View Full Version : !!problem with alpha and mental ray shader!!

03 March 2004, 10:22 PM
hi guys

i have a problem, and no more ideas

iīve connected a lambert with a mr defraction shader and my alpha channel get totaly messed up in the rendering ?

does someone know how to connect a alpha channel correctly ?

the defraction calculates a glossy effect / kind of blurred rays into the transparency of my shader - the color channel is pretty well but i do need an alpha ????

(iīve tried already to make a green screen render an key it in the post without a satisfing(?) result)

pleeeeeeeaaaas help needed :-(

03 March 2004, 09:28 AM
or is there a different solution for glossy refractions ???

03 March 2004, 09:57 AM
Well, you can always use DGS Shaders instead...

But, Why do you use a lambert with diffraction shader?, it might render better with phong or blinn shaders..., o well, use DGS shaders, they make a beautyful alpha channel.

03 March 2004, 10:00 PM
Hey Joie, i cant get "beautyful alpha channels" from any mental ray node either. maybe you could point us to some more specific information. were not talking bout transparency in the color channel. Could you put up a scene file so that we can see?

03 March 2004, 11:11 PM
Mmm, I donīt remember the last time I used a DGS shader in order to refract something blurry, but the Alpha it makes through reflections is totally perfect, Iīll try transparency tomorrow, but I donīt remember if I had any problems with that... (although I tried dielectric materials and they make ugly alphas..., like checkerboards or so, iīm confusing).

Sorry for not being able to help more... (and for my english)

Daniel Whitton
03 March 2004, 03:36 AM
try this thread

03 March 2004, 10:20 AM
hmmm it seem that we have a little missunderstanding here :)

i donīt have any troubble to get textures with alpha or stencil 'file's into my shader - my major problem is that the final result, which i need to compose in ae or combustion, has non correct alpha (rgbA) channel - and i went out of any idea how to pipe transparency to the rendered alpha channel... and i also tired the 'opacity' node and i didnīt got it wont working

03 March 2004, 10:23 AM
I know that thread but he is looking for transparency in the color channel.

What you are discussing there is to load a texture map with alpha channel to show only a part of the texture. We dont even have a texture here in our shader. We have a glossy refraction glass shader that simply puts out a totally weird alpha channel. And the dgs for example only shows up as totally white or totally black. That wont work for compositing. So if theres someone out there that manages to get an mental ray lambert for example half transparent in the color channel to show up as grey in the alpha channel please let me know.

03 March 2004, 10:39 AM
So, here is a little example of what i mean. If you get that result with mental ray lamberts or whatever, please let me know.

03 March 2004, 11:13 AM
that image is absolutely correct.



03 March 2004, 11:49 AM
lol, thats because its rendered with the maya renderer.
I just wanted to show how it should look like. mental ray only gives me totally black or totally white as long as im using mental ray materials.

here is an example of both renderes to compare.
That "Pass Surface Color Alpha" Thing is turned on in mental ray render globals otherwise everything is just plain black

03 March 2004, 11:52 AM
sorry, forgot the file

03 March 2004, 11:56 AM
even more sorry, wrong file size

03 March 2004, 12:24 PM

this looks a bit like premultiplied and non premultiplied images.

i know that softimage did the same with the images that mental ray rendered.
it is just a matter on how you comp these images.



03 March 2004, 12:46 PM
??? what
theres nothing composited right now.

When i take that picture into an compositing app there will be no box anymore. Or when i disable the the alpha channel you wont see anything behind! I just wanna have a correct alpha channel from mental ray but i really start to belief that its not possible in this lousy integration of mental ray materials. Maybe thats why alias is hiding them.

03 March 2004, 12:55 PM
ok.. i think i am talking BS..

nevermind :)

03 March 2004, 04:57 PM
Hi there,
Have you tried to click on "Pass Surface Color Alpha Channel"
under custom entities in MR render globals?


Daniel Whitton
03 March 2004, 07:49 PM
Sorry about the mix up. I have the Rendering with Mental Ray handbook, on page 256(if you have it). To write a correct greyscale alpha image you need to specify s_16. Which is under Image Format of the render globals. Select mentalray Alpha [st] from the drop down menu.

Hope this helps:thumbsup:

03 March 2004, 09:11 AM
hmm.. on page 244 i read:
...if type s or s_16 is specified then a grayscale intensity image is generated..

all this is wired stuff :/



03 March 2004, 01:27 PM
well, it looks more like a luminace channel or something. The Sphere is no solid white as it should be.

I have a sample scene if more guys of you wanna try around with that. Left are maya materials whereas on the right are mental ray materials. When you render it you can see the differece in the alpha.

Have fun

03 March 2004, 02:51 PM
I had a look at the file and the problem seems to be that the MIB_transparency need RGBA info while the slider is only providing RGB info. I plugged in a one colour ramp (50% grey) and the rendered alpha behaved as expected. Having an texture with alpha probably forces the alpha through to the rendering.. what ever!! obviously other texure nodes and file textures will work to give you different effects..

This from Highend 3D is kinda related (

03 March 2004, 07:20 PM
oh that turns out quite nice, for the lambert at least.

For Daniel Rinds defraction shader i cant get it to work. :(
but thx guys, i got some really helpful answers

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