Alpha compositing related question

[Steve Bentley]

I’m not sure where the statue is – is it part of the C4D scene or is it part of the plate? Is that black silhouette a match moved object in 3D or just the 2d matte from the plate?

But you could render out a multi pass (we use EXR’s) and include a depth pass. I assume you want some of the explosion to be behind the statue and some to be infront/overtop of the statue, yes? The depth matte will allow you to comp the explosion up to a certain depth behind the statue and then overlay the explosion from that depth forward in front.
Does that help?

Here’s a tutorial that not only shows you how to do it but also corrects a long standing problem with the depth pass in C4D.
https://www.youtube.com/watch?v=hej9gSpXKqQ

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[Christian Balistrari]

Hi Steve, thanks for your reply.

Yep the statue is a 3D object in the C4D scene and the explosion collides/interacts with that object. The black silhouette you’re seeing is the actual statue .OBJ in C4D with the rest of the scene muted/deleted.

Yep that’s correct, Ideally I’d love to render the explosion with the statue cut out so it could be dropped into AE ready. Have tried using the compositing tag on the statue so it’s not seen by the camera, but doesn’t seem to work.

Just trying your depth pass method now, would I need to render out the explosion and statue as different passes?

Thanks again for your help Steve,
Cheers
Christian.

arthrmusic.com
Music | Animation | Live

[Steve Bentley]

See also Method 2 below which might be better depending on why you need to render the explosion separately.

So the initial problem is that once you get into 2D and you put that mask of the 3d statue over the explosion (itself now just 2d pixels) there is no way to reveal stuff that is “over top of” the statue in 3D space because that depth info is lost. In 2D, the explosion is behind and below the statue as far as AE (or whatever comper you are using) is concerned.
So the idea with a depth matte is that you can split the explosion element into two parts in the 2D comping stage. The part that is behind the statue and the part in front. But you can’t do that with normal masks because that split point is in depth or Z-space and the comper only knows about x,y space. So instead you can render a depth pass which is in its simplest terms an alpha of the explosion but it has a gradient that travels from the camera to some set distant point as a sort of third axis. There are no object “edges” in this depth axis, just a gradient like a foggy cloud (actually thinking about things emerging from the fog is a great analogy – the fog is a white gradient getting thicker further away from you. When you see something emerging it just has less of that white gradient between the object and you.) By creating a clipping or cut off on that gradient you can cut off all the bits of the object that are either beyond that point or in front of that point. The depth gradient is done at super high bits depths (hence the EXR 32 bit format) otherwise there wouldn’t be enough steps to the gradient to get clean edges all the way along the length of the gradient (or should I say depth of gradient). To be clear, the alpha of the explosion has x,y cooridinates but the depth information goes in and out (sometimes called a Zpass).
So in the end you put one copy of your explosion above the statue in your 2D comp and one copy behind. Then use the 3D depth matte tools in your comper (in AE its in the 3D effects menu) to cut off the Z mask where the statue is in both copies of the explosion, but you just invert the cut off (or the gradient) for one of the copies so that one copy reveals whats behind and the other reveals whats in front with the statue sandwiched between the two.

Method 2 – no depth pass required.
The other way to do this is to render the explosion in 3D with the statue visible, but put a composting tag on the statue. Turn off the receive shadows in the tag and turn on the bottom “matte object” button. Render away. The statue will be self matting in the explosion volume. It will show up as a black silhouette shielding any particles that fall behind it but letting particles in front show up. It will also cast shadows on the explosion (if you want) because its really there in the scene blocking light , but since the statue will have no color information we turn off the receive shadows since there’s no point having those calculated. Make sure “seen by camera” is on in the tag; a lot of people turn this off since they think they don’t want to see the object, but the matte object button looks after this. Oh and you have to render with alpha – since the statue will only really exist there. This can be rendered in any 8bit format that supports alpha – no EXR required.

[Christian Balistrari]

Hi Steve,

Thank you so much for that detailed insight, explanations are great!
I’ve managed to make Method 2 work, the ‘matte object’ selection was the bit I missed. Will definitely implement the depth matte suggestion also.

Thanks again,
Christian.

arthrmusic.com
Music | Animation | Live