Preparing a render with Alpha

[Ed]

What is the best way to prepare a render for TFF output with alpha? 

I need to compost my model on top of a photo background layer in PS.  I don't want to bring this photo in as a background in Keyshot.

To get the best result along the anti-aliased edges, should my render background color be black, white, a color that matches the intended background in PS, or does it not matter?  Also, would TFF 32 bit output be noticeably better?  I'm rendering the real-time version.

Ed

[JeffM]

If using regular tiff, try black or a color close to the intended background.

Tiff 32 might give a nicer alpha and should be less dependent on the background color.

Another option would be to do the render with a white background, render to regular tiff with alpha, use the alpha to select just the model (don't select the ground shadow), cut the model out to its own layer, then put a background on the bottom layer and set your original layer (acting as a shadow layer) to multiply to blend out the white.