After watching the tutorial on layering, I decided that I should give the technique a try. So, I created transparency layers with a black background, played with the morph brush to distort the figure's geometry so it looked like she'd been in a fight, and then applied everything to a render.

Overall, the effect works pretty well. However, I had to turn the main specularity off on all the layers, otherwise it seemed to multiply on the skin. That issue was easy to solve, but I can't figure out why my character's lips have a black line around them. This makes no intuitive sense, as I've plugged the layer nodes into the transparency channel, and the background for the layers is completely black.

Any ideas?

Take a look at your lip texture ... and the possible mask involved ... just a thought.

I didn't put the transparency mask on the lip texture. It's on "skin face," and the part where the lips should be has no transparency applied to it.

Hmmm ... the black looks like a mask overlay ... Are you using two different face masks in the face layers or the lip layers that could have a different skin tone? Also ... what render engine are you using? This looks like a preview render, Are you using Superfly? The shadows look a little off and that could be part of the issue. The shadows are harsh where the hair meets the face on the upper right forehead (unless that is one of the injuries).

This is Superfly. The shadows to which you refer are bruises and scratches.

Here's how I did this:

I took the face texture into Photoshop and set up several different layers. At the bottom, I created a black layer. Then, using the face texture as a reference, I used a white brush to create transparencies for each of the various layers. I turned visibility off for the face layer and saved each transparency separately. I saved these into the appropriate texture folder, then added layers for the character's face as needed.

Given that every separate layer is plugged into a transparency channel, I don't understand how that thin line around her lips could possibly form. It's odd . . .

I've never played with layers, so this might be completely wrong. If so, I'm sorry for wasting your time.

With transparencies on objects, there are two settings; edge and falloff. I usually set these to zero, so that they don't appear. This looks like one of those settings needs changing. If I saw this on, say, a clothing item I wanted to apply trans maps to, the first place I'd go is there two settings.

Just a thought.

There are some bugs relating to transparency and scatter using Cycles SubsurfaceScatterBsdf (or whatever it's called - don't have Poser open right now). Are you using that? This might be a variation of it.