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

The ABCs of Digital Preservation

April 27 through May 1 of this year marked the American Library Association's 5th Annual Preservation Week. To celebrate, I gave a talk for library staff titled The ABC's of Digital Preservation. The purpose of the talk was to introduce some basic digital preservation concepts, such as choosing file formats, file naming best practices, and the basics of preservation metadata. I also discussed some tools and models for managing digital materials, and tried to demystify some of the acronyms I throw around casually in meetings ("If we follow OAIS, the SIP could contain a TIF and some PREMIS and or METS, and of course the AIP will need an md5...").  You can view the slides of the talk here. 

The talk was well attended, and I got a lot of good feedback and follow-up questions. In this blog post, I'd like to address these questions, and talk about some resources to learn more about topics discussed in my presentation.

First, I'll start with some general resources...

In creating and organizing my presentation, I was inspired by the North Carolina Department of Cultural Resource's Digital Preservation Best Practices and Guidelines. It's a great resource and an excellent starting point.

Read through the archives and follow The Signal, the digital preservation blog of the Library of Congress. They highlight a lot of amazing projects covering all aspects of digital preservation.

Also, the North East Document Conservation Center has some great resources.

Now, on to your questions...

1. Is there a list of file formats defining their function that I could reference?

Yes! The Library of Congress developed this great resource for file format descriptions. The South Carolina Department of Archives and History also has a nice resource on File Formats.

2. Could you point me toward a bibliography of good guides for organizing photographs (or perhaps a workshop)?

Library of Congress's Personal Digital Archiving project has a good, succinct guide to archiving digital photos. Here's another great resource created by MLIS students at Catholic University of America for Preservation Week. Mike Ashenfelder also wrote a great blog post on The Signal answering questions about digital photo archiving (you should check out the webinar he references as well!) And finally, here is an amazing resource for digging into embedding metadata in digital images.

3. Can you tell me more about the Digital POWRR Tool Grid?

"The POWRR Tool Grid v2 provides a set of interactive views designed to help practitioners identify and select tools that they need to solve digital preservation challenges. This Grid is based on the Tool Grid first developed by the Digital POWRR Project, and combines the form and function of the original POWRR grid with the far greater coverage of tools provided by the COPTR data feed."

4. What are the current archival standard for image scans- resolution and bit depth?

The Federal Agencies Digitization Guidelines Initiative (FADGI!) has an excellent document that goes into great detail providing best practices for image scans:  Technical Guidelines for Digitizing Cultural Heritage Materials .  ALTCS, a division of ALA, has also published a helpful guide for Minimum Digitization Capture Recommendations for many types of media.

5. What are the best practices for preserving video files (ideal formats, codecs, etc.)?

Choosing a file format and codec for video preservation master files is complex. A lot depends on available resources, technological expertise, and the context in which files are created and managed. FADGI has done an excellent analysis of the issues involved in their report Digital File Formats for Digital Tape Reformatting. For best practice guidelines for creation and management of digital video, I really love the Activist's Guide to Archiving Video.

6. Bagit …. wait, what?
Check out this video from the Library of Congress. Another place to learn more is Bagit's wikipedia page. And if your ready to use BagIt, you can download it on Github.

7. What to do if a preservation copy has changed? Can you fix it?
Hopefully, you'll have multiple copies of a file, and the preservation copy can be replaced with a "good" one. For situations where this isn't the case, someone with expertise in the file format should be able to address the issue, as explained by @dericed:

8. What are some best practices or tools for data preservation?

A good place to start would be the How-to Guides and Checklists from the Digital Curation Centre. Then read through the rest of their website -- its a gold-mine of information on curating digital research! Library of Congress has recommended formats for datasets and databases, as well as a report on evaluating file formats for datasets and information on types of dataset file formats. Stanford also has a good guide covering many aspects of digital curation for research data. SCAPE is a project of the Open Preservation Foundation that develops software tools and training materials for large scale data preservation.

Written by Jenny Mullins

Merging Images in Photoshop, Part One

One of the most common problems in digitization is how to deal with an image that is too big for your camera or scanner. The simplest solution is to photograph or scan the object in separate pieces, then merge those pieces together, however this can present its own set of problems to those unfamiliar with imaging software.

In this post I will be describing my own method for merging together images. There are many other ways to accomplish these tasks, and if you have a way that works for you, I encourage you to keep using it, but also be aware of its potential pitfalls. The main benefits of my own method are the ability to quality-check your work as you go, and make simple non-destructive edits that can be changed or reversed as needed. Also, for simplicity’s sake, I will be referring to my own Mac OS based workflow for menus and keyboard shortcuts.

Here is the whole image that we’re trying to assemble, and for whatever reason, it’s been captured in two side-by-side pieces in the standard .tiff format. It is crucially important, when capturing, to make sure there is overlap between the captures. This is going to help us check how well-aligned our merging is, so the more overlap the better.

Notice how each side is wider than half of the image

Now that we’ve got our two images, open both in Adobe Photoshop and choose whichever one you want to start working on. I usually go from left to right for simplicity’s sake, so here I will be starting on the left side of the image.

In Photoshop, select the Image drop-down file menu, and select “Canvas Size…” (or use the keyboard shortcut: option+command+C). Click on the canvas width field, and double it. In the “Anchor” field, select the leftmost column of the grid so that Photoshop knows where to put the empty space.

You should be left with an image like this:

It will end up a little wider than is necessary, but it’ll be easier to trim it down after the fact than to add more space. This will now become our “master” file. Do a “Save As” at this point and designate it as such.

Next, go to the second image that we are going to merge into the master (in this case, the right side image). The next step should be familiar to most computer users: select all of the image (command+A), and copy it to the clipboard (command+C). Then go back to the master file and use paste (command+V) to add it into the image.

If you’re paying attention, you’ll obviously notice that this new image is not in the correct position. However, by looking at the Layers panel on the right side of Photoshop you’ll see that the new image is on its own layer, resting on top of the background (if you do not see the Layers panel, select the “Window” drop-down menu and enable “Layers” there). Thus we can edit it without disturbing the original “bottom” layer.

Now, with the top layer selected, click on the “Opacity” field in the Layers panel and set it to 40%. This will make the top layer semi-transparent and allow us to line it up with the bottom layer.

Then, with the Move tool selected (V), begin moving the top layer around and trying to find where it lines up. Look for any solid shapes that are shared by both images, or where the borders intersect. Letterforms provide nice clear and easily-spottable shapes, which is why I have used them in this example, but it can be anything so long as it’s shared by both images.

We’re getting there, but it’s obviously still not right. At this point, find an area of overlap and zoom in closely. Then, with both the top layer and the move tool selected, simply “nudge” the top layer into place using the arrow keys. The arrow keys will only move the layer one pixel at a time, so obviously this is for the finest level of adjustments.

Almost...

Nailed it!

Now for the final steps! In the layers panel, set the top layer’s opacity back to 100%. Then inspect the images along the borders, making sure that it looks seamless. While checking for quality be sure to zoom in and out.

At this point you can crop the image down to its original size, and it will be ready to go. However, one important piece to remember is that layered .tiffs, in addition to simply being larger files, are also not commonly supported by web or other software. What I like to do at this point is to save the “Master” file with both layers, and then create a new version for common use. The common use version will get flattened (Layer -> Flatten Image) then do a Save As in whatever format is required such as .jpeg or .pdf. This way, if any changes need to be made, we can always go back to the Master version.

And there you have it! A nice, seamless image. In the next post in this series, I will go into more detail for dealing with other problems, such as skew and mismatched backgrounds or details.

Written by Ryland Ianelli

Attachments: Book Arts Style

My last post discussed attachments with a conservation focus. Sometimes the creative possibilities are more important than the archival ones, or piercing holes in items is part of the artwork itself. In that case, here are some further ideas to attach loose items onto a page or in a book.

Just keep in mind that some of the methods below may create permanent changes in the mounted item and are not recommended for valuable, historic, or borrowed materials. Consider the long-term effects of any of these before using in your own projects.

As we saw in the last post (July 22) these were some options for attaching loose items to pages:

Paper clips:

Historically paper clips were made of metal, but those can be prone to rust over time in certain environmental conditions. Alternative shapes to the classic double loop include this dog and the circle. Those made from plastic or coated wire are also commonly available. The binder’s clip is useful for thicker materials. Both paper and binder’s clips come in a variety of sizes.

Eyelets:
 

This creates a hole in both the page and the item attached. It requires an eyelet setter such as this one often available where craft or sewing notions are sold.

Brads: 

Brads usually come in a brass colored metal, but are also available in mini sizes and in a variety of colors. Some even have a shaped head, such as a star or square, like this one.  Brads, like eyelets
and staples, create holes in both item and page.

Staples: These can be hard to remove without causing damage.

Adhesive: Paste and Glue

Adhesives are generally a permanent method of attachment, whether glues, double-sided tapes, or dry-mount adhesives.  Pastes (as opposed to glue) are generally reversible, however they often leave evidence of their application and use.

Pockets & Envelopes:

Loose pockets or envelopes can be attached by a variety of methods, like any loose item. They can also be incorporated into an album during its creation by sewing them in as part of a section or in binding of single sheets. Extensions (or guards) can be attached to the binding edge to allow for ease of use, like in this example where the purple extension is sewn in on the binding edge of the brown envelope. This album is bound with screw posts.

Slits and slots:

Photo corners:

Snaps:

 

These plastic snaps are a scrap-booking item, and operate much like a traditional sewn snap, but are attached more like a brad. A tiny hole is made in both item and page. The two parts of the snap are inserted through from front and back and “snap” together holding them in place.

Screw posts:

Screw posts are much like eyelets, as holes must be made in both item and page. They can hold thin to very thick items, and come in a variety of metals. These are usually used for binding, with extensions available to adapt the original to hold a collection of expanding material. But they can be used for putting a single item onto a page. These are often available at hardware and stationary stores.

Adhesive: Tape

All kinds of tape can work: traditional adhesive tapes such as medical paper adhesive tape, electrical, masking, double-sided, or cellophane. Newer products like colorful washi tape can also be used highlighting the attachment or construction while at the same time adding decoration. Just keep in mind that all tapes have adhesive that is extremely difficult if not impossible to remove.

Needle and Thread:

By machine or hand sewing a needle and thread can attach paper together not just cloth. Paper is not as forgiving of mistakes when sewing, but it works well for many things. I’d recommend testing the paper, needle size and type, and thread combination with the intended materials before embarking on a big project.

Buttons:

Attached with thread, buttons can also be used with a paper page to attach items. To help avoid tearing through the paper, sew a small piece of Tyvek behind the button to reinforce the attachment. Old shipping envelopes can be cut up for this purpose. The button can be used to help secure the thread attachment, sewing through the item and page, or it can be used with a buttonhole if the item can be cut into and has enough flexibility and strength to do so. Often flat buttons are most useful, especially if they will go into some kind of book or album. Stores that sell scrapbook supplies often have such flat, decorative buttons, and these are available at sewing stores as well.

Paper Frames:

Like pockets and envelopes, paper frames can be attached to a page as a means to hold a photo or bound in as pages when binding a new album. These frames can be folded to fit a photo and hold it in place without using adhesive.

This photo shows the reverse side of the frame above.












As I mentioned in my last post, with all these attachment methods remember to keep the spine and foredge of your book balanced with regard to the thickness of your added items. Doing so will help you avoid the foredge splaying out and the book not closing. When creating a new binding, stubs can be added at the spine to accommodate the addition of items over time.

I hope these suggestions prove helpful. I often collect small pieces of paper or ephemera, especially as reference for future projects. Sometimes I just toss these into a box, but perhaps I ought to create a “book of inspiration” using some of these attachment methods. Like commonplace books of the past, my book would be both personal and useful. What kinds of uses do you see for these attachments? What other attachment methods do you find helpful?

Written by Stephanie Wolff

Attachments: Conservation

This spring a scrapbook compiled by the Dartmouth Educational Association came in for some minimal repairs. This archival volume is a bound book with numbered pages. It holds a collection of loose items that have been added to the book over time. Scrapbook compilers have not always given consideration to the long-term effects of their methods, often choosing materials they have on hand. That’s likely the case with this volume, because only one of the methods (paper clip) doesn’t alter the item. Its contents, though, are in pretty good shape. Despite its overfilled pages, the book needed little more than a spine repair. I thought I would highlight some of the attachment methods used in this volume, concerns about their use, as well as share some tips for attaching loose items to pages with an eye toward longer-term preservation of those items.

Paper Clips and Brads:

On this page, both a paper clip (the triangular silver metal piece) and a brad (the brass-colored item) have been used. Both are made of metal, which can be prone to rust over time in certain environmental conditions. (Though no rust was found here.) The brad also requires a hole in both the item and the page in which to place the flanges. Modern paper clips are available in plastic or coated metal that should not rust. Be careful of causing creases in the paper if using paper clips, and choose the proper size to accommodate the thickness of the papers.

Eyelets:

Interestingly, eyelets were used in this album. I imagine they were attached using an eyelet setter like this classic Bates Eyeleter. This method requires a hole in both the page and the item attached, similar to the brad attachment. Even without moisture to create rust, it’s possible for metal to discolor or damage the paper on a facing page as a result of friction or pressure.

Staples:

 

Some of the items were stapled onto the pages. Staples create holes in the item as well as the pages and can be hard to remove without causing damage to both. They are also made of metal and therefore can rust in a humid environment.

Adhesive: Paste or Glue

It’s hard to tell whether this card was attached with paste or glue. Adhesives are a pretty permanent method of attachment. Some pastes (as opposed to glues) are often reversible, however they can leave evidence of their use.

The items that were attached to these pages used some kind of permanent alteration to the item itself (application of adhesive or holes in the paper) except for the paper clip. When considering which attachment method to use in a similar type of album these days, here are a few other options that do not require applying glue to or piercing holes into the item, though they require glue or holes in the album page itself.

Slits and slots:  

To mount a photograph or other flat item onto a book page, make cuts in the base paper (a slit) or remove a narrow slice of paper (a slot) for thicker material. By locating these cuts diagonally at the four corners, the item can be slipped in and will stay without any adhesive.

Photo corners:

Photo corners come in a variety of styles: self-adhesive, gummed, clear, black or white paper, and they can easily be made by hand. Look for ones that pass the P.A.T. test (photo activity test) or have archival qualities, such as being made from acid-free materials. To make your own corners take a thin strip of acid-free paper (about 1/4-inch), at the center of the strip fold one side up at a 45-degree angle.

Then fold the other side up to meet and match the first side. Now you have a corner. Vary the thickness of the paper strip to change the size of the corner for larger or smaller photos. Once the corner is created, use acid-free tape or adhesive to attach to the mounting page or board. These can be used for other flat items as well as photographs.

A simple way of positioning the corners is to use the photo (or a facsimile of the photo of identical size and thickness) as a placeholder. Determine the location of the photo on the page and place a clean weight on top of it to hold in place. Next slip the four corners onto the photo. Then remove one corner, add or activate the adhesive and place back onto the photo and press down to attach to the page. Then do the same to the corner diagonally opposite the first corner. This will stabilize the photo. Continue with the remaining two corners and remove the weight when done.

Pockets & Envelopes:

Pockets or envelopes (choose ones made of acid-free materials) are a great way to add loose items to a page. They can be handmade or purchased and can be paper or clear Mylar. Mylar allows a view of the item without removing it. Use an acid-free adhesive (like glue or double-sided tape) to attach pockets and envelopes to the page.

With all these methods remember to accommodate the thickness of the items added to a bound volume by balancing the binding edge thickness with the foredge thickness to avoid the foredge splaying out and the book not closing. For just a few items this shouldn’t be a problem.

In my next post, on August 19th,  I’ll explore attachment methods more suited to a creative or book arts application.

Written by Stephanie Wolff