Shadowcatcher alpha issue

[hsa013]

Hi,

Just a quick question regarding the shadowcatcher material. 
Attached is an image with identical scenes (geometry) comparing Vray and Corona.  Corona is fantastic in every way in terms of quality of the beauty pass.

However I could not get Corona to render the alpha correctly with the shadowcatcher material behind glass.  In theory it should be like the Vray version.

1. How can I get it to render the alpha correctly behind glass (basically same invisible / grayish)?
2. How can I get it to render the alpha completely black except for the areas where there is cast shadow?

Many thanks

[juang3d]

I agree with the alpha behind the glass, but I don't agree with the surrounding alpha, it is fine as it is because what you see there is the irradiance generated by the object, of course an option to avoid this could be great, but having it is good too, specially if you have the backplate also.

Cheers.

[hsa013]

You are right, sometimes it can be useful but for me mostly it is not necessary and at the moment I would have to roto it out manually.
An option to select either would be the best as you say.

So, is there no remedy to this problem?

[juang3d]

Have you tried to extract a matte for the object and a shadow pass? I have not played yet with too much elements in corona, but this could work if there is no indirect effect in the shadow pass.

Cheers.

[hsa013]

thanks, yes that would be a solution - I haven't tried it yet.  Was kinda hoping that it was me using the wrong settings, being a newbie to Corona.

extracting and merging mattes and passes usually works but it's time consuming compared to the insant solution of just merging the rgb pass onto the backplate using the alpha.
(Plus, sometimes the shadow pass doesn't completely work in itself)

[juang3d]

I know, I have experience with this since I suffered the same problem before with iRay, I think it's a side effect of unbiased render engines, they try to be as realistic as possible, and that irradiance is great for scene integration in post production, you can feel how the object affects it's environment :)

Cheers.