Wait a sec. You asked how to "make objects cast shadows on everything but the ground". Now we learn that this is "not just the ground plane". Your original phrasing implied that we were dealing with the typical shadow-catching ground plane.

So, you have a prop that you do not want to see shadows on? This has nothing to do with the "Poser Ground" plane, right?

I'd have to say I'm curious why your lights should not cast long shadows, if you've placed the lights low. Is this an outdoor scene, i.e. is the low light the sun? The sun casts long shadows in real life.

Or is it indoor, or night with local lights? With local lights, long shadows fade due to blurring - the real light source is not a point. If that's the case you should be using raytraced shadows and blurring, and they will fade with distance as in real life.

Also, are we really talking about the shadow making too much contrast? Most Poser users are not doing gamma correction, so shadows look over contrasty. The solution is not to get rid of the shadow in that case, but to make it generate the proper brightness as a photo of a real shadow would do. Do you have Poser Pro? Have you enabled gamma correction?

Or is realism not relevent, and you just want the ground to not react to light and shadow at all? If that's the case, it would help if you showed us what you're working with. I'm wondering if the prop in question is shaded simply, basically just image mapped? Or is it a procedural shader with some complexity? Does it need to react to light at all?

Sorry for so many questions, but there are solutions (different ones) depending on your answers. I know how to make a shader for a prop that does not react to light at all. I know how to make a shader seem to react to light, while having no lights in the scene whatsoever. I know how to make the shader react to lights and shadows, but to decrease it's response to the shadow alone. I know how to gamma correct in any version of Poser, not just Poser Pro. I could explain any of these once we know that's how you want to go. They all require multiple postings to explain. I'm not going to write them all here, as I've already written all of these things elsewhere in this forum.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)