Render object only through stain | Official Unity Discord | Page 1

zenith needle Jul 28, 2025, 2:07 PM

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I have made an interactive Unlit shader with shader graph where you can drag stains of liquid to the center of the screen (att.1). I apply its material to 3D planes that I position in front of the camera.

I want to be able to render a specific object in the scene only when viewing it through one of the liquids. I have seen tutorials on clipping masks and stencil buffers but I'm unsure how can I output the mask for the liquid from inside the shader graph (att.2) and use this information for the stencil buffer / clipping mask.

If you know how I could achieve this effect or have another suggestion, please shoot me a message or respond to the thread. I feel pretty lost on how to proceed and it would help a ton. Thank you!

zenith needle Jul 28, 2025, 3:19 PM

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Render object only through stain

spice kettle Jul 28, 2025, 4:30 PM

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zenith needle I have made an interactive Unlit shader with shader graph where you can drag sta...

If you use/enable alpha clipping on the shader, stencil operations would mask based on that shape.
Since it's a graph you'd likely need to use RenderObjects features with stencil overrides to handle the stencil, since shadergraph doesn't support it directly

zenith needle Jul 28, 2025, 4:31 PM

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many thanks for the response!! How could I research more on the matter? I'm very new to this

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where would be a good place to start?

spice kettle Jul 28, 2025, 4:36 PM

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This page shows an example using the renderer feature, though with Depth overrides rather than Stencil ones
https://docs.unity3d.com/6000.3/Documentation/Manual/urp/renderer-features/how-to-custom-effect-render-objects.html

zenith needle Jul 28, 2025, 4:37 PM

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oooo thank you!!! :D

spice kettle Jul 28, 2025, 4:39 PM

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Might need to google around for stencil tutorials. I mostly only know older tutorials for shader code like https://www.ronja-tutorials.com/post/022-stencil-buffers/
Same idea, just implemented in code instead. The stencil params are the same

Stencil Buffers

Summary The depth buffer helps us compare depths of objects to ensure they occlude each other properly. But theres also a part of the stencil buffer reserved for “stencil operations”. This part of the depth buffer is commonly referred to as stencil buffer. Stencil buffers are mostly used to only render parts of objects while discarding other...

zenith needle Jul 28, 2025, 4:42 PM

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I have checked out some stencil tutorials but I was unsure if this was the solution to my problem since the edges of my liquid stains are a smooth transition to transparent and I've only seen stencils either render what's behind them or not. So you're saying there's a way to alpha mask the stencil?

spice kettle Jul 28, 2025, 4:43 PM

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Stencils can't do a smooth transition / partial transparency, but the fully transparent areas can be alpha clipped which means those pixels don't render to the stencil buffer

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Another option could be to render your objects with a custom renderer feature to an offscreen texture, then sample that texture (using screen position as UV input) in the liquid shader. That would allow for the partial transparency / fade out at edges

zenith needle Jul 28, 2025, 4:52 PM

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I'll have to reread this thread several time over the course of today to fully understand what you mean but I think I know what you're suggesting and it just might work. I was blindsided trying to use the node from the shader and get it into some other shader/procedure to do the clipping when I can just sample the clipping into this shader where I have the mask and apply it right there. I've never considered this approach before and I'm starting to feel it's the best suited

#Render object only through stain