What is this shadow? (Particle Simulator)

NormanPCN Posts: 4,088 Enthusiast

I noticed something I don't understand and I stripped the relevant comp into a separate project to demo here.

I am seeing a shadow(?) of some kind in the camera looking at a particle sim render. The emitter is a layer emitter (3D) using Alpha. The particle sim layer is 2D. The layer emitter is an atomic particle layer using a fractal setting to create a wiggly line to use for the layer emitter.

In this following video (1:30 long) I show the shadow. I looks like a flying saucer. Strange description but what the heck. It is always in the same position of the camera view. I rotate the camera around to demo. I show active camera and perspective view. I show a little bit of the particle sim settings used. 


Here is a ZIP of the project for any interested in investigating. The project was created with Hitfilm Pro 8.1.


Windows 10 (1803), 16GB ram, GTX 1080 (driver 397.64)


  • Triem23
    Triem23 Posts: 20,238 Ambassador

    Huh. That is odd. I'll download the project and take a look at it myself as soon as I'm done typing up Cook-Torrance parameters for Zach in his thread.

  • Triem23
    Triem23 Posts: 20,238 Ambassador
    edited July 2018

    Ok, this is odd, and I haven't figured it out yet, but let me tell you've what I've done.

    First, drop in  background plane with a color. I used a pink.

    This shows that you have a lot of dark particles. "Aha!" I think, "Norman's spawning black particles that are occluding the green! I'll turn the Emitter Alpha Threshold from 0.1 to 0.9!" Nope, that doesn't fix it.

    "Hmmmmm," I ponder, "Let's look at Norman's emitter comp!" Then I changed the comp dimensions from 1080x1080 to 250x1000. It cuts off the very top and bottom of your wiggly line, but it ALSO eliminated all those wasted parts of the comp that don't generate anything green, which frees up resources. Well, that made the particle sim faster to render on my machine, but didn't fix the issue.

    After adjusting the emitter comp and adding the background plane it looks less like a "shadow" and more of a "blind spot" where particles aren't being rendered...

    Edit: 30 minutes later I found it.... You have a mask on your particle sim layer. Delete the mask and you're good to go.

    Also, I recommend you change your AP line comp to 250x100 like I did. I can't confirm, but I'm 95% certain a "Layer" emitter is really a Quad emitter pulling alpha and color information from the source layer's "texture" (I'm also 95% certain that all video and photo layers in Hitfilm are actually planar polygons being textured. This would explain much about Hitfilm, including global anti-aliasing). So having a 1080x1080 source layer with the wiggly line is actually generating a lot of invisible particles, but from the full 1080x1080 position array defined by the layer. 250x1000 doesn't cut off the line (ok, the tips), but reduces the number of wasted particles.

    I like the idea you have here of using a wiggly line, tilted on it's side, to spread out the aurora particles. Final version will look sweet!

  • tddavis
    tddavis Posts: 5,154 Expert

    @Triem23 @NormanPCN I've said it before, but you guys (and others) blow me away with the depth of technical expertise.  I think I grasped only a mere fraction of the overall concept here, but read it with great interest.

  • NormanPCN
    NormanPCN Posts: 4,088 Enthusiast
    edited July 2018

    @Triem23 "Edit: 30 minutes later I found it.... You have a mask on your particle sim layer."

    Oh man do I feel dumb. Or just a good case of CRS. Thanks a bunch for the slap up side the head.

    I stripped this out of another comp and I now remember why that mask was there (for that other comp). The mask was positioned to put a curved shape across the bottom of a "ribbon" of particles. I completely forgot about that mask when I copied the comp to something new. I my mucking around the mask ended up where it is for this example. The mask has no place in this new comp.

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