PDA

View Full Version : Trasnparent Mental Ray Discussion


Blazer
02-04-2005, 10:27 PM
I'm going to try and revivie the discussion around lights shining through (semi)transparent objects in Mental Ray.

The thread should involve all manner of caustics, volume caustics, transparent shadows, etc'

My two issues at the moment are getting nice transparent shadows, and doing the whole focusing light to a point, creating a nice visible beam in volumes.

If anyone would like to pitch in and spread the knowledge it would be appreciated!

MunCHeR
02-04-2005, 10:33 PM
err I hate to be pedantic but transparent isn't spelt trasnparent, just to alleviate any confusion :D, wasnt there a tutorial at motiondesign that covered some of this, ofcourse nobody knows if they are on the dvd.

MunCH

Blazer
02-04-2005, 10:39 PM
Yes, there was, and it doesn't work very well, or make much sense. Especially when you load the scene up and it doesn't work :)

All the said, let's try and have discussion/examples that go beyond "I think there was info about this somewhere else."

thanks :)

MunCHeR
02-04-2005, 11:10 PM
well the scene did work, did you have your shaders connected properly?

MunCH

PS: If I find any more info I'll be sure not to point them out, that what you want aiigh?

Blazer
02-04-2005, 11:12 PM
I downloaded the scene file......

azshall
02-04-2005, 11:16 PM
well the scene did work, did you have your shaders connected properly?

MunCH

PS: If I find any more info I'll be sure not to point them out, that what you want aiigh?

<smartass>

You are right .. The scene did work.. as long as you were in Maya 5. I too tried that setup in Maya 6 and it didn't work at all. I even downloaded his prebuilt scene, rendered... actually rendered slower than a wolf trying to take an icy dump in the tundra. Great thing about the render was. There was no caustic...

</smartass>

Honestly though .. I would like to know more about this subject ... I am not really familiar with the volume caustics side of things in maya and would like to understand and possibly even get them to work, myself....

Blazer
02-05-2005, 02:17 PM
Alright, so I got the thing working.

I set all the volume /volumephoton materials as made sense to me, and it didn't work. Then I added the "photon collector" as described in the motiondesign tutorial (with different settings) and it now works.

Does anyone know what the purpose of this transparent plane is? It obviously works, but I'm still confused as to why it is needed, and simply shining the light through the glass ball doesn't generate the volume caustics.

floze
02-05-2005, 02:35 PM
Alright, so I got the thing working.

I set all the volume /volumephoton materials as made sense to me, and it didn't work. Then I added the "photon collector" as described in the motiondesign tutorial (with different settings) and it now works.

Does anyone know what the purpose of this transparent plane is? It obviously works, but I'm still confused as to why it is needed, and simply shining the light through the glass ball doesn't generate the volume caustics.
The 'trick' is, it turns the photons travelling through it into volumephotons. This is because a plane doesnt have a real volume, which would some kind of 'enclose' the volumephotons otherwise. That's all you have to worry about.
And besides: 'Photoncollector is a misleading term imho.

Blazer
02-05-2005, 02:41 PM
I was just using the term he used at motiondesign (photon collector/generator).

So essentially, you're saying once they go through the plane with volume shader they become always volume photons as well because they never leave the volume?

It just doesn't make much sense that simply shining a light inside a cube with a volume material won't give you volume photons.

How about the other issues like transparent shadows?

J-Wicz
02-05-2005, 02:56 PM
I followed this tutorial at motiondesign a year ago, and it wasn't really very straightforward. It's not really a good tutorial but rather a proof of concept. A lot of stuff there is not explaind (e.g. the mentioned why this geometry collection photons).

I do architectural visualisations and having volumes of air/gas/dust in MR running properly would be great, but I haven't found resources (though there are two good threads on cgtalk afaik (the one with the colured tiles glass)).

there is also a free mesmer XSI/MR video called "Volumic Lighting in XSI", which i found useful / inspirational
http://www.mesmer.com/

The MR documentation is not a big help either. I always wonder where the feature movie guys get their information from. Obiviously not from the offical docs, not internet forums, not training dvds.

It would really be great if for instance this thread could produce a working logical maya sample scene.

But now it's three o'clock Saturday night in Sydney and I'm to drunk to keep up

cheers
Philipp

floze
02-05-2005, 02:56 PM
I was just using the term he used at motiondesign (photon collector/generator).
I know! :sad:

So essentially, you're saying once they go through the plane with volume shader they become always volume photons as well because they never leave the volume?
It's actually like being in one big volume. You can have the same effect by placing a huge cube, enclosing your objects and your cam, but the light is placed outside it.
It just doesn't make much sense that simply shining a light inside a cube with a volume material won't give you volume photons.
That's actually exactly what you're doing. :D
You just need to push that little mental switch (hey this was two jokes in one!).
How about the other issues like transparent shadows?
What problems do you have with them?

Blazer
02-05-2005, 03:21 PM
That's actually exactly what you're doing.
You just need to push that little mental switch (hey this was two jokes in one!).

I have both a large cuve surrounding my scene, AND the little plane. Is the issue with this that the light has to be outside of the volume?



My issue with the shadows is, shining a light on a transparent green ball still casts black shadows.

BTW, attached is a bonus shot.

floze
02-05-2005, 03:51 PM
I have both a large cuve surrounding my scene, AND the little plane. Is the issue with this that the light has to be outside of the volume?
It's a bit confusing, I know. Somehow the photons need to 'enter' the volume. But if you rudimentary understand how this works it's a piece of cake to get the effect.
Btw: You could apply a parti_volume to your camera to avoid the huge surrounding cube.

My issue with the shadows is, shining a light on a transparent green ball still casts black shadows.
You may apply a shadow shader to it! Set the shadow shader's color to the color you want and set the transparency, then set the 'Mode' to 1 (inclusive) or 3 (no dependency).

Blazer
02-05-2005, 05:52 PM
I have done that... bit it still seems wrong....

I suppose only way to get it right is shadow shader + caustics.

I just hoped there was a better way of doing things.

TBud
02-06-2005, 07:15 AM
Hey for transparent shaddows, I think you have tick the little box under the Mental Ray Render globals: Global Illumination: at the bottom "Direct Illumination Shaddow Effects"

Along with this you need the Mental Ray Transparent shaddow shader... I'm using Maya 6, and I just came accross this issue tonight. But I don't remember this feature being there in 5? not sure though.

Hope that helped

Blazer
02-06-2005, 02:48 PM
figured I'd post another test. You guys are the only ones who would appreciate this :)

floze
02-06-2005, 03:22 PM
Looks interesting! :thumbsup:
What shader do you use for the dispersion?

Blazer
02-06-2005, 03:50 PM
the l_gem shader.

The diffraction shader works great too. Both are slow as ass, but look great :)

grafix
02-07-2005, 01:31 PM
I wonder if you guys have seen this thread:

http://www.cgtalk.com/showthread.php?t=101813

many of your issues have been discussed already there.

@ azshall: to get the motiondesign scene to work under Maya > 4.5, its enough to uncheck no_globil_where_direct in all the parti_volume nodes of the scene. I've no idea why this was set, I guess the feature wasn't even supported in the older versions of Mayatomr...

Blazer
02-07-2005, 02:31 PM
If you'll notice they have a lot of the same issues as are being discussed here.

Anyway, It's also a matter of having discussion, showing better and nicer techniques. Not just troubleshooting. So please guys, bring examples!

floze
02-07-2005, 02:48 PM
If you'll notice they have a lot of the same issues as are being discussed here.

Anyway, It's also a matter of having discussion, showing better and nicer techniques. Not just troubleshooting. So please guys, bring examples!
Dont know if I posted this before, anyhow, that's a humble scene I worked on a while ago:

And sorry, it's very small, didnt find the time for a fullres yet...
http://www.floze.de/pics/cgtalk/assorted/atelier.test.2.jpg
There's a 'photoncollector' in front of the single lightsource, and the windows act pretty much like the glass ball in your scenes, Blazer.

anthonymcgrath
08-15-2005, 02:51 PM
there were some fantastic tutorials online somewhere that step by stepped how to set up this light/photon tracing thru glass objects and the like - does anyone know where they might be?
wouldn't mind researching into this 'true light' stuff a bit more :)

Joss
08-16-2005, 12:49 AM
That's cool Floze. Every artist should have a creative studio like that! :thumbsup:

Yep, you have an easle in there in case you tired of the computer... yeah right.

I'm really amazed by all these cool shaders!

CGTalk Moderation
08-16-2005, 12:49 AM
This thread has been automatically closed as it remained inactive for 12 months. If you wish to continue the discussion, please create a new thread in the appropriate forum.