The DPI Debate (and Anti-Aliasing)

Okay. This is something that I really need to talk about. Although this is mainly geared towards people breaking into the comics industry, the concept is also very important for anybody interested in illustrating or working with digital images. If you are that type of person, listen up. We’re gonna do some learnin’!

All right. So there are two subjects that are really rather inter-related, and these are image resolution and antialiasing.

Image Resolution vs. Image Size.

A lot of people get these two things mixed up, and a lot of software doesn’t help with this either. Image size is not always the same thing as resolution.

Image Size – The physical dimensions of an image, particularly when the image will be printed. This is often defined in terms of inches or centimeters. This can be anything you want, as long as you know it is set to the maximum size you will want to print your image at. Unless you have no other option, do not determine your image in terms of pixels unless you are specifically creating a graphic for use on the web or in video that will not be used in print.

Image Resolution – The resolution of an image is related to the pixel density, or pixel count, given in a particular image. This is traditionally defined in terms of DPI, or Dots Per Inch (The metric equivalent being Dots Per Centimeter or DPCM). DPI is defined in terms of print or video resolution in which the number represents the number of dots that can be displayed in an inch. The higher the DPI, the more detail is stored in an image. This resolution also comes into play when you are scanning in an image.

Update: As one of my readers commented below, the proper term for resolution is actually PPI, or Pixels Per Inch. But this is typically used interchangeably with DPI.

There are a couple other related terms that are also important to know as they relate to the file size and quality of your image. These are Bit Depth and Layers.

Painting at various bit depths.
Painting (150 x 150 px) in 24-Bit Color (40 KB), 8-Bit Grayscale (16.5 KB), and Bitmap (2.7 KB).

Bit Depth – This is where the real file size starts to come into play. Bit Depth is how much data is stored per pixel in your image. Images are often split into different “channels”. In a color image, this is Red, Green, Blue, and Alpha (Transparency). The higher the bit depth, the more color details can be stored, but also the larger the file. Optional bit depths also include Bitmap (1-bit) for use with line art, and Grayscale, which only uses a single channel to store gray values.

Layers – Some software supports layers. Each time you add a layer, it’s like creating a new image at the same size and resolution as the original image. Lots of layers means a huge file, so it can be good to try and minimize these.

What Resolution Should I Work At?

Although this can be a slightly objective question, if your work will ever be printed, you should do color work at a minimum of 300 DPI and line art at 600 DPI or higher. I will explain more of the “why” shortly.

Why not 72 DPI? The file is smaller!

Well, yes, the file is smaller the lower DPI you have. But to be perfectly honest – hard drives  are relatively inexpensive these days. You can get a multi-terabyte external hard drive for less than $100. It is good practice to have an extra hard drives for backing up your work anyways, and the investment is well worth it.

Also, if you are worried about space on your hard drive, and you want to work at high resolution, remember that high resolution line art can be done in grayscale (grays) or even bitmap (black or white) color modes. Bitmap (not to be confused with the image format) just stores 1’s and 0’s for each pixel, for White or Black, so the files are incredibly tiny.

When Lakewood was being published in the newspaper, I would send the printer my 5.75″ x 2.125″ comic strips at 600 DPI (3450x1250px). These files were usually less than 120 KB in size, because they were bitmap images. How’s that for saving space on your hard drive?

So what is this thing about Anti-Aliasing?

Okay, so I’ve been doing a lot of preaching without a whole lot of examples. But that’s what we’re going to get into now. There is this little effect in computer graphics called aliasing which is directly related to image resolution. When you draw a line in a picture and you only have a limited number of colors to work with, you can get a jagged, stair-step type effect on the line.

 

An Aliased Line.
An Aliased Line.

Anti-Aliasing takes the line and attempts to make it look smoother by interpolating gray values.

An Anti-Aliased Line.
An Anti-Aliased Line.

Anti-Aliasing is a wonderful tool for producing smooth final images. Often, however, artists tend to apply anti-aliasing too soon, when it should be applied at the very end of producing the image, when the final piece is being scaled down and exported for viewing on the screen or in print. The reason it should be saved for the final step is because using anti-aliased lines too soon can (in some cases) cause problems or create slowdowns in the illustration process.

Anti-Aliasing in Brushes

A lot of brushes in drawing programs come with an anti-aliased feature, but this often leaves the edges of the brush stroke looking rather soft and undefined. At low resolutions, the brush looks soft on the edge. This can look okay for a soft brush effect, such as imitating a paint brush, but often doesn’t look good in the case of imitating an ink pen.

Aliased vs. Anti-Aliased Brushes.
Aliased vs. Anti-Aliased Brushes. Note the soft edge on the right.

The higher the aliasing on the brush, the less it will look like a pen, and the more it will start looking like a pencil. Which is ironic considering the Pencil tool in popular programs like Photoshop and GIMP is typically what is used to create aliased lines.

Anti-Aliased Lineart and Coloring

Some common “Fast Coloring” methods with line art in programs like Photoshop and GIMP often make use of the Magic Wand tool and the Flood Fill tool. Anti-aliased line art is difficult to select unfilled areas in when coloring because of these inserted gray values. This means that flood selections then have to be expanded before flood fill can be performed on the color layer.

Example of Aliased Flood-Fill Coloring (left) with Anti-Aliased Flood-Fill Coloring (right).
Example of Aliased Flood-Fill Coloring.
Example of Anti-Aliased Flood Fill Coloring.
Example of Anti-Aliased Flood Fill Coloring.

Note: An experienced illustrator will probably tell you never to flood fill on your original line art. This is true if you have layer support. What is important to pay attention to, however, is how the regions fill. If you’re using the Magic Wand tool to select a region, it will select the same region as the flood fill would. Magic Wand and Flood Fill use the same algorithm to detect open regions of similar pixels. So if you’re wanting the ease of producing an image that is quick to color but also have smooth lines, the actual key is in dealing with higher resolution images.

How does Resolution fix the Aliasing Problem?

The higher the resolution, the less the eye will actually see the stair-step aliasing effect. This is actually a key reason why the resolution of most monochrome (Black and White) digital printers starts at 300 DPI. They only have one color they can put down – black. And at 300 DPI, unless looking incredibly closely, the stair-step effect is not noticeable. You simply get a crisp, smooth line.

Aliased Comic Panel.
Aliased Bitmap Comic Panel at 300 DPI. Good for print. Too large for practical viewing on a computer screen.

Without the anti-aliased gray values, it is much easier to color lineart. You can either flood fill directly, or select with magic wand and fill. And for those who don’t enjoy how long coloring takes (me) this time saver is incredibly important.

The other added benefit to working with high resolution art is that when you want a lower resolution, all you have to do is export it smaller. The scaling down in software such as GIMP and Photoshop will automatically smooth (anti-alias) the lines so that they look good on a computer screen, and then you have a clean, crisp final image.

So what am I supposed to do?

Color art is almost always printed at 300 DPI, so it is okay to color images at 300 DPI.

Lineart is best done at 300 to 600 DPI or higher. The reason for this is that when the lines are scaled down, the resulting lines look far smoother than the original ones do. This is why comic artists often ink on 11″x17″ sheets of bristol board, and the image is then scaled down to comic book size for printing.

So… when is 72 DPI a good thing?

72 DPI is great when you are exporting an image to be viewed online. It’s also great if you are designing graphics that will be used exclusively on the web, such as an internet banner or a header for a web site. What it’s not good for is creating images for print.

Anti-Aliased Comic Panel at 72 DPI (down-sampled from 300 DPI source image). Bad for printing. Good for viewing on a computer screen.
Anti-Aliased Grayscale Comic Panel at 72 DPI (down-sampled from 300 DPI source image). Bad for printing. Good for viewing on a computer screen.

My program won’t let me set DPI – only the size of the image in pixels.

That’s okay! Below I have put together a table of common print resolutions to give you a general idea of what the pixel size of each image actually is.

Table of Common Print Resolutions

Paper Size W x H (In.) 72 DPI 150 DPI 300 DPI 600 DPI
Playing Card 2.5″ x 3.5″ 180 x 252 px 375 x 525 px 750 x 1050 px 1500 x 2100 px
Postcard 4″ x 6″ 288 x 432 px 600 x 900 px 1200 x 1800 px 2400 x 3600 px
Small Print 5″ x 7″ 360 x 504 px 750 x 1050 px 1050 x 2100 px 2100 x 4200 px
Manga 5.5″ x 7.75″ 396 x 558 px 825 x 1163 px 1650 x 2325 px 3210 x 4650 px
Half Letter 5.5″ x 8.5″ 396 x 612 px 825 x 1275 px 1650 x 2550 px 3210 x 5100 px
Trade Paperback 6″ x 9″ 432 x 648 px 900 x 1350 px 1800 x 2700 px 3600 x 5400 px
US Comic Book 6.75″ x 10.25″ 486 x 738 px 1013 x 1538 px 2025 x 3075 px 4050 x 6150 px
US Letter 8.5″ x 11″ 612 x 792 px 1275 x 1650 px 2550 x 3300 px 5100 x 6600 px
US Tabloid 11″ x 17″ 792 x 1224 px 1650 x 2550 px 3300 x 5100 px 6600 x 10200 px

The best general piece of advice I can give you for achieving good print quality are these four simple steps:

  1. Work at the highest resolution you can that doesn’t make your computer lag (300 DPI for color, 300-600 DPI for line art).
  2. You can always use a smaller bit-depth. Especially when working with line art.
  3. If you are inking a piece, use a non-anti-aliased brush if you want crisp lines and faster coloring time.
  4. It is easier (and looks better) to scale down an image than to scale it up.

Anyways, I hope this post has been helpful and informative!

Cheers!

-Nathan

4 Comments

  1. Bram said:

    If you really want to drill down, DPI is a printing term, dealing with the output resolution of a printer/device — PPI (pixels per inch) is the measure of the resolution of an image on your computer. They’re used pretty much interchangeably and without much confusion these days, but there is a technical difference.

    That chart of measurements is great, but always be aware of bleed (for any art outside the trim area) and live area (where all your important text and art needs to fit). Setting up art properly at the outset avoids so many problems later on.

    And I know you made the point, but it can’t be emphasized enough — you can’t enlarge an image after the fact. I mean, the program will allow you, but it’s not going to deliver a quality result.

    November 20, 2014
    Reply
    • Ah yes – you are correct, Bram! Thank you very much for that clarification! And yes, bleed is vitally important to bring up as well. I’ll be doing a future article about actual page layout, bleed, and starting images that you intend to get printed.

      Thanks for your comments!

      November 20, 2014
      Reply
  2. Nathan! This is great man! Nice to see this all put together for artists getting started. I learned ALL of these lessons the hard way!

    Great examples and visuals to emphasize the points! Well played sir!

    November 20, 2014
    Reply
    • Thanks Ben! This is all stuff that I think is really important for people to know… and not necessarily the best kind of thing to learn after you make a big print order for books and they all look terrible. 😀

      November 20, 2014
      Reply

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