Render shadow on transparent background with Blender Render [duplicate]

Question

I need to render a model on a transparent background. The models have a light source so they should be casting a shadow on a surface (the XY plane). The purpose for this is to overlay the rendered images (out of blender) on an image of the actual surface, so I want to generate the render including a semi-transparent shadow.

I don´t have any problem rendering the models on transparent background, but I can not make a shadow appear. I tried adding a surface below the model so the shadow can fall on it, but now there's the problem of keeping the surface from rendering.

How can I render the shadow on a transparent background?

Answer 1

On the shadow catcher object, add a material and enable Shadows only in Material > Shadow:

This will make it so that only the shadows cast on the object are shown, the rest of the surface will be invisible, showing the sky behind it.

If you set the alpha to Transparent in Render settings > Shading, the sky will be rendered as image transparency.