Select the hair in the Surfaces pane, scroll down to the PR Hairs Density parameter and see if there's a map in that slot. If so, open that image in your preferred image editing software and make the places where you want more hairs lighter. If not, you'll have to create one for yourself, painting in greyscale over the UV templates with lighter shades where you want more hairs and lighter where you want fewer. That takes care of that problem. Also check for Additional PS Hairs Density and drop the modified map in that slot as well. I'm not at a computer and get PS and PR hairs confused, so I might have the names switched around, but that should be enough for you to go on.
For length, the parameters are PS/PR Hair Length. You can turn that dial up to make all the hairs longer, but if you only want to change the length of certain parts of the hair, you'll need an opacity map like for density.
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Select the hair in the Surfaces pane, scroll down to the PR Hairs Density parameter and see if there's a map in that slot. If so, open that image in your preferred image editing software and make the places where you want more hairs lighter. If not, you'll have to create one for yourself, painting in greyscale over the UV templates with lighter shades where you want more hairs and lighter where you want fewer. That takes care of that problem. Also check for Additional PS Hairs Density and drop the modified map in that slot as well. I'm not at a computer and get PS and PR hairs confused, so I might have the names switched around, but that should be enough for you to go on.
For length, the parameters are PS/PR Hair Length. You can turn that dial up to make all the hairs longer, but if you only want to change the length of certain parts of the hair, you'll need an opacity map like for density.