car reflection continuity issue.. (pics in thread)

[anthonymcgrath]

hi guys
okay this one has given me and half a dozen other members of staff the red ass... take a look at the pics below.. the reflection is a simple giant area light reflecting in the car bodywork but as you can see the reflection is broken in its continuity. I've attached a render and a couple of pics of wireframes.

one of the guys who models cars instantly claimed vertex normals are the issue so i sat about tweaking those - however Mental Ray doesn't care when rendering the model once i've switched to smooth mesh. there's no difference.

another of the guys even took the model and tried all manner of poly editing - combining the two together and still got no luck.







as a test i turned both the door and the rear quarter into one smooth panel.. this is the result below... however this isn't ideal at all as i need to have the door separate...




Can anyone suggest anything that may help with this? I'm using the carpaint shader on the car btw.
Is there some setting in Mental Ray or some kind of lens shader that will assist with this? I've been battling this issue for days.

any thoughts and advice welcome
ta
ant

[Remydrh]

I've been doing a lot of cars lately. . .

Most of it is CAD data unless it needs remodeled.

Very subtle changes on topology will shift your reflection. Especially in a car, the reflection will stretch or compress based on the bend of the sheet metal much like a funhouse mirror will distort your mirrored view.

I also notice in shaded view some of your vertices darken, it's possible there are normals that aren't correct in the model.

If replacing the object with something like a plane reflects correctly, then the topology still isn't correct and there's not much mental ray can do to fix that. mental ray doesn't understand your intention.

[anthonymcgrath]

I've been doing a lot of cars lately. . .
Most of it is CAD data unless it needs remodeled.
Very subtle changes on topology will shift your reflection. Especially in a car, the reflection will stretch or compress based on the bend of the sheet metal much like a funhouse mirror will distort your mirrored view.
I also notice in shaded view some of your vertices darken, it's possible there are normals that aren't correct in the model.
If replacing the object with something like a plane reflects correctly, then the topology still isn't correct and there's not much mental ray can do to fix that. mental ray doesn't understand your intention.

thanks for your reply
vertex normals seem to make no difference once the model is smoothed - i did try that initially.

i'll take a look over the geometry and see if theres any imperfections and see if i can get the reflection to be smoother - the irritating bit is that it shifts so much at the edge of the panel to the door - not so much the slight wave across it :/

[Remydrh]

In many cases for cars, where the door meets the body, the body will then curve away from the edge of the door. Most notable in things like a Camaro. The door is relatively flat but the rear quarter panel often bends out.

[anthonymcgrath]

In many cases for cars, where the door meets the body, the body will then curve away from the edge of the door. Most notable in things like a Camaro. The door is relatively flat but the rear quarter panel often bends out.

you got an example or diagram so i can better understand what you mean? I think mine does that but not too sure

[steve]

Hi,

is there any chance we can get this model or the relevant pieces, like the two surfaces with all its settings and shaders attached ? Would be great for us to take a look.

Are you using any render-time mesh smoothing ?

Best, Steve.

[anthonymcgrath]

Hi,

is there any chance we can get this model or the relevant pieces, like the two surfaces with all its settings and shaders attached ? Would be great for us to take a look.

Are you using any render-time mesh smoothing ?

Best, Steve.

I actually feel really really stupid - turns out the verts on the rear quarter needed moving in X (towards camera) the TINIEST amount... i literally had to zoom in a few times to the point of cam clipping. I then shifted them a bit and the reflection has pretty much mated up!!!?!



its not perfect but compared to what i had its a zillion times better!

[Remydrh]

Yeah I think the darkened patches in the viewport are a sign the normals on the model are odd.

Steve might be able to be more specific about it if you can get it to him.

[anthonymcgrath]

as i said it just turned out to be vertices that needed shifting a TINY amount - unbelievable. I tried playing with normals - they make no difference once its switched to smoothMesh. There are some options in the attribute editor for the shape node (things like propogate edge hardness etc) but i couldn't see them making much difference.