I'm trying to achieve a stylized ink wash or watercolor effect where the image ground remains visible. Since freestyle generates such energetic lines, I want to use them to mask my render during compositing, defining the areas where the ground will show through. So far I haven't had much success.
With the node set-up below, I'm able to apply transparent lines to my image by inverting the alpha of my freestyle render layer before combining my render layers with an "alpha over" node. Unfortunately, flipping the alpha blocks out all areas of the image that aren't freestyle lines. I'm sure I'm missing an obvious step, but so far I haven't been able to figure it out. Does anyone know what I should do?
An ID Mask seems like it would be just about perfect, but as far as I can tell, it's not possible to create one for freestyle lines.