Interesting effect, and I hadn't thought about it before.... Poser doesn't "know" that the boxes are tiles in a single surface, so it treats each box as a separate object, shading it in the way that would look natural if it were standing on its own. Each flat surface is effectively bulged just a bit. (This is actually compensating for an oddity in human perception, in which edges are overemphasized.) The only way to make the roof and wall look "solid" is to build them as single solid objects, either by scaling up a single box or by building in another modeler like Anim8or.

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