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Footage In 3D Space In Front / Behind Particles

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Glenn Dawick
Footage In 3D Space In Front / Behind Particles
on May 9, 2020 at 8:11:10 am

Hi everyone, this has been doing my head in for quite some time..

I have some rendered footage from Houdini which I've bought into After Effects, along with the exported 3D camera.
I'm using Particular to build some atmospheric dust which is looking nice, but it is only showing in front the footage (obviously because that layer is sitting on top of the footage).
My question is, how do I get my footage into 3D space so the particles move around in front and behind my footage?

I have tried turning on the 3D layer option for the footage, but because of my moving camera I need to reposition / parent / scale the footage to sit infront of the camera.. Surely there is a much easier way to get the footage into 3D space where it is locked to the camera movement and is automatically set to the screenspace size of the comp?

Am I setting this up wrong, or is this just the difficult problem of working with footage in 3D space?


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Richard Garabedain
Re: Footage In 3D Space In Front / Behind Particles
on May 10, 2020 at 3:13:39 pm

what is houdini...I know i've heard of it but a quick google search comes up with nothing relevant.

But exported 3d cameras i have had some experiences with...

Sometimes the position gets wonky when importing in a camera. the starting point might not be 0,0.
Also sometimes the scale is wrong, maybe try adding a 3d null at 0,0 and attach the camera and everying else to it. sometimes scaling that null can fix problems because the import path is too small for ae to recognize..and then insert in your footage

Also i cant quite picture your setup but can you parent the footage to the camera.

Im not sure if this will help but I do have an expression that makes your footage always face your camera's poi. it goes into the y rotation

delta = toWorld(anchorPoint) - thisComp.activeCamera.toWorld([0,0,0]);
radiansToDegrees(Math.atan2(delta[0],delta[2]))


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Glenn Dawick
Re: Footage In 3D Space In Front / Behind Particles
on May 10, 2020 at 8:47:25 pm

Hi Richard, thanks for your reply

Houdini is a 3D VFX application: https://www.sidefx.com/

I understand what your saying about imported cameras coming in at strange positions. I've done a few different exports from Houdini with different settings, and as you suggested with a null at 0,0,0 and all seems to be working well there.

I have parented my footage to the 3D camera as per your suggestion, but was hoping for an easier way to move the footage back and forth in 3D space without the footage resizing in screen space. For now I have just zoomed in really far and manually scaled the footage until it just touches the very edge of my composition. This method doesn't seem very accurate.

The biggest problem when parenting the footage to the camera is because it's parented to the camera, the footage moves around with the camera. So now the footage is not sitting at the same depth in the scene as my original 3D object was (static object with moving camera) - Is this something that I just need to manually push back and forth in depth at the same time trying to scale the footage to stay the correct size in my composition?

I've built a very simple example of my scene to help show what is going on. This is inside Houdini:
https://gfycat.com/dishonestinfamousamericanwigeon

Another showing inside After Effects:
https://gfycat.com/indolentreflectingamericangoldfinch

So the issue here is that the footage doesn't stay in place, where the original 3D objects were. If I had a null sitting at the center point of the 3D objects, is there a way to reference that depth in scene from my 3D camera and tell the footage to aim toward my camera (like the script you mentioned), but also scale up / down to stay matched to the edges of my composition? This all feels very confusing!


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Graham Quince
Re: Footage In 3D Space In Front / Behind Particles
on May 10, 2020 at 5:00:06 pm

Have you tried an obscurantism layer in Particular? That’s designed to limpet you put footage in the middle of your particles.

http://www.YouTube.com/ShiveringCactus - Free FX for amateur films
http://shiveringcactus.wordpress.com/ - FX blog


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Graham Quince
Re: Footage In 3D Space In Front / Behind Particles
on May 11, 2020 at 12:19:11 pm

Yikes, ignore my autocorrects. I was talking about Obscuration layers in Particular. Which now that I’ve seen your Gifs was the wrong thing to suggest.

You’ve rendered out a 3D animation from Houdini. You should now treat that as a 2D video that happens to have camera tracking. So, just like with a live action video, you don’t need to place this in 3D space, instead you need to have 3D effects interact with it. You have a 3D camera and you have Particular which responds to the 3D camera, but now you need to tell Particular that some particles need to be obscured depending on proximity to the camera.

Easiest way to do that is to have Houdini render out a Z-Depth pass. This tutorial should cover it: https://www.sidefx.com/tutorials/learn-how-to-render-and-composite-z-depth-...

http://www.YouTube.com/ShiveringCactus - Free FX for amateur films
http://shiveringcactus.wordpress.com/ - FX blog


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Glenn Dawick
Re: Footage In 3D Space In Front / Behind Particles
on May 12, 2020 at 3:06:22 am

Ahhh ok, so even though the footage is bouncing around in 3D space, the Z depth pass helps place the particles in front and behind? I shall give this a try. Thank you!


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Darby Edelen
Re: Footage In 3D Space In Front / Behind Particles
on May 13, 2020 at 6:03:06 pm

What do you mean by bouncing around in space? The footage layer should not be 3D. The camera is used to create the correct perspective motion in the particle rendering. The depth pass allows Particular to know whether the particle currently being rendered is nearer (visible) to the camera than the rendered surface or farther (hidden) from the camera.

Darby Edelen


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Glenn Dawick
Re: Footage In 3D Space In Front / Behind Particles
on May 13, 2020 at 11:58:59 pm

OK right, I'm understanding this now, and feeling slightly dumb for not being able to work this out earlier.


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