i've created a little scene here to test and understand matte shaders.

basically, i'd like to render and see an alpha mask created by the torus.

neither the vexmatte shader and the shadowmatte shader seem to create
an “a” channel in mplay.

it is my understanding, that for rotoscoping, any geometry i model to match the live footage needs to be masked out for compositing.

for example: live footage that includes a building. i model a simple building that tracks the livefootage building. i have some particles collide with the model… say for some rising smoke. i only want the rising smoke. BUT part of the smoke dissapears BEHIND THE BUILDING, so the model of the building should now mask those particles.

if i can render the model AS A MATTE with an alpha channel, then when the smoke is composited over the live footage, it will appear as though the smoke disappears behind the LIVE FOOTAGE BUILDING.

From your description, I would imagine that you would want to matte shadow or to phantom the building, not the torus. You could just use a constant shader set to black on the building if those other two methods arent giving you an alpha.

You could also render the building separately, and then use the alpha though that wont mask the particles in render, just in comp.

i did comp in photoshop…

this might clarify… if i do not render the buildings, then i loose mask for the pyro.

if i do render the models i do get mattes, but no alpha. see i want the remove the pyro where it disappears behind the pole, but is a little ALSO infront of the pole.

what i really want is for my cgi elements to
GENERATE ALPHA mask content.

and yes, the pyro is only lowres. i am not looking forward to figuring out
how to generate an uprez pyro without destroying my autodop explicit cache file.

UPDATE
i finally found the very simple solution.
on the vex-matte shader, ‘main’ tab, i had to set ‘alpha’ to zero.

lol.