This is from Octane, not Poser (my poser node fu stinks), but it does show that realistic shadows are attainable using hdr as the only light source. I used Monument Valley Lookout from HDRLabs. You can see the sun position in the chrome ball.

The "trick" with Octane to get sharp shadows wirh hdr images is to raise the gamma for the HDR. The default is value is 1, and that gives very soft lighting like in the first image I posted, in this image it was at 2.2.

I'm not sure how you would replicate this in Poser, but it should be possible.

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