Shadow pass technique explanation

[LanceClayton]

Hi

I'm having a difficult time getting the shadow pass technique worked out within Octane/C4D.

When I render a full image from cinema my shadow is appropriately balanced (darker at back), but when I render the shadow pass and try to composite it appears that the front/back shadows in the shadow pass have almost similar darkness, with the front being perhaps even more dominant.

I used this tutorial as guidance:

https://www.youtube.com/watch?v=mST_-dmZYHc

Could someone more experienced take a look and try to guide me on the process for getting a decent shadow pass that is in line with appears when the whole scene is rendered out in full.

Project file and reference image attached. Thanks

[niestudio]

The shadow you're compositing is rendered from a flat surface, not from the grass.
When you disable the grass and render a shadow on a flat surface, Octane cannot take the grass into account when creating your shadow therefore it looks different that what you see in the live viewer.

Watch the inlifethrill series on vimeo, but specifically this one https://vimeo.com/119848103

[LanceClayton]

Thanks for the quick reply.

The trouble with the scene is that the grass just takes really really really long to render on my machine.

Do you have any thoughts on how to speed up the render of the grass? I've already dropped the count as low as it can viably go and reduced the hair segments too.]]


Alternatively, is there a workflow that you could suggest to achieve breaking the grass and the shadow into separate passes so that the grass is only rendered once then simply used as a pre-rendered backplate as new assets and shadows are composited on top to visually look the same as rendering out in full without passes?

Thanks a tonne.

[LanceClayton]

Maybe I could try rendering out the grass Shadow Pass will a lower hair count so that it's close to what it would have been but takes lets time to render because of lower hair count?...

[niestudio]

could you create a secondary patch of grass that is the full blade count, but is just big enough to catch your shadow?

[TonyBoy]

Also, it appears to have two light sources which accounts for the fact that the shadow next to the object is darker because it's lit from two directions and the overlap shows darker. How much video RAM do you have?