Hi all,

again a houdini noob question:

Is it possible to render a glasmaterial (I tried the provited glasshader from H12) with Fresnell Alpha? With the Houdini shader I get a complete white alpha!!!

I made an example with Softimage to explain what I mean…

Thank you for your help.

Greets
Nico

No one …. any idea?

How do you solve such a problem …. when you need it to comp and don't have the backdrop?

I know …. it is not physical correct …. but sometimes you had to cheat …. :-)

Use Opacity falloff or if it is Alpha falloff. Dont remember exactly which one does what and is at work atm so can not test.

Thank you for your tip …. sadly it doesn't work how I would expect!

So I should dive into the shader tree and try to build a solution …. I don't know if I'm able off ….

Thank you for your tip …. sadly it doesn't work how I would expect!

So I should dive into the shader tree and try to build a solution …. I don't know if I'm able off ….

You can drive whatever value you want in to the alpha output channel.

Try appending a Luminance VOP to the Surface Color CF output tab of the surfaceModel VOP and then wire that in to the Af Alpha output inplace of the alpha para/perp logic.

One reason as to why the alpha is rendered solid is that the compositor generally doesn't know how to refract rays through a transparent object but the render engine does so let the renderer do it and set alpha solid to block out background objects.

jeff
.. and set alpha solid to block out background objects.

yes , thats exactly how i deal with ‘glasses’ .

@nicoM : i isolate the whole silhouette with the Alpha value 1 and feed into Opacity whatever ‘color i like’ . usually optimized reflections and refractions are separate passes too . i materialize glasses in post and almost never straight from the 3D render . since it is an expensive procedure i found this workflow best for almost every case .

so , in short , if you want colored Alpha , just use Opacity .



.cheers

Thank you jeff and zarti!

Thank you both for your tips …. this are good ways to manage this!

At the beginning the huge flexibility of houdini scares me …. but the more I learn houdini the more I like it!!!

Greets
nico

Hi
I am encountering the same issue at the moment, where I would need black (or shades or grey) in the alpha whenever a ray doesn't hit anything.
Right now I am getting solid white everywhere I have refractions.
This is okay for most of the time when working with backplates. But sometimes I need to put glass-like objects on a background later in comp, and therefore need transparency.

I tried the luminance VOP that Jeff suggested, but it just takes the average of the luminance from C and pipes it into AF, therefore ignoring if the ray continues and hits a solid pixel “behind” a refractive pixel. This introduces transparency where it's not supposed to be.

Any other advises then the ones mentioned above?

cheers,
Daniel