You're correct - the blending of semi-transparent or edge pixels will be influenced by the background. For perfect post-compositing, the background should be rendered black. Then the true background (or other overlapping objects) will be correctly blended into those pixels in your post-compositing tool, such as Photoshop.

The blending formula is this:

Blend(back, fore, alpha) = (1 - alpha) * back + alpha * fore

Where I'm denoting back = background color, fore = foreground color (from your object), and alpha is the alpha channel value for the pixel, or in other words the opacity. Opacity is 1 - Transparency, FYI. The color values stored in the PNG file are the result of doing this Blending step. In addition, the alpha value is recorded.

If you follow through with the math of doing a blend with BLACK, you'll see this does not add anything to the color. A second Blend operation (done in Photoshop for example) will pick up the pure color of your object from Poser. But if you blend in poser with other than black, then some of that will be in the file, and mix with your object's color when you blend it in Photoshop.

I'm just saying I don't get the point of rendering a gradient background if you intend to composite outside Poser. Not only will you be discarding it, but it will erroneously color your fringe pixels.

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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)