as you can see in the image the borders are not as accurate as the original render. Does anyone know how to solve this problem?

These screenshots are not yours and were taken from a rather early demo-test. Have you actually tried it yourself in Poser 11?

But when making comics for real with Poser, you would output multiple lineart renders (e.g. Firefly toon, Comic Book, Sketch into Comic Book lineart) and combine them in Photoshop over the top of a line-less 'color flats' render. You would also manually re-ink / erase the inks, which would be enabled by having your inks on a separate layer.

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Learn the Secrets of Poser 11 and Line-art Filters.