Subtractive compositing workflow using vrayRawPasses

[danwnewlands]

Recently we have worked on a project and we adopted a subtractive workflow to keep compositing clean and quick!

One issue that arose, which I'm now looking into, now the dust has settled, is getting correct control over shadows.

Basically, I'm trying to eradicate any shadows and it works on some parts in shadow but not others as you can see here:



And this is the nuke script:



Any ideas?

Best,

Dan

[danwnewlands]

**The top image shows the rawShadow pass and my beauty pass with the shadow removed. As you can see, its removed the parts that are white in the rawShadow pass, but the underside of the cover is still in shadow. Here is just the beauty for reference:

[Svetlozar Draganov]

I am not very deep in compositing and I don't really know what is subtractive compositing workflow but I assume is that instead of adding render elements from zero you are subtracting them from the Beauty pass, let me know if my assumption is correct or not.

Is there a chance to send us that composition along the elements so we could better understand it's setup and what is the issue with the shadows?

You mentioned that the approach works for some of the shadows but not on all of them, have you noticed on what type of shadows works and respectively not works?
There is a difference between self-shadows and shadows cast from other objects, the outlined area look to me like a self-shadow area and I would like to know if the issue is related only to those type of shadows?
Have you got a non shadow pass from V-Ray so we can compare better what are the differences between the result from the compositing and the alternative one from V-Ray?

[danwnewlands]

Hello,

Thanks for the reply, your correct about the workflow its to keep things less expensive when compositing and only adjusting what's needed out of the beauty render.

I will send the comp script along with the multi channel exr over in about an hour to inspect.

I imagine its something to do with what you mentioned, I kinda expected something like that, as the rawShadow pass is showing shadows cast by objects it seems. Is there another pass to control self-shadows, or both together?

If I can completely remove shadows in comp then we have complete control, which is what we are after, so far everything has worked, just the shadows I can't quite figure out.

If you can suggest another pass to try out I'll take a look once I'm in the office before sending files over.

Thanks for your time

[danwnewlands]

Here is the download link for the nuke scene and render: https://we.tl/eN9qtMS0Na

Thanks mate

[instinct]

First of all, i am actually unsure that "order does not matter" holds true. Especially if you need multiple adjustments. Different discussion though

The areas you are referring too are not shadow but shading. They are in the lighting pass rather than part of the shadow pass. And at this point i wonder why not go additive? The diffuse pass is what you want already ("
If I can completely remove shadows in comp then we have complete control, which is what we are after, so far everything has worked, just the shadows I can't quite figure out.").

Oh, one more thing: Order does not matter in additive either for the parts that are additive. Only parts that need to be multiplied need to be multiplied before adding. But that holds true for both approaches.

Cheers,
Thorsten

[danwnewlands]

So the idea behind working this way is say the beauty render is perfect but just the shadows need to be lifted (or any other tweak) you can make the adjustment with a few nodes, rather than breaking out all passes and stringing them back together again to get the same result making the comp tidier, easier to read and faster. I'm not a compositor but our comp lead said it helps a lot with alpha issues also so I'm trying to get it to work with vray/maya.

Also just testing further, I imagined that if I set all the 'plus' nodes 'mix' down to 0 then I should be left with a black image as all aspects that make up that images are being removed. But it still leaves some edge lighting and what seem to be reflections in the windows?

If this does end up being a pain then we will go with additive, just thought it was worth investigating.

Thanks for your time Thorsten

[joconnell]

Thorsten is quite right in this one - the vray shadows pass isn't really the way to go, you're better off subtracting the lighting pass, dividing it by the diffuse filter to get only the light contribution, grading that as needed, remultiplying it with the diffuse and adding that back over.

Also you're quite right on subtractive for shots where the main render is close, double negative pretty much always do subtractive comping.

[instinct]

Spot on . Forgot to mention that. As we always tweak a lot that hardly happens with the type of work we do.

[danwnewlands]

Thanks joconnell, so I'm using vray's rawPasses which aren't already multiplied by the diffuse so I just need to adjust after my rawLight pass before multiplying which is doing the trick for the self-shadows but not the object cast shadows, am I missing something else? So my lighting nodes now looks like so:

[joconnell]

Yep looks okay. The main reason I do the lighting and divide thing is that the odd time if I'm using sss or a weird blend shader, the rawlighting passes can be incorrect so it's safer to make your own.

[danwnewlands]

Ah okay, so instead of using rawLight use just the lighting pass along with the diffuse to do what you said previously?

[joconnell]

Yep - my normal list of passes to render are:

VrayDiffuse
VrayLighting
VrayGlobalIllum
VrayReflection
VrayReflectionFilter
VrayRefraction
VrayRefractionFilter
VraySpecular
Zdepth
Velocity
Mattes

SSS if needed.

For lighting it's always divide the lighting by the diffuse filter to make your own raw light (this is how the maths of it works anyway - if you turn on the raw light but also have light and diffuse, vray internally divides light by diffuse to make raw anyway, or it's supposed to.). You can grade the Raw lighting to change hue or intensity of your lighting, grade the diffuse filter to change brightness / hue / saturation of the colour of objects. Admittedly doing big changes in either of these should really be a re-render - like if you changed a white object to near black, you'll have too much bounce light coming from it in your GI so it'd be more accurate to re render.

[danwnewlands]

Thanks for all the information mate, I'll try a re-render when I get a chance this week and test with the different passes, I'll report back!

Thanks again to both of you

[danwnewlands]

Hey joconnell,

Just tested and all is working great with the method your using. We now have the control we are after, so thanks again

Dan