Quality assessments can be extended beyond capture requirements to the presentation and timeliness of delivery options. We began our benchmarking for conversion with the attributes of the source documents. We begin our benchmarking for display with the attributes of the digital images.
I believe that all researchers in their heart of hearts expect three things from displayed digital images: (I) they want the full-size image to be presented onscreen; (2) they expect legibility and adequate color rendering; and (3) they want images to be displayed quickly. Of course they want lots of other things, too, such as the means to manipulate, annotate, and compare images, and for text-based material, they want to be able to conduct key word searches across the images. But for the moment, let's just consider those three requirements: full image, full detail and tonal reproduction, and quick display.
Unfortunately, for many categories of documents, satisfying all three criteria at
once will be a problem, given the limitations of screen design, computing capabilities, and network speeds. Benchmarking screen display must take all these variables into consideration and the attributes of the digital images themselves as user expectations are weighed one against the other. We are just beginning to investigate this interrelationship at Cornell, and although our findings are still tentative and not broadly confirmed through experimentation, I'm convinced that display benchmarking will offer the same advantages as conversion benchmarking to research institutions that are beginning to make their materials available electronically.
Now for the good news: it is easy to display the complete image and it is possible to display it quickly. It is easy to ensure screen legibility-in fact, intensive scrutiny of highly detailed information is facilitated on-screen. Color fidelity is a little more difficult to deliver, but progress is occurring on that front.
Now for the not-so-good news: given common desktop computer configurations, it may not be possible to deliver full 24-bit color to the screen-the monitor may have the native capability but not enough video memory, or its refresh rate cannot sustain a nonflickering image. The complete image that is quickly displayed may not be legible. A highly detailed image may take a long time to deliver, and only a small percent of it will be seen at any given time. You may call up a photograph of Yul Brynner only to discover you have landed somewhere on his bald pate.
Benchmarking will allow you to predict in advance the pros and cons of digital image display. Conflicts between legibility and completeness, between timeliness and detail, can be identified and compromises developed. Benchmarking allows you to predetermine a set process for delivering images of uniform size and content and to assess how well that process will accommodate other document types. Scaling to 72 dpi and adding 3 bits of gray may be a good choice for technical reports produced at 10-point type and above but will be totally inadequate for delivering digital renderings of full-size newspapers.
To illustrate benchmarking as it applies to display, consider the first two user expectations: complete display and legibility. We expect printed facsimiles produced from digital images to look very similar to the original. They should be the same size, preserve the layout, and convey detail and tonal information that is faithful to the original. Many readers assume that the digital image on-screen can also be the same, that if the page were correctly converted, it could be brought up at approximately the same size and with the same level of detail as the original. It is certainly possible to scale the image to be the same size as the original document, but most likely the information contained therein will not be legible.
If the scanned image's dpi does not equal the screen dpi, then the image onscreen will appear either larger or smaller than the original document's size. Because scanning dpi most often exceeds the screen dpi, the image will appear larger on the screen-and chances are that not all of it will be represented at once. This is because monitors have a limited number of pixels that can be displayed both
horizontally and vertically. If the number of pixels in the image exceeds those of the screen and if the scanning dpi is higher, the image will be enlarged on the screen and will not be completely presented.
The problems of presenting completeness, detail, and native size are more pronounced in on-screen display than in printing. In the latter, very high printing resolutions are possible, and the total number of dots that can be laid down for a given image is great, enabling the creation of facsimiles that are the same size- and often with the same detail-as the original.
The limited pixel dimensions and dpi of monitors can be both a strength and a weakness. On the plus side, detail can be presented more legibly and without the aid of a microscope, which, for those conducting extensive textual analysis, may represent a major improvement over reviewing the source documents themselves. For instance, papyrologists can rely on monitors to provide the enlarged view of fragment details required in their study. When the original documents themselves are examined, they are typically viewed under a microscope at 4 to 10 ³ magnification. Art historians can zoom in on high-resolution images to enlarge details or to examine brush strokes that convey different surfaces and materials. On the downside, because the screen dpi is often exceeded by the scanning dpi and because screens have very limited pixel dimensions, many documents cannot be fully displayed if legibility must be conveyed. This conflict between overall size and level of detail is most apparent when dealing with oversized material, but it also affects a surprisingly large percentage of normal-sized documents as well.
Consider the physical limitations of computer monitors: typical monitors offer resolutions from 640 ³ 480 at the low end to 1600 ³ 1200 at the high end. The lowest level SVGA monitor offers the possibility of displaying material at 1024 ³ 768. These numbers, known as the pixel matrix, refer to the number of horizontal by vertical pixels painted on the screen when an image appears.
In product literature, monitor resolutions are often given in dpi, which can range from 60 to 120 depending on the screen width and horizontal pixel dimension. The screen dpi can be a misleading representation of a monitor's quality and performance. For example, when SVGA resolution is used on a 14", 17", and 21" monitor, the screen dpi decreases as screen size increases. We might intuitively expect image resolution to increase, not decrease, with the size of the monitor. In reality, the same amount of an image-and level of detail-would be displayed on all three monitors set to the same pixel dimensions. The only difference would be that the image displayed on the 21" monitor would appear enlarged compared to the same image displayed on the 17" and 14" monitors.
The pixel matrix of a monitor limits the number of pixels of a digital image that can be displayed at any one time. And if there is insufficient video memory, you will also be limited to how much gray or color information can be supported at any pixel dimension. For instance, while the three-year-old 14" SVGA monitor on my desk supports a 1024 ³ 768 display resolution, it came bundled with half a
megabyte of video memory. It cannot display an 8-bit grayscale image at that resolution and it cannot display a 24-bit color image at all, even if it is set at the lowest resolution of 640 ³ 480. If I increased its VRAM, I would be bothered by an annoying flicker, because the monitor's refresh rate is not great enough to support a stable image on-screen at higher resolutions. It is not coincidental that while the most basic SVGA monitors can support a pixel matrix of 1024 ³ 768, most of them come packaged with the monitor set at a resolution of 800 ³ 600. As others have noted, network speeds and the limitations of graphical user interfaces will also profoundly affect user satisfaction with on-screen presentation of digital images.
So How Does Display Benchmarking Work?
Consider the brittle book and how best to display it. Recall that it may contain font sizes at 1 mm and above, so we have scanned each page at 600 dpi, bitonal mode. Let's assume that the typical page averages 4" ³ 6" in size. The pixel matrix of this image will be 4³ 600 by 6 ³ 600, or 2400 ³ 3600-far above any monitor pixel matrix currently available. Now if I want to display that image at its full scanning resolution on my monitor, set to the default resolution of 800 ³ 600, it should be obvious to many of you that I will be showing only a small portion of that image- approximately 5% of it will appear on the screen. Let's suppose I went out and purchased a $2,500 monitor that offered a resolution of 1600 ³ 1200. I'd still only be able to display less than a fourth of that image at any one time.
Obviously for most access purposes, this display would be unacceptable. It requires too much scrolling or zooming out to study the image. If it is an absolute requirement that the full image be displayed with all details fully rendered, I'd suggest converting only items whose smallest significant detail represents nothing smaller than one third of 1% of the total document surface. This means that if you had a document with a one-millimeter-high character that was scanned at 600 dpi and you wanted to display the full document at its scanning resolution on a 1024 ³ 768 monitor, the document's physical dimensions could not exceed 1.7" (horizontal) ³ 1.3" (vertical). This document size may work well for items such as papyri, which are relatively small, at least as they have survived to the present. It also works well for items that are physically large and contain large-sized features, such as posters that are meant to be viewed from a distance. If the smallest detail on the poster measured 1", the poster could be as large as 42" ³ 32" and still be fully displayed with all detail intact.
Most images will have to be scaled down from their scanning resolutions for onscreen access, and this can occur a number of ways. Let's first consider full display on the monitor, and then consider legibility. In order to display the full image on a given monitor, the image pixel matrix must be reduced to fit within the monitor's pixel dimensions. The image is scaled by setting one of its pixel matrixes to the corresponding pixel dimension of the monitor.
To fit the complete page image from our brittle book on a monitor set at 800 ³ 600, we would scale the vertical dimension of our image to 600; the horizontal dimension would be 400 to preserve the aspect ratio of the original. By reducing the 2400 ³ 3600 pixel image to 400 ³ 600, we will have discarded 97% of the information in the original. The advantages to doing this are several: it facilitates browsing by displaying the full image, and it decreases file size, which in turn decreases the transmission time. The downside should also be obvious. There will be a major decrease in image quality as a significant number of pixels are discarded. In other words, the image can be fully displayed, but the information contained in that image may not be legible. To determine whether that information will be useful, we can turn to the use of benchmarking formulas for legible display.
Here are the benchmarking resolution formulas for scaling bitonal and grayscale images for on-screen display:
Note: Recall that in the benchmarking resolution formulas for conversion, dpi refers to the scanning resolution. In the scaling formulas, dpi refers to the image dpi (not to be confused with the monitor's dpi).
Let's return to the example of our 4" ³ 6" brittle page. If we assume that we need to be able to read the 1-mm-high character but that it doesn't have to be fully rendered, then we set our QI requirement at 3.6, which should ensure legibility of characters in context. We can use the benchmarking formula to predict the scaled image dpi:
The image could be fully displayed with minimal legibility on a 120 dpi monitor. The pixel dimensions for the scaled image would be 120 ³ 4 by 120 ³ 6, or 480 ³ 720. This full image could be viewed on SVGA monitors set at 1024 ³ 768 or above; slightly more than 80% of it could be viewed on my monitor set at 800 ³ 600.
We can also use this formula to determine a preset scaling dpi for a group of documents to be conveyed to a particular clientele. Consider a scenario in which your primary users have access to monitors that can effectively support an 800 ³ 600 resolution. We could decide whether the user population would be satisfied with receiving only 80% of the document if it meant that they could read the smallest type, which may occur only in footnotes. If your users are more interested in quick browsing, you might want to benchmark against the body of the text
rather than the smallest typed character. For instance, if the main text were in 12-point type and the smallest lowercase e measured 1.6 mm in height, then our sample page could be sent to the screen with a QI of 3.6 at a pixel dimension of 300 ³ 450, or an image dpi of 75-well within the capabilities of the 800 ³ 600 monitor.
You can also benchmark the time it will take to deliver this image to the screen. If your clientele are connected via ethernet, this image (with 3 bits of gray added to smooth out rough edges of characters and improve legibility) could be sent to the desktop in less than a second-providing readers with full display of the document, legibility of the main text, and a timely delivery. If your readers are connected to the ethernet via a 9600-baud modem, however, the image will take 42 seconds to be delivered. If the footnotes must be readable, the full text cannot be displayed on-screen and the time it will take to retrieve the image will increase. Benchmarking allows you to identify these variables and consider the trade-offs or compromises associated with optimizing any one of them.