final colored image in Adobe PhotoShop

How to Separate Line Art in PhotoShop

This is a ripoff of an article that I read approximately over 15 years ago in Macworld magazine. How to separate line art in Adobe PhotoShop is an extremely simple operation, but it had opened up my mind to what could be accomplished in computer graphics.

This is a sort of non-destructive way to cleanly separate your line art for easy coloring. The nice thing is that your line art layer will be opaque, flat, and consistent no matter how you color. You won’t have to risk potentially screwing up your inked work due to the fact that it will be on a separate layer.

System Requirements

The requirements for this tutorial include any version of PhotoShop. I highly recommend an Adobe Creative Cloud subscription with PhotoShop. If you haven’t already, download a trial version of Adobe Photoshop before you proceed any further. A scanner or high-resolution digital camera to input your line art. (I use VueScan, but any software will work.)

Now let’s jump into it!

Digitizing Your Line Art

If you’re using a scanner, use the following settings:

  1. Set your Bits Per Pixel (color space) to 1 bit B/W (or Black & White). In PhotoShop, this is referred to as Bitmap colorspace.
  2. Set the Scan Resolution to 600 dpi.
  3. Set your scanning program to save to TIFF format.

In #1, we use 1-bit B/W color space because we ONLY want your line art. We don’t want to scan in paper fibers, the mounds of white paint, etc. If you happen to use a non-photo blue pencil, we don’t want to pick that up either.

Side Note About 1 Bit B/W:

Back when I was working for a small-time graphic design firm, I learned all about the 1 Bit B/W colorspace. 1 Bit B/W is great for lineart, logos, and type. In graphic design, it’s almost as flexible as vector art in the sense that it can be scaled up and down. (Not infinitely.) In Adobe PhotoShop graphics using 1 Bit B/W color space look on screen like something out of an 8 bit video game. It’s extremely blocky. But when the image is printed out, the blockiness that you see on screen gets smooth out. Clients would often not possess a source file for their logos and/or graphics. We would take a printed sample of a client’s logo and scan it at a high resolution in 1 bit color space to avoid having to recreate the logo/graphic.

Getting back to the tutorial, we’ll scan the line art at 600 dpi (dots per inch) or higher, to allow us to grab as much detail as possible. The more resolution, the easier it’ll be to work with the image. Yes, we’ll end up with a larger file size (and possibly a slower machine) due to the number of pixels within our scanned image, but we’ll always have the option of lowering the resolution.

In #3, we set our scanning software to save our image to the TIFF file format because it can handle a high-resolution image. It’s lossless and compatible with all graphic design programs. TIFF files will make it easier for any company that will be printing your book, poster, etc.

Scanning Using a Mobile Phone

I would honestly recommend against scanning with a mobile device. Yeah, they’re capable of doing lots of things, but 9 out of 10 times, the app that you’re using will only be capable of saving your image as an optimized JPEG with a resolution of 72 dpi. Your mileage may vary depending upon what device you own and which app you use.

Also, I (as of writing, Dec 2019) currently don’t use smart devices such as iPhones, iPads, etc for digitizing my line art outside of taking photos for social media. I haven’t completely fooled myself into thinking that my humble iPhone XR is on par with the most advanced SLR digital camera, but it’s good enough to serve my needs.

Going From Photo to Lineart

Step #1 import your image from your camera or smart device to your computer like you normally would. Open your photo in Adobe PhotoShop.

Step #2, we will be stripping the photo of all of its color data. Click Image >> Mode >> Grayscale…

Step #3, we need to flatten our image to the point where we’re getting closer to only having just black and white pixels. You’ll have one of two options:

In option #1, we can squeeze all of the gray pixels out using Levels. Go to Image >> Adjustments >> Levels. Simply drag all three of the slider together towards the middle.

Option #2 utilizes Posterize. Click to Image >> Adjustments >> Posterize. Bring the Levels slider down to 2. This strips out all of the pixels except for the blacks and whites.

Step #4, because everyone here is as super-anal as I am, we’ll change the image mode to 1 Bit Black and White. Click over to Image >> Mode >> Bitmap. Now we proceed to the next step of cleaning the line art up.

Clean Your Image Up

At this step, you’ll have some garbage that shows up in your scan. It’ll come in the form of random marks, mistakes that you made, etc. We can clean all of that up using the Brush Tool (keystroke B) in PhotoShop. It’ll also help to reset your brush colors to the default black and white by tapping the D key.

You can also use the marque and/or Lasso tool to select what you don’t want and fill it with either black or white pixels.

Changing Color Spaces

Now I know that I have just sung the praises of 1 Bit B/W color space. But it has just served its purpose and we need to move on to the RGB colorspace. 1 Bit B/W doesn’t allow for layers and we can’t apply any color within this colorspace.

In PhotoShop, go to the Image menu >> Mode >> Grayscale… You’ll see a window asking to set the Size Ratio. Just use the default of 1. Then click back into the Image menu >> Mode >> RGB Color. Now we’re in the RGB colorspace.

Now I know some of you are asking, “Why the heck do we need to be in RGB colorspace?!”

First off, this is my tutorial, and I said so. Second, even though you can separate line art in Grayscale mode, I am assuming that you’ll want to color your line art. That’s why. But the same ideal can be applied in either color space, including CMYK.

Let’s Separate Line Art in PhotoShop!

Now for the moment that you all have been waiting for, we will separate line art in PhotoShop from your background layer.

At this point, we’ll select the Red channel using the keystroke Command/Control + Alt + 3. This selects the white areas of the channel.

Side Note:
All channels are in greyscale within Adobe PhotoShop. You can actually use any channel due to the fact that our image only uses 2 colors, black and white. Grayscale Mode only uses one channel. You can use the same keystroke mentioned in the previous paragraph to select just the white areas.

View the Channels palette to see. (To bring it up, go to Window >> Channels.) Since we’re currently within RGB Mode, the top channel within the palate is RGB. Below it are the separate channels, Red, Green, and Blue. Each channel shows a keystroke to view just that channel. Just add the Alt key to your keystroke to select just that channel.

Channels pallet in Adobe PhotoShop.
Channels pallet.

Right now, we currently only have all of the white pixels selected. We’ll reverse the selection so we’ll only have all of the black pixels selected. Use Command/Control + Shift + I to inverse the current selection.

Now you’ll have three different options to create a layer only containing our line art:

  1. Create a new layer via cut using the keystroke Command/Control + Shift + J.
  2. Create a new layer via copy using the keystroke Command/Control + J.
  3. Create a new layer that you can rename via copy using the keystroke Command/Control + Alt + J.

Regardless of which keystroke that you use, you’ll end up with a new layer containing only the black pixels from your image, a.k.a. your line art.

Just Add Color To Separated Line Art

Now you can color to your heart’s content in as many layers as you desire. Because we separated our line art, we now have more freedom and flexibility to color without damaging our it. Enjoy!