The problem with Poser Sketch renders, amazing and fast though they can be made, is: i) there is no transparent background in Sketch renders; and ii) the outline of the subject often 'wobbles' slightly due to the Sketch effect, which means it's then difficult to make a fringe-free cut-out from the Sketch render (by later using the Comic Book mode real-time render .PNG or Firefly .PNG as a mask in Photoshop).

One way to do it is to render on white, then use a 'knockout white to transparency' Action in Photoshop. https://www.deviantart.com/mutinate/art/Knockout-All-White-for-PS-889467682 Then manually resize until its edges more or less fit just within the edges of a standard .PNG in the layer below. Mask the lower .PNG, and fill with solid white.

Alternatively you can sketch a straightforward render in a filter, although in more limited ways. The only ones worth using for that are the standalone StudioArtist v5.5 or higher (vectorises, then does the effect) and the Photoshop plugin Redfield Sketchmaster 2018 or higher. Both are expensive.

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