How to Scan Line Art the Obsessive Way

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How to Scan Line Art the Obsessive Way

What you'll need:

Adobe Photoshop v4.0 or greater
a scanner or access to one
some line art to scan

Here's the original scan. A lot of scanner software has options to scan grayscale images (or black-and-white photos) and line art. Use the grayscale option; the line art mode uses a high contrast setting, but we'll have more freedom by fixing it in Photoshop instead.

I usually doodle stuff on the back of old homework assignments, and they tend to show through, so there's a lot of gray noise. (Plus, the only scanner I have access to is kind of old and lousy.) What we want is to make the background white and the pencil lines black.

To do this, I create a Levels adjustment layer. (I could use the Levels command directly, but by using an adjustment layer instead, I can preserve the original scan and modify the settings should I change my mind later.) The histogram represents the tonal range and distribution of the image; blacks shades on the left side and white shades are on the right. Ideally we would like a full tonal range with a fairly uniform distribution of grays. In this case, however, there is clearly a deficiency of black tones and an abundance of light gray ones.

If you're reading this, you're probably not familiar with the Levels command. The three triangles below the histogram are, from left to right, the black-point, gray-point, and white-point sliders. All pixels corresponding to those to the left of the black-point will be made completely black. Similarly, everything to the right of the white-point will be made completely white. Dragging the white- and black-point sliders toward each other increases the contrast of the image. The gray-point slider determines the gamma of the image; it controls the relative lightness by setting the tone to be perfectly gray. Drag the gray-point slider to the black side, and more pixels become lighter than perfect gray, lightening the image; dragging the gray-point slider to the white side darkens the image.

... after applying Levels.

To clean up this scan, I drag the white-point slider to the left until most of the background noise turns white and disappears. There's a trade-off between reducing the noise and preserving image detail. You'll need to find a point that you're happy with and that doesn't require too much additional work (if any) on the image.

Next I drag the black-point to the right to make the pencil lines darker. If you want a "pencil" look you might want to leave the black-point alone. If necessary, I then adjust the gray-point slider to tweak the line-weights slightly.

Basically, experiment with the settings until the image looks good. =)

... viewing with Threshold.

I could stop there, but here's where I start to get obsessive. I next create a Threshold adjustment layer above the Levels one and set the threshold value to 255. This causes all pixels that aren't perfectly white to appear black. I can then see all the stray pixels that weren't completely removed with Levels. I create a new layer below the Levels one, and using the brush and the lasso on it, I cover those pixels with white. By doing this on a separate layer rather than directly painting on the original scan, I can go back with the eraser later to undo portions as necessary. I now delete the Threshold layer.

Again, I could stop there, but here's where I get even more obsessive. After applying the Levels, the image has an incomplete tonal range. As we can see from the current histogram above, the resulting image has lots of spikes and valleys. This causes the image to appear a bit oversharpened. I create a new layer on the top of the stack. Holding down the Alt key, I go to Layers > Merge Visible. This will create a flattened copy of everything in the current layer. The current layer stack is shown at right.

I expand this layer to 200% of its original size using Edit > Transform > Numeric in Photoshop 5 (Layer > Transform > Numeric in version 4). Make sure the interpolation mode is set to Bicubic in the preferences (File > Preferences > General).

After applying the transformation, I shrink the layer back down to its original size by applying the inverse transformation.

Enlarging and shrinking the image using bicubic interpolation fills in the tonal gaps and averages pixels together, blurring them slightly. Since this sometimes causes the resulting image to be a bit too blurry, I set the layer opacity to 50% to blend it with the underlying layers.


original, after Levels	anti-aliased (100% opacity)	anti-aliased (50% opacity)
(magnified 200%)

As we can see from the histogram, the tonal range is now fuller, and we're done.