Reflect on backside + refraction pass issue

[seandunderdale]

Hey guys. My problem is being able to render a car headlight outer lens, with "reflect on backside" turned on, but also get a clean refraction pass of whats behind the lens for a retoucher. Now, how it works now does make sense....the lens has thickness, with reflect on backside on, it sees through the front faces, then creates a reflection on the backside (inside) faces of the other side of the lens as the rays exit.(I hope that makes sense). My problem is this "inner reflection" comes out in the refraction pass (which again seems accurate), but is there a way to be able to render with a lens on, with reflect on backside on, and not get any of the lens reflections in the refraction element? The only fix I can see is to not use "reflect on backside" and I get a clean show through inside the lens with no outer lens reflection over the top.

[hermit.crab]

You may need to render twice, with and without the 'reflect on backside' option, to get the desired refraction pass.

[seandunderdale]

Fair enough. Thanks for the quick response.

[kosso_olli]

The only fix I can see is to not use "reflect on backside" and I get a clean show through inside the lens with no outer lens reflection over the top.

That is how we do this as well. There is no way around this, it is the same in other renderers.

[seandunderdale]

Thats a shame. While not essential, when I do a comparison with and without, I always prefer with. Has that extra detail. Oh well.

[kosso_olli]

Thats a shame. While not essential, when I do a comparison with and without, I always prefer with. Has that extra detail. Oh well.

Same here. I have to say we do not rely on render passes as we used to in the past. Today we treat the RGB like a photo, and retouch on them.

[seandunderdale]

Totally agree. For higher end stuff, or brochure work I totally go for rendering passes like a photographer would shoot and using elements only to supplement areas. I talked our retouchers out of total beauty rebuilds a long time ago, but on some jobs where its more quantity over quality, and the product has to be accurate, being able to whip the lens off and darken / brighten the chrome etc is handy. But on those type of jobs reflect on backside is probably not required anyway.