Object Matte

[pefon]

Hi,

I've just started using Houdini so apologies if this is a stupid question.

I have some particles flying over a simple background, i'd like to get an alpha of just the particles to help with compositing.

I have solved this by changing the alpha value of the background to black, this works ok but the alpha isn't particularly clean, I do have quite a lot of motion blur on the particles so i'm unsure if this may be causing it?

I've tried upping the pixel samples etc but it won't come into After Effects clean.

I have seen that you can use takes in Houdini and use the object Matte option at the obj level but i'd rather not have to render the scene multiple times.

I'd appreciate any help.

[beeemtee]

Hi,

you can use extra image planes for exporting object mattes. You should export

a parameter from the shader with a value of 1.0 and include it in the mantra

ROP's parameters/output/extra image planes section. This way there is no need

for takes or extra render passes, your image can hold all the mattes you need.

You need to use a image file format and compositor that supports extra image

planes. OpenEXR is a natural choice. I don't know much about AE, but I guess

it supports OpenEXR and extra image planes.

Without seeing the image you rendered it's a bit hard to guess the source of

the noise. Can you show us the rendered picture, or the scene file? Paricle

motion blur can be tricky if the number of paricles changes between frames

or in frames. Check out 'velocity motion blur'.

hope this helps

Mate

[pefon]

Thankyou for the reply, it works great. I've been doing some studies into getting the clean alpha and i think it may be a problem more with After Affects interpreting the matte.

Thanks again.