[HOU-1672] Major Refraction Issues

[Pipekk]

I'm on Houdini 18 and am having some major issues in the latest as well as previous version builds.

Mainly Refraction & Reflection with both material builder and complex materials are showing some really odd behavior.

Looking at the image "refraction full", on first glance make me feel like the dragon and gemstone were incorrect.
So I did tons of testing and experimenting between custom materials and vray presets. And every single one seemed off.
I come from a faint Cinema 4D background and just started with Houdini about a month ago. So I hopped into Cinema and tested the same material presets and settings as well as lighting conditions and sources. As you can see in the image with only the 2 gems, there its working like a charm.

After further testing I noticed that for some reason, if I use geometry which is not imported but generated by Houdini it looks correct. Using imported models be it, obj or stl doesn't seem to matter. I played with the import parameters but none seemed to fix this issue.

Also I noticed that reflections do not appear at all on imported models see "reflect channel only".

I'm not sure if this is user fault and just very user unfriendly design or if its actually a bug.

But if anyone here has a answer or know more.

I'd really appreciate any help!

[corestudio]

I wonder if the normals are wrong or not set. have you tried adding a Normals node to the mesh? Set to 0.

[Pipekk]

I wonder if the normals are wrong or not set. have you tried adding a Normals node to the mesh? Set to 0.

I tried that and I also tried adding the facet node and enabling "unique points", I even tried both in combination.
While with some settings I get vastly different visual results like reversing the normals.
In no case I get a remotely realistic visual result.

Considering the default should look like the cinema 4d one, which is identical to what you get in Maya and 3ds Max from what I have seen.

I'm starting to think that this is a bug which for some reason flew under the radar.

[Gosho.Genchev]

Using imported models be it, obj or stl doesn't seem to matter. I played with the import parameters but none seemed to fix this issue.

From V-Ray's point of view, there is virtually no difference between rendering Houdini geometry generated in the scene or importing obj or bgeo files from a File SOP. The only exception to this is VRMesh/VRScene files, Alembics and VDBs.

Here's my test files: https://drive.google.com/file/d/1yFr...ew?usp=sharing

Here's a the result:



If I had to guess, I'd say the scale of your objects is different between C4D / Maya / Max and Houdini. Exporting an obj from Maya and bringing it into Houdini would make it 100 times bigger if you started off in Centimeters in Maya.
If you'd like to, please share one of your Maya/Max files so we can check what's causing the difference and message you back - you could do it via e-mail if posting on the forum is not an option. Your files will not be shared.

Regarding the render elements - could the difference in what you're seeing be related to the V-Ray version used. With V-Ray Next, some changes were made - please check the yellow note here: https://docs.chaosgroup.com/display/...VRayReflection

Hope that helps!

[Pipekk]

From V-Ray's point of view, there is virtually no difference between rendering Houdini geometry generated in the scene or importing obj or bgeo files from a File SOP. The only exception to this is VRMesh/VRScene files, Alembics and VDBs.

Here's my test files: https://drive.google.com/file/d/1yFr...ew?usp=sharing

Thanks for your help.

With your test scene I allowed myself to do some further testing and created a test scene involving different kinds of imports as well as some native models. https://drive.google.com/drive/folde...to?usp=sharing

For some reason 99% of the time no matter how I export and what model I load it gives me the bugged result.
Only when I exported a entire model scene, from Cinema 4D it worked as it should... just like you showed with the maya scene.

However as soon as I export a single model with the exact same c4d settings, it gives me the bugged result again...

[Gosho.Genchev]

Seems the vertex winding order is reversed for "gem obj test c4d export.obj".

Can you please try dropping a Reverse SOP for it to check if that fixes things on your end ?

[Pipekk]

Even if I reverse sop the "gem obj test c4d export.obj".
While I then get a different looking result. That reversed result is still nowhere near accurate or how it should be.
On the right you can see the same material and same 3d model with a export which works just fine. And on the left the exact same mat and 3d model with a different export suddenly breaks.
I added some additional light to make the issue more apparent and updated the test file https://drive.google.com/drive/folde...to?usp=sharing

[chakarov]

Hello Pipekk , as I can see from your houdini scene and obj files, both geometries are "different". They have different point order for their primitives, that's why one of them should be "reversed" and other one not. Also because of that different order one of them has also reversed normals /pointing inside the object/ it's normals should be also reversed. Reverse SOP only reverses point order per primitive, but normals can be reversed from Normals SOP "Reverse Normals" option! You could see how I repaired both geometries in provided file.

For "wrong" object you could also first use Reverse SOP and then Normals SOP, in this case normals will be calculated and saved over the "right" point order.