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Shalinar

Unexpected results: Particles with Geometry Lights

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Hi all,

I'm getting some unexpected behavior when I try to light my scene. I've got a particle sim, and I want those particles to illuminate the scene, so I assigned a Geometry Light to them. However, it's not acting as I expected.

For one, cranking up the Intensity of the light does nothing. In the viewport, I can see the scene getting brighter, but in the render there is zero difference between an Intensity value of 1 vs 1000.

Another odd behavior is that the light in the render seems to be dependent on where my display flag is set in the particle network, rather than the node that I told it to point to. I'll upload my hipfile to illustrate better, but basically I've got two null coming after my particle sim. One has a vop where I'm cranking the Cd way up, and the other is just the vanilla result of the particle sim. Even though I set my Geometry Light to point to the one with the overblown Cd, if I move my display flag to the other null then that is what gets used for the render.

This illustrates the third strange behavior to me, which is that cranking up the Cd values makes my light brighter. Like the Cd is acting as the Intensity parameter, rather than the actual intensity. So in order to make my light actually illuminate objects, I have to completely overblow my Cd and ruin the color of my particle sim.

Are these supposed to work this way? All three seem odd to me. If I've got something set up incorrectly though, please let me know.

Thanks,

-Chris

particle_lights.hipnc

 

 

 

Edited by Shalinar

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Geometry light needs a GEOMETRY (not just points). You could potentially instance (copy sop) spheres on top of your points. Also don't forget to add material to your geo light.

But you do not have to do that in order to have light out of points, you could just crank up Ce values.

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2 hours ago, tmdag said:

Geometry light needs a GEOMETRY (not just points). You could potentially instance (copy sop) spheres on top of your points. Also don't forget to add material to your geo light.

But you do not have to do that in order to have light out of points, you could just crank up Ce values.

Hey Albert, thanks for the helpful response! I'm not sure about the first part. I was following one of Steve Knipping's particle tutorials, at the end he just adds a geo light and points it to his particle sim, and everything worked fine without instancing actual geometry onto the points or changing anything in the geo light. Also I don't really know what kind of material to add to a geo light... I tried creating a principled shader with Ce set to Use Point Color, but it didn't make a difference.

In the end, controlling the Ce value on the particles' shader did the trick, so thank you for that! I would still like to understand why the geo light doesn't behave like I would expect it to, though.

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1 hour ago, Shalinar said:

Hey Albert, thanks for the helpful response! I'm not sure about the first part. I was following one of Steve Knipping's particle tutorials, at the end he just adds a geo light and points it to his particle sim, and everything worked fine without instancing actual geometry onto the points or changing anything in the geo light. Also I don't really know what kind of material to add to a geo light... I tried creating a principled shader with Ce set to Use Point Color, but it didn't make a difference.

In the end, controlling the Ce value on the particles' shader did the trick, so thank you for that! I would still like to understand why the geo light doesn't behave like I would expect it to, though.

If you're not sure about the first part, here is an example (check before and after copy sop). Also take a look what objects i am rendering under mantra rop.

Your geometry will become like small area lights. In case of polygons - each polygon is a small area light. 

As for material - same, Ce is fine (I've created new one). Not sure what tutorial you have seen and what render engine it was (raytracing?micropoly?pbr?) but I'm using PBR in for this

particle_lights_geo.hipnc

Edited by tmdag

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The geometry light doesnt need geometry per se, but it does need a primitive. So instead of copying geometry to the points, you can use Add SOP and turn on "Add Particle System" and geo light should work fine. The size and color can also be controlled per-particle with pscale and Cd which is useful too.

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On 3/13/2019 at 10:03 PM, tmdag said:

If you're not sure about the first part, here is an example (check before and after copy sop). Also take a look what objects i am rendering under mantra rop.

Your geometry will become like small area lights. In case of polygons - each polygon is a small area light. 

As for material - same, Ce is fine (I've created new one). Not sure what tutorial you have seen and what render engine it was (raytracing?micropoly?pbr?) but I'm using PBR in for this

particle_lights_geo.hipnc

Thanks mate,
I should have been clearer: I know that instancing spheres onto particles "works", but I want to avoid that if possible since instancing geo onto tens or hundreds of millions of particles can get quite heavy. Following Steve Knipping's tutorial "Applied Houdini - Particles 1" (https://www.cgcircuit.com/tutorial/applied-houdini---particles-i/video/ah-particles-9b-last-changes-illumination?time=53) around the 55 second mark is when he adds a geo light to his particles. No instancing spheres, it just "worked". I believe he was using PBR in the video, either that or micropolygon PBR.

 

On 3/13/2019 at 11:08 PM, Skybar said:

The geometry light doesnt need geometry per se, but it does need a primitive. So instead of copying geometry to the points, you can use Add SOP and turn on "Add Particle System" and geo light should work fine. The size and color can also be controlled per-particle with pscale and Cd which is useful too.

Interesting. I got it working with the Add SOP like you suggested. It's bizarre to me that "Add Particle System" would produce what I need when an actual particle system from a real POPs simulation doesn't. I'll have to rewatch the tutorial I was following to see if there was a primitive with the particles somewhere that I missed, because he definitely did not use this Add SOP method to get the geo light working.

Thanks for the help guys :)

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7 hours ago, Shalinar said:

Interesting. I got it working with the Add SOP like you suggested. It's bizarre to me that "Add Particle System" would produce what I need when an actual particle system from a real POPs simulation doesn't. I'll have to rewatch the tutorial I was following to see if there was a primitive with the particles somewhere that I missed, because he definitely did not use this Add SOP method to get the geo light working.

Thanks for the help guys :)

Well what you get from a POP simulation is not a particle system. If you middle-click you can see that its just points basically. When you "Add Particle System" on those points you do get a primitive and an actual particle system, so the points will belong to this primitive. I think this particle system thing is a bit legacy from the old POP system before it was moved into DOPs, it's not really used for anything else that I know of.

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