Output render, Only ground shadow, and Only ground reflection at once

I can’t think of a way to do it, but thought I’d ask just in case.

Say have I have a product consisting of lots of parts, and maybe 15 different materials. I’ll need a finished render of that. But we want to output what Just the ground shadow would be, even all the way under the product (to avoid antialiasing issues once composited back together), and the same for a ground reflection. In the end I’ll want 3 images output when hitting render only once. If there’s a format that blender outputs to put those into a single layered file, that’s awesome also. In keyshot, I’ve found that the PSD format is Completely uncompressed, so it’s not worth the loss in storage. I just save out all 3 as png files and put them together afterward.

I couldn’t think of a way to use the compositor to do that since I can’t think of a way to accomplish that in one render. The product needs to cast shadow, but be completely invisible. In the product main render, the shadow needs to be visible to GI and reflection, but not render (that one should be fairly easy), and the reflection is the same. Honestly, I currently cheat that one by just flipping the product upside-down on Z. It’s not perfect, but works Most of the time ok. But doesn’t work for blurred reflections in keyshot at least. I’m trying to think of how to do it in blender. If I can figure out some time saving things in blender, I think we can start switching to blender for all our renders.

I’m predicting some “why” questions on the shadow and reflection. It really doesn’t need to be the entire shadow under the Entire product. But we’ve found that we usually get a slight halo of pixels between the shadow and the product, caused by antialiasing, if you just split them in compositing. The end composite has to end up in photoshop because that’s what I’m delivering. And they will want those separate so they can change the opacity and look of the shadow, and same with the reflection. I render those as 100% reflective, and then it usually ends up at only 2-7%, but occasionally they’ll want that photographed on glass kind of look so it’ll be up at 30-40%.

I just thought I’d ask you folks. So far, there has always been some technique in blender that can be done that I didn’t know about. Like, in this case, maybe it’s setting up those three “scenes” in a way that lets them all 3 render at once when you hit render. I haven’t found a way to do that, but perhaps I’m searching with the wrong terms.