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I have a scene that scatters over a million grass plants on a landscape and also an animated river that runs through the landscape. As the river is the only thing hats animated is there any way I can render the landscape and grass once instead of over and over each frame along with the water?

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You can render the first static frame, save it as image and then plug into Alpha Over node in compositing.

  1. Disable animated river. Uncheck Disable in Renders icon in the Outliner tab: enter image description here
  2. Render only Statics and save on disk: enter image description here
  3. Enable the river in Render and disable all statics (same as 1)
  4. Check Fill>Transparent (available for both Eevee and Cycles):enter image description here
  5. Set up compositing Alpha Over node:enter image description here
  6. Render the river. Compositing will be executed for each frame of the animation: enter image description here
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  • $\begingroup$ thanks but because the river water is inside the the river bed on the landscape the wtare seems to be floating and doesn't look right $\endgroup$
    – user118535
    Mar 9 '21 at 18:45
  • $\begingroup$ You can split what's closer to camera than the river and what's further into two main objects, render them separately and use two Alpha Over nodes. Nodes: i.stack.imgur.com/UgOnZ.png Result: i.stack.imgur.com/8AAEq.gif If I misunderstood you could you please show an example of your floating river? $\endgroup$
    – Zubmariner
    Mar 9 '21 at 19:31
  • $\begingroup$ some part of the water should be hidden behind landscape, how to add image in comment? $\endgroup$
    – user118535
    Mar 9 '21 at 20:57

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