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Thread: Strange Instancing slow down?

  1. #1
    LW Junkie squeegie's Avatar
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    Aug 2003
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    Central Florida
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    Strange Instancing slow down?

    I'm running into an issue with instancing grass... maybe.

    I have a residential house model I am working on, and the front yard has about 50k instances of grass clumps. I've reduced the grass to around 300 or so polys, with just an image map for the blades.

    So my house scene has tons of trees and plants.. all of which are instanced using points as my placement anchors.

    The grass however is placed on a large poly with a few subdivisions.. nothing crazy, and I am clipping the area with an alpha map ( to keep it from growing through the driveway).

    I have the grass model GI settings around 8 primary/ 4 secondary at 50%

    Here is my issue... when I render out the scene without the grass turned on, I can get the whole thing to render in about 20 minutes ( trees and all)... but when I turn on the grass instances, it shoots up to 8 hours.

    Even when I lower the GI for the grass to 1 primary ray and 0 secondary, it still grinds to a crawl.

    Here is the kicker though, if I pull the base poly for the landscaping, and the grass instances into a new scene with the same exact settings ( light setup and everything), it renders all 50k instances in about 2 minutes. I have even cranked them up to 250k and have had zero issue. Its only when I turn on the grass with everything else that I get this major slow down.

    I'm stumped... Any idea what I am missing? I've checked the polys in the grass and everything is good. I made sure all my geometry is clean. The image map for the grass is very small ( less than 18kb).

    I'm also using a single Environmental light with the new 2019 Sunsky.
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  2. #2
    Registered User
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    Jan 2005
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    Colorado Springs
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    1,768
    Can you COPY the problem scene to a new .LWS file and have the COPY reproduce the problem? Since it probably has to do with the environment / lighting setup, you could then experiment with those settings in the copy without messing up your whole original scene. Also, you could quickly eliminate everything in the scene except the ground plane and instanced grass with the Scene Editor.

    Good luck!
    mTp

  3. #3
    Curmudgeon in Training Ma3rk's Avatar
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    Mar 2003
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    Near Beaverton, OR
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    Do you actually need radiosity for the grass objects? Perhaps having it on for just the ground polys but disable for the blades themselves? Doubt if they're really contributing much anyway.

    OR, they have some sort of reflective param enabled that's probably not needed.
    "Never be a cat in a cartoon. Never." Chief Wiggum

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