Invisible matte object (well sort-of)

[Tim Saunders]

Sorry for the duplicate post, but I thought I would put this in the max section as well, since I might be able to accomplish it based on max settings.

 

I'm wondering if anyone know how I can achieve this.

 

I am hoping to animate a space developing from the ground-up. I thought i would be able to place a large box over the space that was invisible. The box could mask everything it over-laps, but as the box is moved up, you could see everything beneath it. The problem is, I want to see this happen from a view above the space, similar to the second image.

 

The images below are obviously not the actual model, I just wanted to create some test. But I can't seem to make the box mask the objects it covers (physically), but still allow the camera to see the objects it is not physically covering. Obviously, once an objects has a matte/mask property, if it blocks the view of things behind it, they will be masked.

 

Does this make sense at all?

 

I guess another way to describe it would be like animated clipping planes. But only, the clip will be done on the scene's Z-axis, not the camera's.

 

Any ideas?

 

[erickdt]

Right click your masking object> Vray Properties> Check "Matte Object"> Set "Alpha Contribution" to -1.0?

 

Would that work?

[BrianKitts]

I can't think of how you would tell the mask object to only hide the objects in it, and not what it's covering from the camera's point of view, which means you'd also be masking the ground plane.

 

wouldn't it just be easier to put a slice modifier on your model that you want to cut, and animate the slice plane. you can also put a cap holes modifier ontop of the slice to close the objects so they aren't open.

[erickdt]

Another thing you could do is do a separate pass where you apply an animated vray light material to everything that wipes from white to black (from bottom to top) and then use that as a masking channel in AfterFX. You could theoretically apply this to two still images vastly cutting down your render times.

[Nils Norgren]

I think what you are looking for is not possible, a "Matte Object" in this context is camera dependent, you are looking for Geometry clipping, and the best way to do this (to my knowledge) is to use a slice plane, a custom shape via an animated boolean, or carefully animated objects. I have seen a tutorial by Louis Marcoux where he used an animated gradient ramp, and custom mapping to create this effect (with a soft edge) that worked rather well, the main problem is that rendering with this strategy can be slow due to all the transparent objects.

 

Hope this helps,

 

-Nils

[Tim Saunders]

Slice plane, with cap holes is perfect. Thanks everyone for the responses.