Is there a way to render a shadow without the surface the shadow falls onto? The result I want is a transparent PNG of a rendered object with a shadow cast onto an invisible plane so that I can place it, in Photoshop, over any color and it looks great.
If I turn the ground plane off, no shadow gets rendered.
Yes, but do not turn the groundplane off. Just use a color background. Render to PSD. When in PS the groundplane is its own colorfill layer wich you can remove/turn off and there is your render with shadows and/or reflection.
What if it's not the environment / ground plane you wish to remove / render on a separate layer but an object / plane. If you set an object a base color that will be redered in the image. And if you make a fully transparent png image and set that as a base color that will render as white. I wish to use this technique for interior design / photo mapping and make planes to cast a shadow to but still retain the original background image. Could this be done?
The ground plane trick works perfectly - when it's the ground you want to get rid of. But I have the same need, that is placing objects against a wall, instead of sitting on the ground (or a flat surface like a table).
It's so close to providing this feature...something as simple as changing the orientation of the ground plane would go a long way, so that it's a wall instead of the ground.
I suppose you could rotate your image 90 degrees so that the "wall" in the image is the floor in the app...and then just work sideways. But a control to flip the surface in the app would be so much easier and more logical.