[Solved] Visible reflections in refract channel | two sided materials

[Duron]

Hi,
i came across another issue where i need help.

As i'm specialized in automotive section, i am very dependent on proper render elements for editing later on.

I've read that corona makes all shaders by default two sided. But is this also a restriction? Will there be a checkbox at the initial release to force one sided material?

My problem is that i see the backside reflections in my refraction pass which makes it unusable for me.
I've also checked the (raw) component channel but got everywhere the same problem.

Thanks.

[Ludvik Koutny]

All objects in reality reflect from both sides. Corona's not good at fakes, and generally, absence of faking should not prevent getting good result for automotive rendering. Most of specialized automotive renderers are unbiased, so they don't have such toggle, and neither does Corona. I wonder what do you even need refraction pass for.

That being said, what i see on the render looks a lot more like some sort of artifact rather than backside reflection. It looks very weird, in case you mean that bright stripe in middle. Or is that just region render?

If you really need to do such fake (and you should never ever do it) then you can simply plug CoronaFrontBack map into reflection slot, and set front color to white, and black color to black. But it still makes no sense to do such thing, and doublesided reflection should definitely not cause these artifacts. Actually, it's absence of back side reflections what often ruins for example appearance of headlights on cars when using renderers which offer this option.

[Duron]

To split the reflection and refraction is very handy to fix something or retouch in postproduction. Imagine a situation, where you have to add a driver into a car without its beeing rendered. This way you can split the passes and grade each pass easily. I'm with you when it comes to keep things as real as possible. This is something why i keep with corona somehow. I feel that my renderings are more realistic than vray. And this i've noticed in my very first render already.

Your solution will help me out even it's not real for sure. Ive also tried and decreasing the IOR is also removing the backside reflection too. So there are two workarounds now, cool.

Thanks for your support rawa.

[Ludvik Koutny]

I understand, but i am still not completely sure what i am looking at. It's not very obvious from the picture you sent. Maybe if you manage to reproduce it on some simpler scenario (few boxes behind each other), i could get to the source of the problem, and it could possibly be resolved in a more elegant way than putting FrontBack map into reflection slot :)

[Duron]

Sorry for confusing rawa.
In my case, what really would annoy is the reflection in the refraction pass. So i could not easily retouch the inside elements and then just put the reflection back on top again. Without this feature, you would always have to rerender or just leave it as it is, because you can't render that special thing for example.

Sure you can have a look how to solve this in a more elegant way but, i think both known solutions would help already.

Thanks.

[Ludvik Koutny]

Oh, reflection in refraction pass, now it's clear.  Hmm.... this is a tough one, because technically, internal reflection is indeed part of refraction pass. Everything after ray initially enters the refractive object is... 

I guess, in this case, FrontBackMap is currently best solution.

[James Vella]

Is there a solution for this? We are getting the same thing, I would like to remove the sky from the glass by using refraction, but the reflection of the sky is built in. Vray defaults are like this, for pipeline purposes this would be handy if I could select the 'raw' refraction without any reflection....

[James Vella]

I was able to fix this by changing the refraction IOR to 1.0

Im not sure if this is the correct workflow. Any thoughts?

[maru]

Can you provide us with more specific description of what you need to do?

[James Vella]

This is what I get in vray:
Refract


Reflect


Raw


Corona:
Refract


Reflect


Raw


Glass Material


What I want is to be able to remove all reflection & replace sky

[romullus]

For simple glass sheet, you could mark material as thin (no refraction) or set IOR to 1.0, then you should be able to get same effect like in Vray.

[maru]

Do you have "reflect on back side" enabled in the vray material? If not, then the difference is that Corona renders glass correctly.

[James Vella]

I understand what you mean - the backface of the glass is reflecting... but even with the IORefraction set to 1.0 I still get the error. I really shouldnt be seeing any reflection in the refraction pass anyway...Is this a bug?

[PROH]

Hi. If you compare your Vray example and your Corona example you can see that there's no double reflection in the Vray example but in the Corona example there is. Since the second reflection is behind the front glass it'll be visible in the refraction pass. I don't think refraction IOR set to 1 changes that (haven't checked how "Thin" behaves here). So if you wan't to only have the front-reflections, then remove the backside of the glass.

Hope it helps

[James Vella]

Ive tried this with a simple box and a teapot.

box = glass
teapot = reflection in glass

still cannot remove reflection from refraction pass. As a render element, it makes this pass essentially unusable to remove reflections in post

the 'thin' option helps... but then you cannot have a solid piece of frameless glass standing and get the effect you want on the edges of the glass and have the ability to remove reflection.

From the pure standpoint of post production - I should have the ability to remove or add reflection using these render elements. The refract pass is simply not behaving 'correctly'.