How To Render Ground Shadow Only

[Joelaff]

Hi. Yes, familiar with track mattes and think I have tried what you are explaining, the difference being in that my background image has no shadows in it. I am wanting to composite the background image sequence + vehicle shadow sequence + vehicle. When I try and adjust the shadow darkness either by the method you suggest or simply adjusting the shadow layer itself, it doesn't look good when compared to the beauty pass I posted earlier in this thread. That shadow layer has the shadows in the RGB layer (although the background colors is also black, so they're not visible in the RGB layer), it has them in a VRayShadows channel, and in the alpha channel. The shadow pass channels VRay creates are inverted, so I am wondering if that is the reason for this, even though I have tried inverting them?

Also, I am rendering my shadows the way you suggest, but is your env background also black? Mine is, and there is no shadow in the RGB channel but only the alpha.

I think that method I just posted will help with your first issue. Or simply turn off the FP and render the shadow on the ground plane, then displace the shadow matte with the live action luminance (or green channel, etc.)

Yes, black environment BG. Always render with black BG. The BG should always be a separate layer in the comp.

[Joelaff]

Yet another option is to render a second shadow pass that is just the shadow on the ground plane (no FP) and then combine both of those in the comp to your taste (like MAX them together etc before using as a track matte).

It seems like the biggest issue is that the shadow is not fully dark in the middle. That method I just posted above with the images of the teapots should fix that.

[leejk]

Hi,

Just wanted to update this. I reached out to support and they confirmed that the shadow catcher methods in VRay are really meant for simple surfaces. The grass acting as a shadow catcher casts shadows on other blades which creates numerous transparent spots instead of self-shadowing and creating a darker spot, making the shadow appear lighter than expected. Since the shadows would be close to the camera I didn't go with a normal shadow pass on a simple surface since it produces too hard of an edge. Maybe this could be solved in AE, don't know, but decided to just render everything together and be done with it.

[Joelaff]

Did you notice the method in post 15 did solve this issue and give you the shadows from the grass on the ground without the self shadow of the grass on the grass itself?

[leejk]

Did you notice the method in post 15 did solve this issue and give you the shadows from the grass on the ground without the self shadow of the grass on the grass itself?

I did but could not get it to work. See the attached sample project. Note the problem when you toggle off the beauty pass in AE. The taller grass gets lost in the shadow and the shadow also messes up the grass compositing thru at the bottom of the teapot. The 3ds Max file has scene states for each pass setup if you wanna try it... was hard to whittle down to make it uploadable here.

[Joelaff]

Definitely going to be a comp challenge that way because the CG grass will never match the live action grass. That's why most people displace a flat shadow in post by using the live action plate as the displacement map.

[leejk]

I went back to post #15 and re-read that and missed something in my test scene... I had a dome light on which was messing things up. Doh!

So now I have a black and white image in the VFB VRayMatteShadow channel that matches your result shown and seems to work now. Thank you!

When the grass pass, shadow pass, and teapot pass are now comped in AE, it mostly does what I wanted. The shadows are a bit off from the beauty pass for some reason and the teapot lacks lighting on the back side. However I did make the FP object a matte object in the shadow pass because I could see how to use the VRayMatteShadow and being that the grass was visible in the RGB channel of the shadow pass. How was the VRayMatteShadow supposed to factor in with this in AE?