Separate Shadow Pass/Render Element

[Infrared digital]

Hi

I am trying to render a series of passes for comping in Nuke and have managed to render the CG element on its own but cannot for some reason get a good pass/render element for the shadows.

I have tried VRay Raw Shadow but if you see the attached it comes out very grainy and unusable;



Am I doing this wrong or is there a better way to get shadows on a separate pass ?

thanks

N

[joconnell]

Is your ground element set up as a matte object? What can happen is if an object isn't involved in the rendering of beauty pixels such as the case of a ground plane object that's set to matte, it sees that the object doesn't make much contribution and thus it's early termination kicks in. What's lighting the scene? You can try setting your dmc min to 2 so it raises the base standards of the sampling. Personally I always render shadow passes separately with the main cg object hidden so you get all the pixels behind it too - it means you can blur and treat the shadow more in nuke without having any fringing.

Try the vray matte shadow element too.

[Infrared digital]

The ground element has a matte and shadow material applied and I have the dmc min at 2 and also the vray matte shadow element is almost identical to the vray raw shadow.

So really my best option is to render the shadows separately ?

[joconnell]

Are you trying to get everything all in one render - main element and shadows too?

To be honest I far prefer having the two separate and further than that I like to have my gi shadow and my direction light shadow separate too. For example I did a film recently and the cg was mainly a vehicle in a live action plate in each shot. By and large the environment was desert like so you have a hdri for the sky and then a vray sphere light for the sun. We painted the sun out of the hdri and used our own light as it was far easier to match the spread of the sun shadow then. When I rendered the shadow pass though, I used the vray properties to make the ground into a matte object with shadows turned on and affect alpha turned on so I got a black matte for the sun shadow, I also turned off GI and for the soft sky shadow, I used a vray dirt map in an extratex element with a radius that looked good. Then you can use each shadow as a separate grade in nuke and vary the colour and amount of shadow independently, blur each one if needed, and you had a full shadow that went under the cg instead of just the visible pixels of the shadow seen in the final render - if you blurred or colour corrected that you might end up with an odd fringe where it meets your main CG.

It could also be that your shadow is actually quite weak in the main render, again vray won't do heavy sampling on something that doesn't contribute a lot so the above can work nicely.

[Infrared digital]

I appreciate your time in replying to me John.

I have taken your advise and will render all the main passes in one go with the shadows separately. I think it was possible to do it all in one render but hey ho. This is my first forray into a full VFX project and its mental ! One thing I would like to figure out is a better system for rendering from VRay. Although the Pre-Render and Animation-Render are brilliant methods for scenes with moving objects, it is fraught with potential human error when faced with forgetting to tick a box or whatever. Also the number of shots that I am working on is quite significant - very short clips but the setup process is required for each shot and also the rendering process - it all adds up.

[joconnell]

I getcha. I'd make my decision based on what you're rendering as moving versus non moving and then break it up into exterior and interior. Brute force is definitely viable with light cache and you've the benefit of knowing that your GI won't flicker once you're happy with the grain level you're getting.

On the Pre-animate and render bits, that's really territory for scripting - going through repetitive actions for you automatically so that the same process is carried out every single time. I did the same for the file naming conventions on the last film I did and it keeps everything consistent. If you're working on a lot of things then maxscript can really help you out a lot whether it's just dealing with a huge number of objects or automating repetitive actions.

[Infrared digital]

So I have gone down the route of rendering the CGI element as one render then unticking "Visible to Camera" for it and rendering the shadows BUT, the render times for the shadow render are astronomical in comparison to the render of the CGI element.

To render the CGI element is taking 30 seconds - to render the shadows is taking 10 minutes !!!

[Infrared digital]

I managed to sort the render times, what I did was for the ground plane which has a Matte and Shadow material applied I turned off Generate GI, Receive GI, Generate Caustics, Receive Caustics and Matte for refl/refr.

The render times are down from 10 minutes to 1 minute 30.

[joconnell]

Oh yep - what I do is knock off GI, only leave on the light casting the direct shadow and then use an extratex AO element for the sky shadow. Then you've got two totally separate shadows that you can adjust colour and levels of easily. Also what are your sampling settings for your main render?