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HDRIs and reflections

cajhincajhin Posts: 154
edited April 2021 in Technical Help (nuts n bolts)

I can't figure this out. Basically, I want to render dome and object (car with windshield) seperately, and merge them in post.

With "draw dome = off", the object still gets lighted, and it still reflects the dome - *unless* the object is transparent and there is nothing behind it.

In the attached image, you see:

a) dome on: the hdri sun reflected in a glass panel, and on black/white panels that are behind the glass.

b) dome off: the hdri sun is NOT reflected on the glass area, where nothing is behind it (where the dome would be visible if it was drawn).Can't wrap my head around this.

It looks to me, that the dome must always be on when you have a glass window that shows the dome, otherwise the reflections are off?

Bonus confusion: with a bit of 'glossy roughness', the glass becomes a window into the dome (it shows the dome even though it is off). Bug of feature?

hdr reflection background.jpg
1101 x 678 - 63K
Post edited by cajhin on

Comments

  • SevrinSevrin Posts: 6,306

    You can split out reflections from specular surfaces using the Specular canvas in Advanced Settings.  Using alpha masks will always exclude HDRI backgrounds, but not their reflections.

    This is a HDRI with a glass pane - Daz Iray Solid Glass (not a terribly reflective glass surface) - on the left  and a straight mirror surface on the right.

    Full render

    Specular Layer with Alpha mask not much reflection on the glass, some reflected light on the right arm

    Diffuse canvas with Alpha channel - for completeness

    Glossy layer with Alpha - generally handy for adding more sparkle to renders

    All 3 layers combined with Alpha channel - Same as just a Beauty layer with Alpha mask

    You can also just create an alpha mask for anything, and use that to help you composite.

     

     

     

  • cajhincajhin Posts: 154

    Thanks a lot for the detailed answer. I'll explore this tonight.

    Basically I want to adjust the colors for individual items so they blend better. Looks like "alpha mask for anything" will be very useful (and doesn't require 3 renders per object).

    Until now, I challenged myself to adjust everything in iRay, but I think tweaking the domei/car/figure individually requires post.

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