Add another new layer to the top of the Layers Palette. This layer will be used to add shading by first filling a selection with a flat tone of gray, and then adding further shading with the Airbrush tool.

After adding the new layer, use the Lasso tool to make a selection of the face where shading will be added. The selection doesn't have to be very exact, since the bold line art will be trapping (covering) most of the edges. After making the selection, feather it (I used 20 pixels) and fill it with a 35 percent gray. Leave the selection active and use the Airbrush tool to add more shading.

Copy the shaded layer, make a new file, and paste the shaded layer into the new file. Choose Image/Mode/Bitmap, to convert the new file from Grayscale to Bitmap mode. In the dialog box that appears, select halftone Screen for the method and click OK. Now, a Halftone Screen dialog box appears. For this illustration I entered 22 for frequency, -25 for angle, and line for the shape-click OK.

The image is converted to lines, and is in Bitmap mode. Convert the image back to Grayscale mode, and select the magic eraser tool. Set the magic eraser's options to, "0" tolerance, uncheck anti-aliased, uncheck contiguous, and set opacity to 100. Now, click on a white area in the image with the magic eraser. The white is deleted, leaving black lines on transparent.

Drag the new line art back into the original image, and position it. The layer with the gray tones of shading can be removed if you like.

Tip: Add shading with the airbrush tool, using black to feather from the solid black edges of the line art, to tones of gray.

Tip: Change the transfer mode of the shaded layer to Darken, allowing you to see the black line art underneath as you paint.