to make a rectangular light in mental ray?

I'm trying to make an area light that casts light from all over a rectangle shape. This is as far as I have got using a photometric light and, under the 'Shape/Area Shadows' changing the shape to rectangle:



For me, the problem with the above light is that light is being cast from the centre as opposed to evenly from the rectangular plane. How could this be done? In Vray it was so easy...
 
Old 08 August 2011   #2
If an option, you can use a self illuminated material on a rectangle mesh and turn on FG.
 
Old 08 August 2011   #3
Thanks for the speedy reply Swahn. I've tried doing the self illuminate trick but am I right in thinking that a self illuminated material does not actually cast light? From this tutorial it would seem that way...

Illum. plane tut

Therefore, illuminated material are for faking lights and reflections (?) and not what I'm after.
 
Old 08 August 2011   #4
No, it can cast lights thanks to FG. Even better would be to use a mr Sky Portal. These photometric lights are in serious need to a complete overhaul since max 2008 if my memory serves.
 
Old 08 August 2011   #5
Unfortunately all area light sources with mental ray in 3ds Max emit from a center point. This includes the skylight portal although the annoying effect on it isn't nearly as visible as say a photometric area light...but it's still there.

Sure, you can use FG on self-illuminated planes but it's just not the same. It won't emit photons, it won't emit direct light (FG disabled), it won't produce specular if you need that for fake highlights.

If you feel this issue warrants fixing then vote for it HERE
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Last edited by JeffPatton : 01 January 2013 at 03:53 PM.
 
Old 08 August 2011   #6
It's a shame mental ray cannot do that (I have cast my 3 votes!). I'll have a go with the self illuminated material later on as it may be fine for my needs.

I actually asked the original question because I'm looking to make a fluorescent tube (which would be visible in the render) which would need to obviously cast light and make shadows from the entire tube. Would I do this with a self illuminated material? Thanks.

(I would try it out but I'm at work where the nearest thing to studio max is sketchup...)
 
Old 08 August 2011   #7
Hi

The Trick of Jeff´s PortalLight work here very good. I use the original mia_portal_light Shader, and have more options to optimize the outcoming light.



@Miked08: mental ray can Mesh-Light/Object-Light, but the Autodesk Coder can not, or want not programming the features. I think they can not, because we tell it for years.

mfg
hot chip

Last edited by thorsten hartmann : 08 August 2011 at 09:38 AM.
 
Old 08 August 2011   #8
Originally Posted by JeffPatton: Unfortunately all area light sources with mental ray in 3ds Max emit from a center point. This includes the skylight portal although the annoying effect on it isn't nearly as visible as say a photometric area light...but it's still there.

Sure, you can use FG on self-illuminated planes but it's just not the same. It won't emit photons, it won't emit direct light (FG disabled), it won't produce specular if you need that for fake highlights.

If you feel this issue warrants fixing then vote for it HERE

Thanks for pointing that out Jeff. Didn't notice the Skyportals also had this issue too. Is this true even if you use the blackbody shader?
 
Old 08 August 2011   #9
Did you try changing the Light Distribution to Uniform diffuse? I was able to get something pretty rectangular by doing that:

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Old 08 August 2011   #10
Well, there is a clear difference between V-Ray and Mental Ray when it comes to direct area lights.



These are two area lights close to a surface facing it. You can see that Mental Ray is still projecting all samples from the centre of the node. Not very realistic if you want true area light.
 
Old 08 August 2011   #11
The best way is to use sky portals, just change de source to custom and it will behave like vraylight.
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Old 08 August 2011   #12
Originally Posted by 3DMadness: just change de source to custom and it will behave like vraylight.
Eh, not really. That's just assigning a color (and or intensity) to the light. Best I can tell it doesn't change the programming/math that goes on behind the scenes with how it's emitting the light as it still seems to emit from the center


(used the kelvin shader on the portal light)


Placing a translucent material in front of the lights also illustrates the issue.

The problem mainly comes into play when using large area light sources. If you use smaller, square area lights, then it's not a huge issue. However, if you ever want to do things like simulate a large photo shoot diffuser type thing, like this:


Then you'll pull your hair out, or just switch to a different rendering application.
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Old 08 August 2011   #13
Thanks for the great replies everyone and Jeff, your post really summed it up I think. Portal lights are pretty good and in small scale cases it's possible to hide that the light is coming from the centre, like below:



Could I change the question What if I'm wanting to make a fluorescent tube i.e. a light that casts even light? That probably cannot be done either.... Here's my experiment with a photometric light emitting from a 'cylinder shape' with the 'light shape visible to renderer'. I tried adding a glow effect to the light but of course the glow comes from the centre and forms a sphere, doh!



Thorsten posted something about this but I didn't understand really.
 
Old 08 August 2011   #14
That's amazing to me that such an integrated renderer has this problem. Anyone know does it occur in Maya too? Is it a Max implementation problem or a MR bug in general??

Edit:

I am remembering now that I did a scifi ship and it had some areas of long strip lighting. I was pulling my hair out too about this problem. I thought it was just me at the time!
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Old 08 August 2011   #15
Originally Posted by JeffPatton: Eh, not really. ...

As usual I agree with you jeff... its really not the same, but its better than the rectangular photometric light and closer to the vraylight.
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