few questions from a returning user with a bad memory

[3di]

Hi, I've recently returned as a corona user but am having a few issues.  Can anyone help?

1.  When using the shadow catcher material with alpha mode set to compositing, i'm getting very stepped gradients in the alpha when saving as png (rgb 48 bit) exr 16 bit half float seems ok ...think windows 256 colours.  Direct visibility override is set to black. 

It's also incredibly slow to render, i'm talking at least 50 times slower to reach a similar noise level than if I render the floor with a standard corona material instead of the shadow catcher.  I've spoken with Marcin today who recommended increasing gi vs aa and light samples, but that didn't really make much difference apart from better balancing the direct/indirect noise levels.  The scene is lit by a single hdri which i've tried clamping in the output but that made no difference.  All of the albedo's are fine too.  Most of the noise seems to be in the shadows (judging by the cs shadows pass).  The scene is very simple, consisting of 5 materials:

shiny silicone with, diffuse set to 0, reflectivity set to 1, ior set to 1.6, glossiness set to 1, translucensy set to 1 transluceny colour white (value 188), absorption white (value 188) distance 1mm.

aluminium, diffuse 0, reflectivity 1, ior 999 (as per new pbr instructions), glossiness 0.4

plastic reflectivity 1,ior 1.6, glosiness .8.

translucent plastic, virtually the same as the silicone apart from absorption colour/distance.

shadow catcher, alpha for compositing, enviro backplate set to black with no map.

hdri image used as an envornment map for lighting/reflections.

I'd share the scene, but i'm under NDA.


Question 2:  Is there an exhaustive help file?  I can find tutorials, but I could do with a proper reference of all features/dialogs/ render options/materials/lights etc so I can get back up to speed more quickly when the tooltips are insufficient.

Final question (unless question 2 is a yes):  in the stats tab of the vfb at the bottom there are sections for GI caches and performance.  What does this data actually mean, and how can I use it to analyse and correctly configure the renderer on a per image basis?

Cheers

Michael

[3di]

Interestingly I've noticed that the backplate material is only slow if alpha mode is set to "for compositing."  I'd be interested to know why that is.

[Christa Noel]

shortly, that is a known bug.
afaik that fix is already scheduled by team but there is still no any information about fixing progress.
the last related information: https://forum.corona-renderer.com/index.php/topic,14939.msg95040.html#msg95040

[3di]

Oh crap, that's not what you want to hear with a tight deadline pulverising you into 18 hour shifts already!  Thanks for letting me know.  I've got a possible messy workaround:

Instead of using shadow catcher render out 3 versions using two different materials in replacement of the shadow catcher.

1st render out the objects without the ground plane to get the alpha of the objects

2nd pass select the floor and then render selected in viewport, with a pure white non reflective material applied to it.  this is to capture the shadows

3rd pass select the floor and render  seleted in viewport  with a pure black diffuse material and reflectivity set to 999 and the reflectivity overide set to a black map (so it only reflects the objects and not the environment map).

comp the images together with the top layer having the beauty pass, the second layer using the shadow version (inverted) as an alpha mask over a pure black to recreate the shadows, and then the bottom layer the reflections version ,controlling its contribution with the layer opaciity..........will need to mask out the non reflective parts so they dont affect the background layer.

It's messy and requires a lot of work, but the render times will drop by about 16 hours in my case.

[3di]

Update, rendering out the reflection only is also incredibly slow if you change the floor materials reflection overide to a black colour in order to only reflect the scene objects.   Noise at 10 passes is 23% whereas without the environment overide set to a black colour in the floor materials slot the noise at 10 passes is just over 2%........will try adding a black dome instead.

edit:  Using a dome which doesnt cast shadows didnt work either as you cant allow light through an object without using the rayswitcher/object properties which results in the same slow render time...hmm, looks like i'm stumped.