reflection / refraction & alpha channel

[dynaman]

hey folks,

i've encountered this issue before and managed to hack around it, but i can see that it is going to present a problem in my current project. here's the problem:
- i have some modeled 3D context that i am using to generate reflections on a curtain wall
- i don't want to render the context in my image; i will be compositing the rendering into a photographic background
- so in object properties, i make the context not visible to camera, but visible to reflection / refraction
- problem is, in the rendering, the context shows up through the glass areas, and hence doesn't contribute to my alpha channel.

what i need is for the context to be only visible to reflection, and i don't want it to be visible THROUGH the glass. i want to be able to see the photo background through any transparent surfaces.

is there any way to do this using vray? i could be wrong, but i don't seem to recall having this problem using scanline in max.

here's a quick shot of what i am talking about.

thanks in advance for any suggestions.

[DaForce]

in the VRAY properties for the object, visible to reflection/refraction are 2 seperate options.. so just untick refraction and away you go

You might want to try invisible to GI as well.. see if that helps... probbaly now what your after.. the info above should be the trick.
You will probabaly need to re-tick visible to reflection/refraction in the normal properties for the vray ones to have an effect.

[dynaman]

SWEET! that's what i'm talking about!

thanks dude.

[DaForce]

No problemo!!

[dynaman]

daforce,

i tried doing what you said, but i'm still getting the same problem...maybe i'm missing something??

here's what i did (v 1.47)
- select the context group
- go to vray properties
- check "on" matte object
- set alpha contribution to -1
- set refraction amount to 0

this gives me the exact same result as i showed above. i also tried checking and unchecking "visible to camera" in object properties, as the vray alpha setting should overrule the object properties.

here are a few test results (just a few of the scene parts on a black background):

test 00:
vray properties: alpha -1; reflection amount 0
object properties: not visible to camera; visible to reflection / refraction



test 01:
vray properties: alpha -1; reflection amount 0
object properties: visible to camera; not visible to reflection / refraction



test 02:
vray properties: alpha -1; reflection amount 0
object properties: visible to camera; visible to reflection / refraction




as you can see, test 00 and test 02 are identical, and none are giving me the result i'm looking for. any further suggestions? there must be a way to do this...maybe v1.5?

[RErender]

afaik you have two choices. Either use the map you have for the reflection in the background or reflection override slots or render two passes. The first as normal the second without the reflection geometry to get the alpha you need, the second pass can be rendered whithout lights or GI since all you are after is the correct alpha.

[DaForce]

Ahh your not using 1.5, that tick box i mentioned is only in 1.5... it "should" do what you want. But at the same time maybe it does the equvilent of change the refraction amount to 0.0

Hang on, i notice your changing the reflection amount. Is the problem that your seeing the reflection of the object or the refraction? as in the object is behind the glass.
If you only want it to be visable to reflection but not refraction (thru the glass) you need to try and drop the refraction amount to 0.0 or use 1.5 and untick visible to refraction.

make sure you have affect alpha ticked in your glass material as well.

[DaForce]

I just tested in 1.5 if you right click on the object that your using to generate the reflections, and go into the vray properties for that object and untick visible to refractions you will get the visual result your after but the ALPHA is still blocked.

So looks like the 2 pass option that RErender suggestion is your best bet.

[dynaman]

Ahh your not using 1.5, that tick box i mentioned is only in 1.5... it "should" do what you want. But at the same time maybe it does the equvilent of change the refraction amount to 0.0

Hang on, i notice your changing the reflection amount. Is the problem that your seeing the reflection of the object or the refraction? as in the object is behind the glass.
If you only want it to be visable to reflection but not refraction (thru the glass) you need to try and drop the refraction amount to 0.0 or use 1.5 and untick visible to refraction.

make sure you have affect alpha ticked in your glass material as well.

i think i did try everything you suggested here. the problem is that i'm seeing the geometery THROUGH the glass, and i don't want to. i only want to see the geometry reflected in the glass. i would have thought turning the reflection amount to 0.0 would be the same as what you suggested, but it didn't work (see test00 or test02)

the only way i can get the proper alpha outline is to make the geometry not visible to reflection / refraction in object properties, but of course this means i get no reflection, which is the whole point of what i'm trying to do.

RErender, your suggestion would work, and is what i've done in the past. i just figured there has to be a way to get vray to do what i want.

maybe the only solution is to use 1.5...

[DaForce]

Cool just checking you had done it all right. Seems you had.

Like I said I get it to look right.. by unticking visible to refractions it was only visible to reflections and not refraction. BUT the alpha channel still wasnt blocked out, even after turning the object into a matt object with alpha contrib at -1.... so not sure what was going on there.

But 1.5 certain gets you alot closer to where you want to be anyway

[dynaman]

now i'm wondering what is the purpose of the "refraction amount" in vray properties / matte properties. the name would seem to indicate whether particular geometry is visible through refraction or not, but there appears to be no difference at all between a value of 0 or 1. maybe this is a bug in the software?

[DaForce]

Those settings are all to do with matte objects, and since yours isnt set to be a matte object, they have no affect.

See here for more info.
http://www.spot3d.com/vray/help/150R...l_settings.htm

[jonnybefree]

hi there, what when you're using the mtlwrapper, with a non-refractive but reflective mat. in base-slot? and turning on matte with alpha -1?
don't know, maybe its bullsh.... can't test it at the moment, but maybe it works.

best regards

Jonas

[dynaman]

jonas, i'll give it a try.

daforce, actually, i did set it to be a matte object in my latest tests.

[dynaman]

hey jonas, gave it a shot - same problem.
seems to me the material wrapper and vray properties are redundant? i've only been a vray user for a few months - was one of these systems a throwback to an earlier release?