Hi all, I am using 5.4 and am having trouble finding a way to render alpha transparency of glass.
I am using the movie render graph to render a vehicle with a holdout on a shadow-catcher plane. In post we comp together the scene and lay down the vehicle + shadow catcher on our own gradient background.
For this to work we need the glass of our vehicle to have alpha transparency so the HDRI of our render scene isnt visible through the glass windows. Unfortunately I cant seem to find a solution to this. Other software’s path tracers have pretty straight forward options to fix this issue.
The following image is an example of what I want/need:
I’d appreciate any help I can get. If its a 5.5 feature I’d be happy to download and test the latest version, but this seems like a pretty trivial path tracer feature for most other renderers so I’m suprised that UE doesnt have it implemented yet (I’m aware the PT has only been officially fully supported since the new 5.5).
Hi, were you ever able to find a solution to this? I’m in the same boat and still looking. Reflection passes seem only to exist within the deferred renderer and not the path tracer but I may be wrong.
Thanks so much for your reply! I didn’t mean to imply transparency doesn’t work, just that the path tracer in ureal has a couple of issues for me that I’m finding hard to get around (probably due to my lack of knowledge) which also seemed what OP was after.
When trying to render a vehicle with windows (car with windshield) with a transparent background, the HDRI will be visible when the glass has refraction turned on, so I will always partially see the HDRI used through the windows (how much is visible is based on the opacity value of the glass). Ideally, I would only see the reflections.
For ex - here i can see the trees in the background due to the refraction in the glass, and at .1 opacity of the glass. Technically I would only want to see the reflected clouds (they’re too hard to see in this example)
I can turn off the refraction, and set the BG elements as holdouts, but even then the “black” BG of the holdout will be calculated into the RGB values of the image and “darken” the glass.
I also don’t seem to have a translucency or reflection G-Buffer that I can access in a post process material for example with path tracer. If I had that like is available for the deferred renderer via separate translucency (scenetexture-postprocessInput1), i could do some manipulation to get the affect I want at render time.
Ahh gotcha. Yes as far as I know, this is a limitation in Unreal currently. I am also very interested in the ability to separate passes in a more nuanced way like this, akin to what is possible in VRay, Redshift, Arnold, etc. I like that there are render passes at all, but they are very simplistic. If there is a way to access more detailed buffers, I have never found any documentation on how to do it.
I think part of the limitation is that Unreal folds Specular, Reflection, and Refraction together into one pass. Turning off the specular lighting component in the path tracer would get rid of your refractions, but also would get rid of all the reflections as well. Frustrating.
