Object ID & aliasing & usage within comp program

[stevesideas]

I've been using object ID's and mat ID's for a long time but I don't think I've been using them correctly. I'm trying to isolate teh object perfectly so I can carry out colour adjustments for example. I normally render out the id (Mat or Object) and then save as PNG. Within aftereffects I use extract so I can use the layer as an alpha (or inverted alpha). The edges and aliasing are never correct and I have to fudge it. Shoudl I been rendering out alphas instead or turn on filtering for these ID passes ? Should I be using extract or is there another cleaner way to key this layer to use as a perfect alpha for the object ?

Any help appreciated.

[Zdravko.Keremidchiev]

This should work fine, it depends how your scene is set. Generally it will be best if you could render the object on a black background(BG) so the AA doesn't get affected by it.
Also this workflow should work correctly if the background is the same on the image your are trying to comp and on the original one.
Furthermore make sure you render linearly.
Feel free to upload a simple example scene and we will have a look.

[stevesideas]

Thanks. I have had more luck saving as RPF instead of PNG or JPG. The accuracy seems to be better for object ID.

[super gnu]

are you using multimatte or objectselect to produce your masks? only pure greyscale or pure red ,green or blue mattes will give precise edges, particularly (or maybe only) where two coloured mattes meet.

[DPS]

If you really want accurate corrections then technically you need to render objects separately and unpremultiply before you do colour corrections as even with multi matte the edges of overlapping objects will have their colour/value 'contaminated' by the other.

Best to fudge it.

Or EXR2 and vray's deep output is awesome for this type of thing but not supported in AE.