Image Resolution and You

Picking your resolution is perhaps the most important decision you'll make when digitizing photos, artwork, or any other kind of image. But it can also be confusing if you're not familiar with these terms and their meanings. This is a simple primer to help you choose the right resolution for your needs.

The terms DPI and PPI are both shorthand units for measuring an image's resolution. DPI stands for "dots per inch" and PPI, "pixels per inch." This means that DPI is technically a term for a printed object's resolution while PPI describes an image displayed on a screen, but in common use they are essentially interchangeable.

The system that these measurements describe is called Raster, and it's by far the most common in a modern digital setting. A raster image is essentially a mosaic, collecting dots of color called pixels in a tiny square grid to produce an overall image. The more pixels per inch, the more detailed the image. Simple enough, right? For an easy example, here is the same image at three different standard resolutions: 600dpi, 300dpi, and 72dpi (click to see at full size)

You can always lower the resolution of an image, but it's impossible to raise it, except in a simple multiplying sense. This is why all digital images look blurry if you zoom in far enough. You're making the pixels bigger, but you aren't adding any new information to them.

Another factor you will want to consider is your display resolution. Modern high-definition TVs will often give you this basic measurement, and while computers have a greater variety of resolutions they will generally fall under a few typical values. 480p means a screen is 480 pixels wide, and is considered "standard" definition. 720p is, of course, 720 pixels wide, and marks the beginning of "HD" standards. 1080p is probably the most commonly used HD resolution, and the cutting-edge "4K" resolution is a convenient shorthand for screens 3,840 pixels wide. The screen resolution will determine how "large any given image looks at full-resolution on the screen. If you try to stretch a 480p wide image across 1080 pixels, it will look bad.

While the ideas surrounding pixel resolution, display resolution, and print resolution are quite complicated, they can still be understood easily with a few guidelines. For most purposes you can create images using 3 different resolutions:

600ppi is what we at the Dartmouth Digital Library Program use as the standard for high quality "master" images. Although many scanners can go higher, the size of the file becomes very unwieldy at that point. My advice is to always start at at least 600. Better to have a high-quality image and not need it than to need it and not have it.

300ppi is a common resolution for a high-quality print. Unlike looking at a screen where the resolution can be shrunk or blown up, a printer is rigidly limited in the amount of detail it can put into any given area. While a particularly good printer may get higher resolutions, most will clock in around 300dpi. This lower resolution also makes transferring files for print easier. And of course, it's always useful to keep your higher-res files around in case you need to go back to them.

72ppi has become the most common display resolution on the internet. There are a few things to consider before simply converting your image into 72ppi. Look at your display, and understand what its resolution is. Then consider how "big" you want your image to look on the display. So, if you have a 1080p monitor and want an image that fills the whole screen, you'll want to change your ppi to 72, AND change your image width to 1080p at the same time, with the proportions locked.

Here we can see Photoshop's image size menu (Image -> Image Size), where the pixel width and resolution are changed while the proportions remain constrained.

This is often a confusing concept to grasp. The simplest way I can think of is: if you reduce a 300ppi image to 150ppi, but also double its size, it will essentially be the same image when you see it on your computer. But if you try to print that, it will be half as detailed by virtue of being twice as big.

Fortunately, you don't need to fully understand all of this in order to create and work with high-quality images. As long as you make sure your highest-quality 600ppi master versions are safely backed up, you can play around with these variables in Photoshop or any other imaging program until you meet your own needs. Understanding how screen resolution, print resolution, and image resolution work together is an ongoing process that changes with technology as well as peoples' needs. It's important to be consistent, especially so for an institution like ours, but it's equally important to know how to adapt to your own needs.

Written by Ryland Ianelli

Dartmouth Library Hosts New England Code4Lib Regional Meeting

Early last December, library and technology enthusiasts from the far corners of New England gathered in the Jones Media Center here at Dartmouth College Library for the NE Code4Lib 2015 Fall Meeting.

Thanks to the awesome NDSR-Boston cohort, a lot of digital preservation ground was covered- including getting started with digital preservation, building a digital preservation community, and processing born-digital archival materials.
I gave a short talk about a script I use to QC and package digital materials for ingest into our repository.

You can see slides for these talks, and all of the great presentations from the meeting, on the NECode4lib 2015 Fall wiki page. Also, be sure to check out the Storify for the day put together by Joe Monibello.

The Book Arts Workshop also played a role in the event. My colleague Shaun Akhtar and I made use of the resources, expertise and equipment of the Book Arts Workshop to print custom letterpress give-away posters for the Meeting. It was a really fun way to tie together all of the technological expertise that resides here in the Library!

Written by Jenny Mullins

Digital Production Roundup

Sometimes great content comes through the Digital Production offices and we don't have a chance to share it. Here are a few of my favorite selections from recent shooting sessions:

"The Flayed Angel" by Jacques Fabien Gautier D'Agoty, available to view in Rauner Special Collections. Read more about it here

A design for a theatrical device by Denis Diderot, co-founder/chief editor/contributor to the Encyclopédie with Jean le Rond d'Alembert

Vintage ad from Dartmouth Alum Robert L. May's scrapbook. May created the Rudolph character in 1939; read more about him here

Concert posters from Dartmouth history, scanned for an exhibit in the Baker Berry Library: Fleshtones concert poster, 1983

Spring Formal poster with Duke Ellington

New Hampshire maps from the Evans Map Room, scanned as part of the Granite State Maps project

Map illustrations

Written by Ryland Ianelli

Merging Images in Photoshop, Part Two

While my first post in this series covered some basics of non-destructive image merging in Adobe Photoshop, this post aims to give you some tools to work with less ideal images.

A typical problem faced by many when attempting to merge images is how to deal with slight variations in your materials. These variations can be a result of the hardware you use (scanner or camera), lighting conditions, software, or a myriad other factors.

In this scenario I'll use an image from a scanned book that has a lighting problem. This is a very common difficulty when scanning books on a flatbed; the book's gutter will raise up from the scanner's surface, giving it a darker tone and distorting the content. Applying pressure to the book may work sometimes, but often we do not want to risk damaging our books (or, worst-case-scenario, our equipment) that way. Much safer to work digitally.

The first step is identifying the cutoff point where the gutter begins to negatively affect image quality. As you can see from this image. As always, make sure you've saved a master version before making any edits. 

Use the Rectangle Marquee tool to isolate the "safe zone" of the page, where the page's content is mostly unaffected by the gutter. Copy this selection and paste it into a new document with approximately the same dimensions as the original document. This is our new "base" document. I will usually put the word "EDIT" in its title somewhere so it won't be mistaken for the master.

In situations like this when we are dealing with an off-white paper, we'll want to make sure our background matches the paper's tone. Use the Eyedropper tool to select the page tone, and use the paint bucket to fill it in the background of the new base document.

What we are going to do next is create an amalgam of the image's two elements (page and background) to create a new version with improved legibility. To accomplish this we are going to use Photoshop's Layer Mask tool. Layer Mask is incredibly useful for photo editing, and best of all it is a non-destructive solution, meaning whatever you do with it can always be easily undone. The Layer Mask is very much what it sounds like: a layer placed on top of each image that can be "masked" or "unmasked," concealing or revealing the image's contents. Select the right side layer and click "Add Layer Mask" at the bottom of the Layer menu. You will now see a small white rectangle linked to that layer; that's your layer mask. The color white means "unmasked," so right now it's simply sitting on top of your original layer waiting for you to give it instructions.

Next, be sure you have the correct layer, AND your layer mask selected, or else you will alter the image itself. The Layer Mask functions in grayscale. Black is "masked," white is "unmasked," and grays are everything in between. Set your foreground color to black. Select the brush tool, and reduce the hardness to 0 (use whatever diameter you feel comfortable with). Check again to make sure you're still on your layer mask, and simply begin lightly touching up the problem spots along the gutter with your brush. Use single clicks rather than click-and-drag, that way you will make very minor adjustments while you get a feel for the technique.

As you can see, we've fixed the page to appear legible and flat. But be warned, this exact method won't work for every situation. If you have content that is deep in the book's gutters, you will likely have to accept a less-than-perfect image. However these techniques can still be used to improve the image quality in those situations, it is simply more difficult to get a "perfect" image. Vary your brush settings and color in the Layer Mask (experiment using a 50% grayscale) to find solutions that best suit your situation.

Written by Ryland Ianelli

"The Dartmouth Brut: Conservation, Authenticity, Dissemination"

The Dartmouth Brut (Rauner Codex MS 003183) is the focal point for the latest issue of Digital Philology:  A Journal of Medieval Cultures, edited by Professor Michelle R. Warren, Professor of Comparative Literature.  A conference on the Brut was held in 2011 at Dartmouth College and the presentations are reprinted in the journal, including the photo essay, "The Dartmouth Brut: Conservation, Authenticity, Dissemination" co-authored by Deborah Howe, Collections Conservator, with Professor Warren. If you have ever wondered about the considerations that go into conservation treatments this essay highlights the types of problems to be solved and the collaborative decision making that guides the conservator's treatment solution.

The article in the Fall 2014 issue may be found at: http://muse.jhu.edu/journals/digital_philology/toc/dph.3.2.html

The digital copy of the Brut may be found at: http://www.dartmouth.edu/~library/digital/collections/manuscripts/ocn312771386/index.html

Congratulations Deborah!

Barb Sagraves

Head, Preservation Services