Digital way of detecting and removing pencil traces?

Question

I draw pencil sketches on paper and "ink" them using a variety of pens. (I'm using the term "inking" very loosely here to describe the process of drawing the final lines over a pencil sketch, no matter whether actual ink is used - in my case, I use a variety of felt-tip pens and Indian Ink.)

Of course, the final ink lines will never match the pencil lines 100%. There will be deviations and additions all the time:

To get rid of the pencil lines, one would usually use an eraser before scanning.

However, with some papers and inking materials, this tends to damage the canvas and/or the final lines, no matter how carefully you use the eraser. Also, it's a lot of work.

Is there a clever generic digital method to get rid of the pencilled lines without touching the final lines?

Merely playing with contrast and brightness doesn't produce good results, as it will destroy or alter details of the final lines as well.

I can use a very recent Photoshop (CS5.5 to be exact, but the more generic the advice, the better.)

Answer 1

Lollero's suggestion is a good one. It doesn't help much if everything's gray and black, however.

Here is a technique that takes a little finesse and some practice, but is your best general approach without changing your initial workflow:

• Scan in the usual way, and use a Levels adjustment layer to make the inking fully black (RGB 0,0,0) by dragging the left slider toward the center and the middle slider to the left. These aren't necessarily dramatic adjustments. You're just looking to make the inking black and the sketch lighter:

• Make the canvas pure white by dragging the white slider in a bit (a value of 248 worked for this example).

• Make the art layer (not the adjustment layer) active and switch to the Channels panel.

• Create a new Alpha channel by selecting the channel with the highest contrast (the Red channel in this case) and dragging it onto the new channel icon.

• Choose the Dodge tool, set the Range to Highlights, and the Exposure to 25% or so. (For more aggressive work, use a higher setting; for delicate areas choose 15% or 10%.)

• Set the size large. I used 125 pixels for your example.

• Brush over artwork to remove the sketch lines. The inked lines won't be affected because they're black. Watch your inked lines just to certain you don't accidentally wipe out some detail. Every pass will apply the dodge setting cumulatively.

• Zoom in, and with a small, hard edge Dodge brush take care of any detailed areas that your broad strokes left behind. There won't be too many. Here's what your example looked like after less than a minute with the Dodge tool:

• Invert the channel (Ctl-I)

• Use the Dodge tool to make any grey in your ink lines white.

• Ctl/Cmd-click on the channel icon to make it a selection.

• Switch to the Layers panel. Your artwork layer should still be targeted.

• Click the Layer Mask icon at the bottom of the Layers panel. This creates a layer mask according to your selection, and deselects automatically.

At this point, you can tweak the Levels adjustment layer as needed, and you can fine-tune the mask by brushing with white to reveal part of the layer or black to hide. Tweaking the mask is where a Wacom tablet becomes almost a necessity.

The advantages of this technique are that you never mess with your original pixels, so you can always step back and try again if something doesn't work out quite right. With a bit of practice you'll be able to clean up any drawing very rapidly.

Answer 2

Without resorting to much trickery I would use colored pencil for the sketching.

Lets say you'd use for example a blue pencil for sketching and your regular black ink for line work.

Then you can use (top menu)

Layer > New Adjustment Layer > Black & White

or

Image > Adjustments > Black & White

And slide the Blues and/or Cyans sliders high enough and you can get rid of the blue sketch lines really easily.

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You can also try and change your sketch lines to color lines and then doing the same thing.

( Note that this is asuming that your line work is dark enough.. or that you can make it dark enough without ruining it. )

( Point being that you cant colorize Black )

You can change the color with (top menu)

Layer > New Adjustment Layer > Hue/Saturation

or

Image > Adjustments > Hue/Saturation And remember to check colorize checkbox.

After that you could try the way above this one.

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For the best results.. I would say, buy drawing tablet and put sketch and lineart in different layers.

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Also, I hope that you mean Photoshop cs 5.5 and not Photoshop 5.5 :)

Answer 3

There is no way automated digital way to completely eliminate the pencil lines without doing some damage to the quality of the ink lines. However, it is possible to get close—probably close enough that you could manually fix the problems in a few minutes using a graphics tablet.

Looking at the sample image, it appears that there are two characteristics that could allow a computer to differentiate the pencil from the ink. First, the pencil lines are mostly a lighter shade of grey. Second, the pencil lines are thinner. In signal/image processing terms (what Photoshop effects are really doing behind the scenes), these properties correspond to amplitude and frequency.

Your original approach (using brightness and contrast settings) only took into account the first property. By picking the one that works best, or combining both, it is possible to get a fairly good result that only requires minimal touch-up.

'Lightness' of the Line (Amplitude)

The histogram of the image shows us the relative number of pixels at each brightness level (shades of grey). In the figure below I have labeled some of the features on the histogram of your example image. The pull-out below the histogram is a black-and-white (binary) image showing all the pixels that have brightness levels in the indicated region on the histogram. This mostly corresponds to the pencil lines, but as you can see, it also contains part of the soft edge of the ink lines. When I remove all the pixels with brightness levels in that region, I get the image on the left. As you can see, most of the pencil is gone, but now the edges of the ink lines look a little rough. If I had moved the threshold (cut-off point on the histogram) further to the left, it would have gotten rid of more of the very light pencil markings, but it would have also degraded the ink lines even more.

'Thinness' of the Line (Frequency)

Manipulating the frequency content of an image is done all the time in graphics editing, but relatively few people realize what they are doing, because Photoshop hides all the technical details behind what they call a blur filter.

If I apply a Gaussian Blur filter to your image (using a radius of 2 px) and then apply a levels adjustment, I get the image in the figure below. As you can see, the resulting lines are mostly free of pencil markings, but they are thicker and have less detail than your original ink lines. That is what a blur filter does: it removes detail. This is useful to you because thin lines = higher level of detail and your pencil lines are usually thinner than your ink lines.

The Final Image

Every image is different, and the only way to figure out what will work best is to experiment. When I played around with your example image, I seemed to get better results from the second (blur) method. However, I suspect that given enough time, the best results would come from some combination of the two. I will end with a figure showing a sequence of image adjustments that gave me fairly good results after a few minutes, but that could use more fine-tuning.