Bad geometry only in render

[Shot12]

Hey, total noob here with corona and 3ds max.
I'm working for an automotive aftermarket part manufacturer, we recently switched to corona after seeing some impressive renders.

We used to used keyshot to do quick the renders and when approved we then built the molds, made the parts and then a photoshoot for a catalog.
My boss wants to skip all that, as he is convinced it's best to go digital and have better renders, check the market interest and then if there is a demand build the product.

So one of the big challenges we're facing right now is making the models work for rendering. We use a combination of 3d scans, Cad surfaces, and polygonal models. 3ds max doesn't really like Cad surface data.

We are importing the Cad as STP and letting 3ds max do the conversion to mesh, however it leaves un-welded vertex and some low resolution areas. TurboSmooth or mesh smooth don't really work here. The solution we came up was to import every component as a separate object, instead of an assembly. That way, we can control the poly density while importing the parts and making it more or less dense as needed. This method leaves an Ok looking mesh on the 3d viewport. But when rendered in corona, it shows some artifacts.

What would be the best approach to fix this or the better practices to work in these scenarios?

On the attached pics you can see the comparison between the render and the 3d view

[romullus]

Shading artifacts are common with CAD models in Corona, but there are ways how to mitigate them. You can try to turn off shadow terminator handling in render settings. In the render setup>system tab open system settings window and turn on enable dev/debug mode, then close the window and go to performance tab, expand development/experimental stuff rollout and look for terminator handling - turn it off by setting shadow shift to 1,0 (lower values might work if you're willing to experiment and find what's work best in your specific case).

Also regarding CAD models, you should get much better result if you import them as body objects in 3ds Max instead of converting them to polygonal mesh, but you need to familiarize yourself with specifics of working with body objects, since out of the box they usually provide sub-optimal results.

[Shot12]

Thanks! That worked out! It fixed my cad surfaces, But it introduced noise in the relfections on native polygon mesh.
setting values between 1 and 0 fix the reflections but the pases take more time to render and end up more noisy.

Is it possible to apply the shadow terminator in only certain areas?
Or the best would just render it with and without and combine in post?

[romullus]

Shadow terminator handling is a global feature and it can't be used per object, but it shouldn't increase the noise at all. Maybe there are other reasons for increased noise, like if you accidentally disabled denoising as well, or rendered different amount of passes. Turning off shadow terminator handling may produce artifacts in other places, but those are usually more easy to fix with traditional methods and usually they are not as noticeable as artifacts on CAD models.