IMHO, for each HDR result it should be clearly documented how high the camera was above the ground when the render was made. Because the cam position becomes the center of the Environment Sphere, and the Poser ground level should be that amount below the center when the scene is build (by lowering the floor or raising the Sphere, whatever).

@BB: can the (Diffuse) ProbeLight node be of help? It maps the environmental HDR onto an object. Not investigated further, but might be an idea. We "only" need to map the bottom x% of the image, or so.

For the mapping math: the mapping of a portion of the botton half of the hemisphere onto a (circular) groundplane depends on the distance of the center of the hemisphere to the center of the ground plane, relative to the radius of the hemisphere of course. Not very complex, but every distance brings a different mapping which might make it an issue to implement.

RV: Camera's should be straight forward, please. Horizon, ground level, etc should be straight horizontals.

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though