Shadow catcher speed

[Juraj]

I am not putting this topic into bug because this probably isn't.

Is anyone using this in production ?

I did about 50 tests today and same result, with simplest Shadowcatcher (everything on default), composite mode takes 10 times longer to clear.
Nothing is wrong in stats, same ray/s, samples per pixel, total rays. Same. Except for noise. The image starts from 20+ perc. instead of 6 with any other mode. I tried adaptivity on/off but that is hard to judge since noise ratio is not being shown in latter.

Is it because of the forced black ? I can't wrap my head around why it would need so much more sampling. The shadow alpha is incredibly useful, and the two other modes are totally useless.

For workaround, I am using just the non-compositing modes, and select the whole area by regular alpha and tweak it with levels. Only works because I need just white sweep..

[romullus]

Juraj, why do you think it's not a bug? IMHO, that's really shouldn't happen. For compositing mode is very needed for me, but i simply can't afford to use it currently, because of amount of noise it produces.

I have reported this as a bug some time ago - still waiting for a resolution: https://corona-renderer.com/bugs/view.php?id=1696

[Juraj]

Because I wasn't sure something like this would be a bug in 1.4, sometimes I just don't use it enough to know :- ) I presumed someone would already have reported it since this is rather important feature ?

Or maybe just nobody use it.

The difference is amazingly (in bad) high.

[Ondra]

the thing about shadow catcher is that it is magic. And not the good kind either. It took a LOT of time to make it work reasonably according to expectations in our shading framework. And the way we made it work also produces sometimes fireflies, because the sampling is not adapted to its specifics. It may be possible to improve in the future, but I dont want to promise miracles...

[romullus]

That's a sad news indeed :[
On a positive side, at least it's here and it works.

[Juraj]

Using non-compositing is a feasible workaround for now, just need to get it right in framebuffer then. Done this way, everything is back to its incredible speed, I was almost getting worried what did I setup wrong :- ).

It's funny because I don't need the advanced features of shadow catcher, which I presume cause the complicated programming behind, just simple GI and Direct shadows that fall on geometry, something I could do with some manual post-work selection if I rendered out such passes. Just wanted a simpler solution that does it directly. Now I see the shadowcatcher is quite something more.

As outsider I can't guess what is hard, this would be the last thing I would have expected :- )

[romullus]

Hey Juraj, care to share your experience with shadow catcher? What i want is to render simple product shots with shadow catcher and then to be able to quickly swap background colours (behind shadow catcher) in photoshop.

[Juraj]

Sure, but hope it's not too clumsy.

Since composite can do this directly but it's so slow (and indeed cause some fireflies too), I use 'alpha mode' always transparent. The way I have it setup in 3dsMax renders out the whole background (including matte) mid-greyish.

I then adjust levels of the whole alpha mask (everything except rendered objects) until it looks like I rendered with composite and put white background behind. The result is identical. Now since the background was mid-greyish, perhaps using PS blending modes to get different colors wouldn't cause halo fringing, but I didn't try this since I needed only white sweep in this project.