How can I render a ground shadow as semi-transparent in the final output PNG?

Question

I am rendering a 3D model into a transparent PNG so that there is no background in the final render. All it contains is the fairly well detailed 3D model.

Now I want to include a shadow below the object as though it were sitting on a plane. I want the PNG to contain the model and a transparent shadow below it so that when I compose it over a variety of background images (which will be done on a website in markup) the shadow will appear to be cast on whatever I choose for the background image.

I've tried modeling a plane below with transparency, but then it doesn't seem to receive a shadow at all. What else could I try?

I am using 3DS Max, but I imagine that solutions exist that could be executed in various 3D software tools, though the details of implementation may be different.

So basically, what I am asking for help with is: how can I render a shadow below a 3D model such that it will be rendered as semi-transparent and without the ground plane in my final output?

Answer 1

1. Create a plane where you want the shadow to be cast, like a floor.

2. Right click, vray properties. In -matte properties, tick the “matte object” option, and set “alpha contribution” to -1.

In -direct light, tick the “shadows” and “Affect Alpha” options. When you render the image, save in PNG.