Front Projection & Alpha channels

[mschoenhals]

My students and I are doing an alien invasion of our school. Question: we want to compose things in After Effects so what we want to do is render out only the aliens and ships in Animation Master. How do we get the front projection maps not to render out but keep the models that pass behind them blocked?

 

For instance, when a ship passes behind our school building, how do we get the render to not show the portion of the ship passing behind the school (our front projection map) without rendering the projection map or it's objects?

 

I've been through these tutorials, Ed explains multiplane rendering but not this particular issue.

 

http://www.hash.com/users/ed/tutorials/fpm/fpm.htm

 

http://www.hash.com/Camera_Guy/lighting.html

 

 

Any help would be appreciated!

[robcat2075]

I think this requires a two stage solution.

 

render one frame of just the dummy geometry that matches your front projection material. Use the alpha channel from that as a mask in an AE composition to subtract out the parts of your spaceship render you dont' want to show.

 

Alternatively you could draw a mask in After Effects based on your live footage to clip out parts of your spaceship render. same result.

 

Actually... I'm wondering why you're doing the front projection stuff in A:M at all if your intention is to composite in AE. But that's just me.

[mschoenhals]

Actually, I had that thought while driving home (on the sidewalk, I should pay more attention).

 

I guess we won't be doing any front projection stuff really, it'll all be done in AE. I'm a little new to AE and this project is a bit of a challenge for me and my students but we're up to it.

 

Thanks for the help!

 

Mike

 

I think this requires a two stage solution.

 

render one frame of just the dummy geometry that matches your front projection material. Use the alpha channel from that as a mask in an AE composition to subtract out the parts of your spaceship render you dont' want to show.

 

Alternatively you could draw a mask in After Effects based on your live footage to clip out parts of your spaceship render. same result.

 

Actually... I'm wondering why you're doing the front projection stuff in A:M at all if your intention is to composite in AE. But that's just me.

[mschoenhals]

Hey Robcat,

 

Any quick tips on how to use the alpha channel created in AM as the mask in AE? All the tutorials I've looked at so far show creating custom masks using the pen tool - I'd like to use the image.

 

Thanks,

Mike

 

I think this requires a two stage solution.

 

render one frame of just the dummy geometry that matches your front projection material. Use the alpha channel from that as a mask in an AE composition to subtract out the parts of your spaceship render you dont' want to show.

 

Alternatively you could draw a mask in After Effects based on your live footage to clip out parts of your spaceship render. same result.

 

Actually... I'm wondering why you're doing the front projection stuff in A:M at all if your intention is to composite in AE. But that's just me.

[robcat2075]

Hey Robcat,

 

Any quick tips on how to use the alpha channel created in AM as the mask in AE? All the tutorials I've looked at so far show creating custom masks using the pen tool - I'd like to use the image.

 

i just have AE 5 so terminology may be different for you

 

- you need to turn on the "modes" panel in your timeline

 

-in that panel is a drop down menu for each layer that selects compositing mode and/or targeting for matte effects

 

- this permits you to make the alpha channel of one layer act upon another layer.

 

- the order in which they appear in the timeline has a lot to do with what can act up on what.

 

- look up track mattes in the AE manual for better info