Hi, this is a sample with shadow catcher setup:
https://drive.google.com/file/d/188S8wvb097fdrAGZp3cqNorji6loOMm2/view?usp=sharing

1. Image 001.png. Make a Ground Plane object (higlighted), assign CentiLeo material to it, in material Diffuse settigns check Shadow Catcher flag as highlighted on the image.
2. Image 002.png. Setup your HDR lighting. Select the file of hdr as shown here. You may optionally override the HDR for couple of contributions (coming from Reflection, Camera or Diffuse, etc). In this example we override the texture for Reflections. They will be drawn from some other Sky Override object. We also override Matte Shadow contribution, it will cast the shadows from "Sky Override" lighting to the Ground Plane object.
Also you can change the strength of shadows that are casted on the Ground Plane. The shadows can be casted from HDR Sky objects as described above and from Sun light source (with CentiLeo light tag) and from Omni light souce (also with CentiLeo light tag). To manage the shadow strength of each light source you may change the "Matte Shadow Weight" as hightlight on 002.png
3. Image 003.png. To get the mask of object and shadows just open the CentiLeo render settings, AOVs/passes Tab, and in that place please enable "Alpha channel" flag as highlighted on the image. This way you be able to select Alpha chanel pass in IPR window as you can see. Also this pass will be provided inside Picture Viewer render together with Beauty image.
4. Image 004.png. To get the image with transparent background and the semi-transparent shadows on top of that just enable "Transparent background" option as highlighted on the image.

The shared sample has this .c4d file with textures.

Kirill

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ssjenforcer wrote: Also, I see that there are two types of centileo materials: centileo material and material [node] What is the difference? One only has material editor while the other has node editor. And the centileo material has 'matte' while the material [node] has shadow catcher. I tried both material types and neither give me shadows in the render.

Just material is the old version of CentiLeo materials that was the only option before Cenima R21. Since then Maxon has added node based materials support for 3rd party renderers and we have made the implementation and exposed it into the main menu instead of just materials. "Matte" and "Shadow Catcher" from both versions mean the same thing, we have just changed name in one place forgetting to change in another one.
You probably don't see shadows if you don't setup HDR lighting. With no any lighting it's black. Just learn from our sample.

The link to sample is https://drive.google.com/file/d/188S8wvb097fdrAGZp3cqNorji6loOMm2/view?usp=sharing
It's in my first reply, read it again.

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ssjenforcer wrote: When I used the shadow catcher for the material node and looked at the node editor, there was nothing there about shadow catcher. Is that normal? Shouldn't there be some node reference to it?

When you click to the node in the node editor you see the settings of this node in attribute manager as in this screen. See highlighted settings.

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ssjenforcer wrote: Will it not work with simple lights at all, only when using an HDRi?

The HDR is needed to make a background for engine. If you don't need a background then additionally use HDR Sky setup + transparent background in render settings.
Everything is exposed in render settings.

Select "Transparent background" option in render settings to make transparent image.
Actually it's a responsibility of Cinema 4D to save the image layers that renderer produces. Maybe PSD will get a separate Alpha pass? Actually PNG when saved has alpha, I think it can be extracted in AE. Currently I can't look into AE.

The composite tag doesn't work yet in CentiLeo. I know it's needed very much, but still busy on other features.

I think we have missed the Multi Pass checkboxes in render settings. They are enabled automatically when user enables the Light passes or Beauty passes, but it was missed to do so for Alpha channel. But anyway I recommend to select what is highlighted on the image below: the Alpha channel, Diffuse, Reflection, and Multi Pass / Post Effects on the left menu as highlighted.
Then after re-render in PV you will see the image and layers in Layer tab. If you select "Multi-Pass" option in this tab the checkbox "Layers" will be activated in the Save menu for this image (see at the left side). This works for manual saving after render is done.


But really the Alpha channel output can be enabled if we make automatic render image save. This can be enabled using Save Settings (keep attention on highlighted settings):
1) To save the regular image (beauty main of centileo) you use the usual Save file of the Regular Image tab.
2) To save the layers you have to setup the Multi-Pass Image tab settings and setup additional Save filename, also use layer name as suffix.


3) If you enable the flag "Separate Alpha" in Regular Image tab the Alpha Channel will be outputed as a single separate file with "A_xxx" prefix.


4) If you disable the flag "Separate Alpha" then Alpha channel will be embeded to regular image (our beauty image) and it will get transparent background and semi-transparent shadows:

Why you background is red? Why you don't do it with ground plane + shadow catcher that can be transparent with semi-transparent shadows?

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ssjenforcer wrote: When I tried multi pass non of the layers other than the Alpha were transparent backgrounds. Is that because of using separate alpha?

Yes, this is because of this. I have tested Cinema 4D exactly for this for several hours. So, it has Alpha channel for rendered image and it's applied on not only for main beauty image. It can have Separate Alpha On/Off:
1) In case it's Off then the Beauty image has transparency, but other passes still don't have (Diffuse, Reflection).
2) But in case it's On we get all images without Alpha channel and Alpha channel itself is stored as a separate image.
I think that case 2 is good, because you can re-apply this Alpha image to any image pass in AE. I don't have AE now, but I think it should be possible, it looks as something basic: the 4-th channel

In the future I will make a functionality to determine the images with object masks where you can determine the group of objects that go to any other output mask image. Also render objects or groups of objects separatelly from the other scene in image but with GI influence.

ssjenforcer, as for default light settings it seems you have some non-default Exposure settings or probably scene scale. When you start with a new reset scene in Cinema then CentiLeo light intensity settings produce good lighting for the objects around them without burning them. Also CentiLeo light intensity multiplier is a multiplier in centileo light tag! It just multiplies the Light intensity and color of native cinema light source object.

I hope to implement a system where shadow casts can be controlled by user per object or in some mask system. For example it's possible to make a system where object doesn't cast any shadows on all other objects. Probably it's possible to make a system where object doesn't cast a shadow on selected objects, maybe using some kind of layer system to determine the groups of objects receving or not receving the shadow casts. Indeed this feature is very much needed.