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This topic contains 8 replies, has 2 voices, and was last updated by Pgeorgeadis pgeorgeadis 1 year, 11 months ago.

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  • #2743
    Pgeorgeadis
    pgeorgeadis
    Participant

    I have a scene with a wine glass. When I render the scene with an hdri as the light source, the glass renders as expected with transparent shadows. However, when I render this scene with emitters instead of the hdri, the shadows cast by the glass are much darker, almost as though the shadows arent transparent as if the glass were made of an opaque material. Am I missing some setting? I am using the maxwell for rhino plugin if that makes any difference.

    #2744
    Mihai
    Mihai
    Keymaster

    Examples please 🙂

    #2747
    Pgeorgeadis
    pgeorgeadis
    Participant

    here is an example of the scene lit with an HDRI hdri
    here is the same scene with the same settings but lit with an emitter instead of an hdri emitter

    I let each one of these go to around SL14. The one lit by the HDRI got there significantly faster. Look at the base of the glass. The one lit with the emitter is a lot darker. If I let this scene continue rendering, the area under the base of the glass will accumulate a bunch of firefly artifacts. Here is an example of a scene lit with the emitter that I let go to SL24. (I was using a texture for a base in this scene) sl24
    As you can see, it looks like Maxwell is trying to make the area under the base of the glass lighter, but its not doing it smoothly and its taking a ridiculous amount of time.
    What am I doing wrong here?

    #2748
    Mihai
    Mihai
    Keymaster

    This has to do with the old problem of unbiased render engines which is seeing caustics *through* a specular surface. The reason it is so pronounced in your case is because you are using a very small emitter, which will create a concentrated beam of light which in turn will create more caustic light. Seeing that caustic light that falls on the floor, through the glass has always been a problem in Maxwell. What you can do in this case is simply make your emitter larger. At least 3x larger and you will see it clears much faster. This will “fade out” the caustic light and illuminate the area seen through the glass properly. This is the same reason why the HDR lighting gave you a clean result so fast – it is a very large light source. If however it was an HDR of a bright sunny day with the sundisk visible, that would produce sharp caustics as well and it would render slow also.

    Of course you will get softer shadows with a bigger emitter and if you really need sharp looking shadows in this case, what you can do is make two renders – one with the glass object hidden to camera. This will give you fast caustics and sharp shadows. A second render with a bigger size emitter and a custom alpha created for the glass object. Then composite the glass object render with the one that just has the caustics and shadows of it.

    #2754
    Pgeorgeadis
    pgeorgeadis
    Participant

    Thanks! This was very helpful.
    What if the surface that’s having the shadows cast onto it is an SSS material? Like shadows from a glass cast onto an SSS surface for with large vs small lights for example. Would this behave the same as an opaque surface or is there anything else I would need to account for?

    #2755
    Mihai
    Mihai
    Keymaster

    Yes it would behave in the same way regarding the visible caustics and emitter size, just it would probably be slower to render since the caustics falling on the SSS will in turn illuminate the SSS.

    #2781
    Pgeorgeadis
    pgeorgeadis
    Participant

    Even after hiding the glass from the camera, the shadows cast on an sss surface seem unreasonably dark compared to a diffuse surface. For example, the following 2 images have the exact lighting and camera settings and show the difference between the shadows cast by the base of the glass on diffuse vs. sss:
    Diffuse:
    diffuse

    SSS:
    sss

    Both of these renderings went around SL24. You can see there are some light artifacts in the sss render almost like it is trying to lighten up. What am I doing wrong here and what can I do to fix it?

    Thanks!

    #2782
    Mihai
    Mihai
    Keymaster

    I guess caustics illuminating SSS isn’t working very well….I don’t recall it being this bad though. I don’t think there is a workaround, maybe just mix the SSS with a regular material on top set to about 60% opacity so at least you get some caustics. But why do you need translucent tiles in this case?

    #2783
    Pgeorgeadis
    pgeorgeadis
    Participant

    I don’t exactly need the tiles to be translucent, they just look a lot better with the sss material than anything I was able to achieve without sss. Basically everything except the caustic shadows looks great. I am open to suggestions.
    Thanks!

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