Shadow Catcher material not blocking light - causing double shadows

[jeffjag]

I can't seem to figure out what's wrong with my scene to cause this issue. As you can see, there's a couple things going wonky here.  The concrete platonics in the shade of the building are darker than the ones in the sunlight... but they're also casting shadows onto the ground. Also, the glass tube coming around the corner of the brick wall is casting shadows only onto the shadow catcher material (wall and ground). and it's shadow is cut off by the wall and not casting a double shadow. I know they don't match up quite perfect, working on that too.  The background is a video, so the camera has been motion tracked - which is why I didn't upload the scene file.

The whole thing is set up so that there is geometry for the planes of ground and the building, with an inset for the door - all using the same shadow catcher material.
Shadow catcher is set to environment mode using a material with the MP4 on a diffuse channel.
Everything is a collider object, except the balls and platonics, which are rigid bodies. 
I have lit the scene with both Corona Sun and sky object for off-scene reflections. Sky object with a light grey color on it to tint the shadows to match the ones in my video - but the sky has corona composite tag set to only GI, reflections, refractions.

How can I get rid of those double shadows in the shade of the building?

Thanks,
Jeff

[Frood]

Shadow catcher materials/objects just do not cast shadows on shadow catcher objects. It's a limitation because we currently have no option to control it (here: cast shadows but in exclude list: ground object). You would have to mimic the building shadow with another object (not visible to camera - max user here dunno how to control this in c4d) but you would get double shadows where there are already shadows in the footage, additionally very difficult to match.

I always struggle with this when doing more complex comps to integrate stuff into existing images. Wish we would get some advanced matte material or at least some option to block light as in your title to solve this. I remember a few posts with exactly the same problem and the issue is high on my Corona wish list. Curious if anyone else has a solution for this with the current implementation.


Good Luck

[jeffjag]

Thanks for the reply.  To clarify, my problem is with a shadow catcher object not occluding the light from casting additional shadows from a normal corona material. Currently the shadow catcher materials seem to be working fine receiving shadows, almost too well actually.  The plantonics(5-sided geometry) in my scene are a corona material with a concrete texture map.   

So, I tried the approach of having another solid object with a black material on it, then added a corona compositing tag and made it only "cast shadows" but not visible to camera, gi, reflections, refractions or mask.  That didn't change the result, unfortunately.

I have the corner of the building set pretty precisely with the angle of the sun so that the objects that are supposed to be in the shade actually are in the shade... but they still cast an additional shadow of their own. its so weird.

Again, thanks for the info.

[Frood]

but not visible to camera, gi

As mentioned: no C4D user here. But I think gi has to be on, otherwise you will get no shadow/blocking at all. Also: It won't solve the problem because of the double shadow you get from the building then. Well, it's still a shadow catcher and no full matte material :-/


Good Luck

[jeffjag]

Yeah, I think this is a difference between how C4D and Max handle render/composite tags?... but I agree it seems like there should be a new kind of material to solve the issue.

SO... I figured out a different way around the problem which will take more work and compositing... but that's kind of what I'm trying to do so I might as well do it the hard way.

I'm going to render a pass with alpha that has just all the 3D elements with the correct lighting and shadows applied to them, and render a pass with the background video and the shadow catcher material without the 3D parts, then layer them in After effects.

From that point, I will put the original footage on the bottom, the shadow catcher render above that, then the 3D elements above that. But I will animate a vector mask of the double shadow footage to remove that part of it using motion tracking on the original footage in AE... so there will be normal shadows on the bottom and right, then the top left will basically be just the original footage showing through the mask. 

I think it'll work.

[Frood]

animate a vector mask of the double shadow footage to remove that part

I was thinking about rendering multiple times but did not want to recommend exactly this :) It's a horrible and ridiculous workflow for what I call a standard situation - really does not suit Corona style :( We need matte material or a tweaked shadow catcher urgently.

Would be nice to see the result of your efforts when it's done and thanks for feedback,


Good Luck