Hello,

I was wondering what is the difference between sgi file and pic file when used in compositing.

And how would you get the shadow of only the moving object in one file…for example with the snail tutorial, who would you make it have shadow?

The .sgi picture format is rather old and I think is hardly used anymore. The .pic format is Houdini's proprietary format. If you're using the Apprentice, then you should render out as .picnc files instead. These files are *not* watermarked so that you can use them in Houdini's compositor and they won't be watermarked until converted into an external picture file format.

As for your second question, there's a “Shadow Mask” parameter on the light that casts shadows. By default's set to “*” which means to match all objects. You can change this to only the name of your moving object in the scene so that only that object will have shadows.