Backplate/HDRI Refraction through 2sided Vehicle windshield

[Robert Bodis]

Hi,

I bumped into some issues.
My scene setup is pretty basic. Vehicle with double sided windshields (shell) , HDRI for the lighting and a Backplate.

Now, when I look through my windshield, I see the HDRI and not the actual backplate.
In order for that to show up, I have to check in  " Thin", but if I do so, I lose the nice glass effect from my windshields.

Is there any way to get proper refraction on my widnshields without checking in " Thin " ?

Note;
-I already tried to plug in the backplate into the Enviro Refr slot in the Windshield material(s). (this ended up causing refracted bits of my backplate on unrealistic spots)
-Already tried having normal glass on the outside of the shell, and thin one on the inside. (...nope)
-Tried all 3 projection modes in the shadowcatcher.

What occured to me is basically adding a plane behind my vehicle with the shadowcatcher material, but this might slow down my render, cause shadows (if they cannot be turned off in the object properties) so on, so on.

I'd greatly appreciate if someone has some intel on this. Not sure if it's a bug or not.
Thank you!

Robert,

[romullus]

No, it's not a bug. You need to instance your background to refraction override in render setup>scene>scene ebviroment.

[Robert Bodis]

Nope.
That way I get every single glass messed up. Headlights included, which are not supposed to get refractions from the backplate.

As I said before, the closest I got to a solution is adding a plane behind the windshield and add the Shadowcatcher to it.
However this will generate a dark square where the plane is, even if I turn off receive/cast shadows.

[pokoy]

Yes, facing this often. The solution that works and that you mentioned is to use a plane with the backplate mapped on it but it's a workaround that doesn't work well in some situations (animations or situations where the backplate is too close to the scene's geometry causing visible changes etc.). What would help is another map slot called 'backplate map' or so that would work differently from the refractions override.

[maru]

This should work straight away... Can anyone provide with examples (images, scenes)? I am especially interested in how you are making the backplate, and loading the HDRI.

[Robert Bodis]

Hope this one helps a bit more.

This is my workaround this issue , with the plane mentioned before.
This way I can see the backplate if looking through the windshields.          (thin-not activated)     (both clay and interactive image attached)

If I don't have the plane there, he will not project the backplate behind the windshield, but the HDRI.        (thin-not activated)     (didn't have a image attached of this one)

Third case would be by simply checking in " thin " , but then I lose a lot of detail from the windshields and they basically look flat.


*My windshields have a shell modifier on them

[PROH]

Hi. At bottom of the map rollout in Corona mtl there are two slots for BG reflection override and BG refraction override. So if you create a glass material (solid - not thin) specifically for the windshields and put the backplate into the "BG refraction override" slot, then I believe it should work as needed.

Hope it helps

[Robert Bodis]

Hi. At bottom of the map rollout in Corona mtl there are two slots for BG reflection override and BG refraction override. So if you create a glass material (solid - not thin) specifically for the windshields and put the backplate into the "BG refraction override" slot, then I believe it should work as needed.

Hope it helps

Thank you for trying to help, however,
Please read my first post again,

Done that.
I get incorrect refraction , pieces from the backplate that are not visible whatsoever.

[Ludvik Koutny]

This is not resolved by using refraction override but by adding plate behind the car with ShadowCatcher material on it.

It should never cause dark square (and it does not seem so, on your recent image you posted). Keep in mind that Corona does not support receive shadows option, as it's too much of a fake. So toggling it will do nothing.

If your plate behind the car darkens, then it is most likely due to the plate being way too close to the car, catching a bit of shadow occlusion from the environment. Just put it further away from the car (even like 50 meters) and make it a bit bigger so it covers entire window area.

The plate thing is not really a workaround but rather default workflow. ShadowCatcher kinda works in a way it assumes you do very basic blockout of your scene with geometry. There's unfortunately no better way of doing it given how limited representation of a scene location single HDRI map and single backplate provides.

[Robert Bodis]

Moving the plane further away did stop the blackening.
However this is still a lot of work for such a simple , hate to call it "issue" .

I really hope they will add some sort of a direct override for this in the future.
If I'd have objects around the vehicle, I'd be screwed with this plane.

Thank you,

[Ludvik Koutny]

How is that a lot of work? It's just a single plane with the material you already have made. Plane creation = 2 secs, assignment of existing material on the plane = 2 secs, placement of the plane ~5 secs.

And how does the workflow break with objects around vehicle? You would then just move the plane further, behind those objects.

And how would you suggest resolving it differently? Aside from directly visible environment, the backplate can only be seen by any secondary rays (reflection, refraction, direct) if it has an actual physical position in the scene. Reflected and refracted rays need to know where in 3D space exactly is that point located, so it can be reflected or refracted.

It's easy to imagine it for car windows with a little bit of thickness, because they still visually behave as thin glass. BUT, what if there was a glass sphere, which bends the rays that it refracts parts of the refraction environment that are outside of camera's field of view? What color/information would be there, when backplate projection area is defined by camera view area, and there is no backplate information outside of it?

[Robert Bodis]

I will just ignore your tone there and ask you a simple question instead.

What if I want to animate something.
I.e. a vehicle moving across the scene. I guess you would prefer animating a plane behind the vehicle instead of coming up with a proper solution, don't you ?

"And how would you suggest resolving it differently? "
Doubt I am the one who should have the answers for this.

Hugs,

[Ludvik Koutny]

What if I want to animate something.
I.e. a vehicle moving across the scene. I guess you would prefer animating a plane behind the vehicle instead of coming up with a proper solution, don't you ?

You will never animate camera, because your backplate is stationary, fixed from single point of view. If you needed camera animation, you would either use just HDRI, without backplate, in which case, problem would disappear, or you would use footage (image sequence). In that case, you would have to do some advanced scene blockout anyway, otherwise reflections would not map correctly.

That leaves us with just a stationary camera case. I see no problem with extending the plane to cover entire path of the car from edge to edge of the frame. I guess that would be another about 3-5 seconds of work. I am throwing those time estimates here because I have 3ds Max open with Car scene and testing my suggestions on the fly.

Also, I don't see anything wrong with the tone. It was just a series of questions with answers to cover the cases you may ran into, and the last one was a genuine question intended to make you pause and think for a second how would your suggestion (of some specific override for this case) work.

[redstarcg]

The plane method extension will also project misc reflections onto a car. Also instead of fixing the problem you are making excuses and temporary solutions. Corona is a fantastic engine and quality is great, but this is a very simple request that should not be hard to fix. Vray has the ability to have the refection be normal, corona is beating vray in almost every aspect so why not make this as good also.

Also as far as image sequence backplate. You said some advanced scene blockout would need to be done. That may be recommended but not needed. I have done a complete Ford Commercial and did nothing but animate the vehicle and slightly hdr, client was happy and work looked good. So no need to do something complex if it looks fine.

[Robert Bodis]

Obviously it would have no camera movement.

Let's just say a car spinning around.
Try out another theory with this then. Start scaling up the plate behind the vehicle, simulating the upscaled plane idea you just told me.
Now check the reflections on the vehicle.

You will be able to see the big plane on your clear coat.

Obviously you could haggle it out with a regular vehicle. However if I would have 500 frames to render, it would be hard to spot where that thing reflects up on the vehicle.
For most this would give a stroke after 500 frames.

Then there would be another issue if it's a bigger vehicle, god forbid a Sci-fi thing.