Creating Alpha Transparency through Multiple Objects

[kevinmGFYPQ]

How does one create the ability to composite in background scenes through glass objects in front of one another?  For example, I have an interior rendering whee you are looking through a partial wall of glass block and then through an exterior window half blocked by the interior glass block wall.  If I save the rendered image out with an alpha channel, the composite does not work correctly.  

It is quite easy of there is just the exterior window as the alpha channel for the glass allows me to replace the background HDRI or environment with my own environment in post processing.  However, if there is another glass object in front of the exterior window, things get complicated and do not allow this typical composite to work.

2018 3ds max 

[kevinmGFYPQ]

Why can't I edit my original post?  Also, sorry about the hitting the "need and answer" button.  This is a new setup for me versus what I was used to back when I used to use the AREA forums more regularly.

Anyway, I attached a PDF of a test render showing the issue and a PNG file with the transparency so you can see how it behaves when pulled into photoshop or similar.  If you drop a background image, the exterior windows behind the wavy glass does not seem to be transparent at all since you see none of the background image.

[jon.bell]

Hi Kevin,

Thanks for your post.

I'm curious what renderer/materials you're using, and if you have sufficient G-buffers/reflection and refraction bounces set in your renderer to allow for multiple levels of transparency. If not, then the window glass that's behind the frosted glass isn't going to have a proper alpha channel allowing any background plate to show all the way through these layers.

Also, please note that the frosted glass in your rendering isn't going to automatically distort/refract a composited background. Although it should allow a level of translucency, if you're using raytracing, there's no way for a raytraced material to reflect/refract/distort an environment that's not actually in the scene at render time. (You can set up workarounds where an environment may be visible in reflections/refractions, but not visible to the camera, but I'm not sure if you're doing that with this setup.)

However, if the glass in your scene has enough layers to produce translucent alpha sufficient to see an underlying background image, then you should be able to fake a refracted, blurry background in your composite. (What composting program are you using as well?)

Please let us know, and we hope to hear from you soon!

Best regards,

[kevinmGFYPQ]

Hello Jon and thank you very much for the reply.  I am using Vray at the moment but have used others.  I will be up front and mention I am not a materials expert for sure.  However, I think I actually know what you are talking about with the the material set up to allow deeper layers of transparency.   I think that is adjusted in the refraction and reflection max depth settings.  I think there is a fog setting I need to check into as well.  The more I think about it, there are probably other settings that can impact this such as refraction color, IOR and glossiness.  Since I am not real familiar in customizing such things, I guess I need to mess around with it a bit.

As for your comment on inserting the environment map not showing in the reflections if it is not actually enabled in the scene, I guess I could put a copy of that map in the reflection slot of the material itself.  

I knew this was not an issue of the renderer or 3ds max but I wanted to ask to at least get a direction to look.  I was kind of hoping it was something I could deal with by rendering out passes which is something else I need to learn more about.

I use Photoshop for my compositing of still renders

Thanks again

[kevinmGFYPQ]

Actually, I just checked the material settings and they are set pretty high in the right areas to allow the level of transparency I should need.  I noticed a new problem though that is more troublesome.  If you look at the attached PDF, when I generate an image with an alpha, I lose the information behind the interior window as shown.  Any sense of a reason for this?

[kevinmGFYPQ]

bump

[jon.bell]

Hi Kevin,

Thanks for your update and example images. I'm puzzled why you would be seeing only a transparency channel behind those windows, instead of the geometry that's supposed to be there, unless somehow the rendering properties for the geometry got turned off.

Would it be possible to get an archived version of your scene file for me to look at in our offices? If you want, I could send you a private OneDrive link for your upload, if it's not proprietary.

Please let me know, and I hope to hear from you soon!

Best regards,