The documentation about the shadow_matte AOV's is somewhat limited, in my opinion. (https://support.solidangle.com/display/A5NodeRef/shadow_matte)
"
aov_shadowDirect light shadow AOV.
aov_shadow_diffA difference AOV which can be used to eliminate the shadow from the direct component.
aov_shadow_maskThis AOV can be used in comp to localize and tweak the shadow.
"
Could you elaborate more on those and explain how those three AOV's should be used in a compositing program ?
Thanks in advance
I know it's a very late answer .. but in order to separate the shadows from objects, I do this (using Photoshop):
layer with beauty:
RGBA ..and as mask >>> shadow.mask (inverted)
layer with shadows:
SHADOW.RGB ..and as mask >>> shadow.diff (set transparency on "multiply")
It works very well, also with foreground transparent object (like glass) covering the shadows on the ground 🙂 I use it for many renders showing metal machinery on white infinite backgrounds.