occluding object material?

Started by andy.engelkemier, June 17, 2019, 01:34:55 PM

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I'm replacing only a Portion of an image. We're designing something for an existing product. Some of the photo goes straight through the middle of the product.
Well, no problem, I have photoshop.

Aaaaand, then someone said, "Hey, lets animate it!" So I did that. And so I had to then create a frame by frame mask because the animated portion goes right over the portion that would be masked out. So the mask shape changes over 20 frames. I know, 20 frames, big deal. Still, I'd love to know if there's a better way. Because it Could be more than 20 frames down the road.

So normally, i'd create an occluding object. I'd tell the material on that object that it is to override the alpha to zero. So when a camera ray hits that object, it says, "Oh hey, even though there are things behind you, I'm reporting back 100% transparent." This way, you can punch alpha holes in things.
This works great for animating things in front of and behind things that aren't in your scene, like a 2D photo!

The only workaround I could think of, which is OK I guess, is to create my occluding object, and apply flat white to a new model set, then apply a flat black to the occluding objects. Then use that as a layer mask in AE. But on only 20 frames, it's probably a wash as to whether that would save me time from masking it. I guess it would likely produce better quality though. But it may also pose problems when it comes to ground shadows or reflections, if I were using them.

I'm mostly curious, is there a material I'm missing that might have this feature? Or perhaps a utility that goes into opacity or something?  I feel like it's somewhere, but I'm simply overlooking it.

Ryan Day

Not 100% following the exact thing you're trying to achieve, but have you tried playing with the cutaway feature? You can animate that object in the way it sounds like you're intending with an 'occluding object' to dynamically hide/show areas of a model.


Lets say that's your image...because your client hates you. Your team has designed a new deck on that scooter, which for whatever reason swivels back and forth and you are showing that animation. And all you want is the new fancy swivel deck. You've modeled the wheels and hubs for size, but they are already in the image, and the model you have is just for placement. But it's a close enough shape that you can use it for masking, which is often the case for casters.

The problem is, your deck moves, and it's on both sides of the wheel.
Now, in this particular case, render layers would have worked. I didn't think about that. I just never use them, so didn't think of it.

But that's basically what occluding objects do. It's used to mask an object. As soon as a ray hits the object, it says "Hey, this ray is now 100% transparent, regardless of what's behind this object."

It's not for everyone, but they can be handy to save time in compositing.