Alpha transparency is rendered out of MRQ with different quality results (in most scenarios, not at all) depending on the renderer (Lumen v PT), and the type of shader (Substrate v Legacy). The best results seem to come from the combination of Legacy glass using the UE4 material node with Path Tracer. All tests were performed in Substrate enabled projects over the course of a few years ranging back to 5.3 to varying degrees of testing. Ultimately the goal is for glass to render out with alpha transparency from MRQ, allowing for visibility of any objects behind the glass.
Within the project and files provided, I ran tests with 3 spheres in front of an opaque cube. These spheres have different glass materials assigned (Legacy glass, our Substrate glass, and Epic’s recently released glass from the Substrate Automotive Pack).
There were three main render scenarios this was tested in that yielded different results:
Render out an image of the materials present in the sequence using either Lumen + Raster Translucency, Lumen + Raytraced Translucency, or Path Tracing
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Lumen/Deferred was the first image I posted with the more gradient alpha. If the banding is the issue then you can try and switch to TranslucentGreyTransmittance if you don’t need to transmit color to clean out the alpha channel due to the 16bit.
Actually, it seems I’m getting different results depending on the dome material used, oddly enough. The alpha is actually behaving differently depending on if we use the dome material that is default with HDRI Backdrop versus one of our own custom ones, and I need to find out why that might be. Will need to do some testing on my end.
Yeah its a bit of an issue with Substrate wanting to be a PBR shading system and semi-invisible things don’t really exist in the real world. It wants to be solid objects that just transmit energy. That goes double for Pathtracer. That said, we still need to be flexible enough for artists. I’ve flagged the banding issue in the alpha. And yes, the ThinIOR is still being developed. There were a couple of other smaller things that popped up that were JIRAd while I was messing around with your repro so thanks for reporting it.
No, I’m referring to how most scenarios will not render an alpha on translucent objects at all. You can see in this image that was rendered in Path Tracer, both Substrate glasses render without alpha, but the glass built with a Legacy UE4 node in the master material does. Lumen also yields a variety of results, but the best result is still UE4 legacy glass, with Path Tracer. [Image Removed]
I see, thanks for clarifying. What I am seeing on my end when rendering your Lumen preset and looking at the alpha I do see some banding in the alpha. The transmittance when the BlendMode is set to TranslucentColorTransmittance is stored in a fast, R11G11B10 floating-point format buffer. The dev said this was a conscious choice for performance reasons. That said TranslucentGreyTransmittance uses 16its so that produces a clean alpha.
For Pathtracer all the alphas are solid/opaque due to refraction being enabled. In the path tracer, technically, things with transmission are not partially visible/invisible. They are a solid object transmitting energy. So the alpha is solid on purpose. You can get a non-opaque alpha by disabling refraction in the material. I spoke with the PT dev about this and he said that this is not really a supported workflow and may produce varying results but shouldn’t break anything as far as he knows.
Thanks for the answer. What about Lumen? The only result that actually gave us an alpha we could work with was PT + Legacy shader. Lumen seemed not to produce great results either. At least in our projects.
We hadn’t noticed the banding yet, but good to know. However, the main issue is how the alpha comes across. For example, to further illustrate, here is the render in Lumen with Raster translucency taken as a screenshot from Ps to better visualize how the Alpha is coming across in renders. [Image Removed]And below is with Raytraced Translucency [Image Removed]It’s acting like a cutout. Anything behind the glass is wiped out. Same here with a render of a vehicle, where none of the interior of the car is visible, or even the glass itself.
[Image Removed]If this is caused by a simple setting I am missing, I’d be grateful for any insight.
Actually, scratch that. It was a refraction issue. And I have fixed it. Seems like the problems in the end (at least in Lumen) are a mix of refraction and project settings (that I still need to diff between the project I sent you and the project I was originally testing in.
So to recap:
Path Tracer alpha can be fixed by turning of refraction (not ideal, as it will affect look of glass)
Our Lumen glass alpha issue is fixed by taking the ThinIOR material function in your substrate glass from the automotive pack and using that. I’m assuming this will not be necessary in the future as within Epic’s material function there is a comment around the nodes that says, “Remove it in 5.8”.
Banding can be fixed by switching to TranluscenceGreyTansmittance, though that only works for glass without color.
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