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Regarding GI, i do not know any exact specifics, as my understanding of internal rendering mechanism is limited, but i believe this is where differential shading comes to play. It should be somehow able to compute the difference occlussion or color bleeding make on the backplate. Ondra has managed to do it somehow in Corona, and judging from the recent picture you have posted, you did too.
Regarding matte separation in reflection. I believe that transparency should be always black, to properly composite it. Refraction on the other side, should be maybe toggle-able. With high IOR materials with curved surfaces, it would be probably better to keep original refraction from render, as there's not many post processing workflows which result to matching refraction, on the other side, if IOR was something really low, or surfaces would be straight and coplanar, like car windows, then it would probably be more appropriate to propagate it in alpha and replace it's content in postprocessing too.
So first case would be something like glass sphere, second case would be something like window, each of them requiring different treatment.
But this whole complex thing i wrote above could be easily avoided, if there was an actual matte/shadow material, which user could set up manually, including the mapping projection method, than doing it automatically with enabling matte object property.
It's very confusing in current state. For example even now, i am still not sure what enabling matte object does.
Does it camera-map whatever i see through it automatically?
Why doesn't it work in reflections by default?
If i want to composite it in post, how do i proceed?
How does the surface react with GI or lights?
If i have just gray diffuse material on the matte object that has matte for refl/refr enabled, then is original gray material completely ignored, or does it still impact the result in some way?
IMHO current solution just creates a huge space for confusion and uncertainty, like many other solutions in Vray
Regarding matte separation in reflection. I believe that transparency should be always black, to properly composite it. Refraction on the other side, should be maybe toggle-able. With high IOR materials with curved surfaces, it would be probably better to keep original refraction from render, as there's not many post processing workflows which result to matching refraction, on the other side, if IOR was something really low, or surfaces would be straight and coplanar, like car windows, then it would probably be more appropriate to propagate it in alpha and replace it's content in postprocessing too.
So first case would be something like glass sphere, second case would be something like window, each of them requiring different treatment.
But this whole complex thing i wrote above could be easily avoided, if there was an actual matte/shadow material, which user could set up manually, including the mapping projection method, than doing it automatically with enabling matte object property.
It's very confusing in current state. For example even now, i am still not sure what enabling matte object does.
Does it camera-map whatever i see through it automatically?
Why doesn't it work in reflections by default?
If i want to composite it in post, how do i proceed?
How does the surface react with GI or lights?
If i have just gray diffuse material on the matte object that has matte for refl/refr enabled, then is original gray material completely ignored, or does it still impact the result in some way?
IMHO current solution just creates a huge space for confusion and uncertainty, like many other solutions in Vray
On a more serious note; generally a matte shadow is defined as the ratio of the total scene lighting at a given point and the total scene lighting without shadows (i.e. shadowedLight/unshadowedLight). This is from my experiments; no-one has actually explained this clearly and I had only the 3ds Max Matte/Shadow material as a guide back in the day. For GI, because the shadowed/unshadowed thing is tricky to compute, V-Ray uses the occlusion from the environment.
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