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View Full Version : Volume lights in XSI - Bugs and feature lacking


Buhrmi
08-15-2004, 02:16 PM
Hi :wavey: ,

I am having troubles using XSI volume lights. Maybe somebody of you guys can help me.

First:
Is anybody able to get this example to work in XSI 4? It let's you color the volume light and change it's intensity. http://www.grahamdclark.com/Studentresources/tutorials/colorstexturesspotlength/index.html I managed to color the light, but as soon as i connect something to the "color" input of the light shader the volume light disappears.

Second:
Is there a way to exclude _specific_ objects from affecting the volume light? I want the volume light to penetrate some objects. But all objects in a scene are acting as obstacles for the volume, even it the object is in the lights associate group.

Third:
How can I create an "inverse" volume light? I've downloaded the demo of Cinema 4D to show you what I mean: http://www.buhrmi.de/muell/inversevolume.jpg Any ideas?

:bowdown:

runejw
08-16-2004, 10:44 PM
Regarding the 3.rd question - this may be of interest:

http://www.xsibase.com/forum/index.php?board=12;action=display;threadid=10715;start=6

C4D is probably unique in having this reverse volumetrics so easily accessible, they probably made it directly in their rendering engine.

1.st and 2.nd - dunno, I'm a very fresh beginner in XSI Foundation. I'll leave those to someone more experienced to answer.


Cheers,
Rune

Buhrmi
08-17-2004, 07:49 AM
Thanks for the info. I'll have a closer look this afternoon ;)

Here are two pics to show you the problems 1 and 2 better:

First: http://www.buhrmi.de/muell/exclusive_volume2.jpg This pictures show, that the light coloring works, but as soon as you connect something to the color input, the volume effect dissappears.

Second: http://www.buhrmi.de/muell/exclusive_volume.jpg Here you see that a Grid in the exclusive light's associated group still is an obstacle for the volume effect, allthough it does not recieve any light.

runejw
08-18-2004, 08:52 PM
Not sure if it relates to the problem(s) but I struggled some time today with getting the look I wanted from ordinary volumetric lights. Reading this tutorial
http://www.softimage.com/community/exp/v3/tutorials/tutorial_PDFs/04_lighting.pdf

at least made me discover a couple of important facts about volumic spotlights:

1) The spotlight's Exponent variable (default = 2) affects the volumic property dramatically, so lowering this to 0.2 as in the tutorial made the volumetric light rays much more visible. All the way to the target object instead of rapidly fading off and also improved the colors it took from a transparent colored glass-plate it passed. (Will post it to my gallery when it finishes rendering shortly... http://home.online.no/~ru-j-wi/softimage.cfm

2) The Reflectance (Volumic properties of the scene -> adjust it to your liking via region render) and Umbra=0 (Soft_light shadow) also made a difference.

Otherwise, I hope it won't be too long till I receive my box with the training DVD's, I have the feeling there is much I'm missing out on, just because I don't know what the effect of changing various values will be.


Cheers,
Rune

DougNicola
08-19-2004, 01:12 AM
The Volumic Effect shader might be the better way to go here. Here's my first crack a the Trapcode Shine effect with XSI (no post-processing used, just render tree):

http://www.dougnicola.com/VolumicEffect_shine.jpg

Here's an animated version of the effect (1.3MB):
http://www.dougnicola.com/Volumic_shine.mov

Another version:
http://www.dougnicola.com/VolumicEffect_shine_3.jpg

runejw
08-19-2004, 01:25 PM
Show us the rendertree! Show us the rendertree!

:bounce: :scream:

I tried using the fast-volumic effects, but probably did something wrong - didn't get a result.


Cheers,
Rune

DougNicola
08-19-2004, 07:39 PM
I'm using the Volume Effects shader, not the Fast Volume Effect.

Here's my favorite render so far of this effect (no post, only render tree, I love mental ray and XSI!):

http://www.dougnicola.com/CoolBlue1.jpg

Here's another animation (1.6MB):
http://www.dougnicola.com/CoolBlue_shine.mov

The effect comes from three different areas, the Volume Effect shader, a point light, and a grid abject used to invert the color. The only necessary change on the actual Text object is to turn on incandesence and crank it way up so it will come out completely black when inverted.

Here's the set up of the objects. The large cube is the volume, and the white dot is the point light. The grid goes between the camera and the volume:
http://www.dougnicola.com/Object_page.jpg

Here's the changes made in the volume effects shader (also add the light in the lights page):
http://www.dougnicola.com/VolumeEffect_page.jpg


Here's the changes made with the point light. Note the high intensity and the negative umbra. The negative umbra makes the shadow extra dark, which, when inverted, becomes bright white. The falloff is also important:
http://www.dougnicola.com/Light_page.jpg


Here's the transparent and inverted grid object. The inversion flips all the color coming through the grid:
http://www.dougnicola.com/Grid_page.jpg

One thing to keep in mind with the Volume Effects shader is that the camera cannot be inside the volume! Kind of limiting, but seems to be the way mental ray works right now. Also, you cannot build on the Volume Effects shader in the render tree, but you can animate the values.

mesmer has a really nice free video (www.mesmer.com (http://www.mesmer.com/)) on the Volumic Effect shader. Look under eLearning->course list->XSI 102->Free Tutorials, and find the "Volumic Lighting in XSI" video. Lots of other great XSI vids there too!

I've just started experimenting with the Volumetrics is XSI, and I think it's very possible to get many cool effects, but you really have to dig into it.

runejw
08-19-2004, 08:04 PM
Very Cool! Thanks so much for the mini-tutorial ! :buttrock:

I see my main mistake is trying ONLY to meddle with the spot-light volumic effect.

I had an inkling that maybe XSI could do it, but in a different way, and it sure does...

Also clever use of the inverter-grid.

Totally agree on the Rendertree, one of the best features of XSI.

runejw
08-22-2004, 10:15 PM
I tried the orignal example and think I succeeded.

What I did: I made a cube, fully transparent, put the spotlight inside and attached volume_effects to the cube and picked the spotlight.

Then made the render-tree like in the original post - not totally the same since I didn't find exactly the color_interpolate for example. (Used a gradient instead).

Adjusted spots exponent to 0.2 and got a fairly similar result.

Then tried putting spheres in the light and made light exclusive and associated the spheres - voila no shadows from them. (With inclusive they make shadows).

Then made a polymesh grid which I made semi-transparent - that worked fine also :thumbsup:
The transparency value can be used to tune how much light passes through.

End result like in the picture here (http://home.online.no/~ru-j-wi/softimage.cfm?FuseAction=Album&pAction=PicDet&pAPICID=89)

The scene file raymarch.zip is attached for anyone interested in the rendertree etc.

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