Digital Imaging Tutorial

7. Presentation

Key Concepts

introduction
formats/compression
web browsers
network
scaling
monitors
image quality
guidelines

MONITOR CAPABILITIES
User satisfaction with on-screen images will depend on the capabilities of display systems. In addition to speed of delivery, users are interested in image quality (legibility and color fidelity adequate to a task); full display of images on screen; and to a lesser degree accurate representations of the dimensions of original documents. Unfortunately, given current monitor technology, meeting all these criteria at the same time is often not possible.

Screen size and pixel dimensions
In contrast to scanners and printers, current monitors offer relatively low resolution. Typical monitors support desktop settings from a low of 640 x 480 to a high of 1,600 x 1,200, referring to the number of horizontal by vertical pixels painted on the screen when an image appears.

The amount of an image that can be displayed at once depends on the relationship of the image's pixel dimensions (or dpi) to the monitor's desktop setting. The percentage of an image displayed can be increased several ways: by increasing the screen resolution and/or by decreasing the image resolution.

Increasing screen resolution. Think of the desktop setting as a camera viewfinder. As the monitor setting dimensions increase, more of an image may be viewed. The figure below illustrates the viewing area for an image at various monitor settings.

Increasing Screen Resolution: Viewing area comparison for a 100 dpi (original document size 8"x10") image displayed at different monitor settings. The pixel dimension for the image is 800 x 1,000.

Decreasing image resolution. One can also increase the amount of an image displayed by reducing the resolution of the image through scaling. This figure illustrates the relationship of a monitor's desktop setting at 800 x 600 to an image scaled to various resolutions.

Balancing Legibility and Completeness: Displayed at 200 dpi on a 800 x 600 monitor, one can only see a small portion of the page (left). At 60 dpi, the whole page is fully displayed, but at the expense of legibility (bottom-right). Scaling the image to 100 dpi offers a compromise by maintaining legibility and limiting scrolling to one dimension (top-right).

You can calculate the percent of display if you know the following variables: 1) document dimensions and image dpi, or pixel dimensions of image, and 2) desktop setting.

Calculating percentage displayed

Enter document dimensions in inches: (width) and (height) and enter resolution of image: dpi

Enter image pixel dimensions horizontal vertical

Screen size	% displayed	% width displayed	% height displayed
640 x 480
800 x 600
1024 x 768

Dimensional Fidelity
At times, it may be important to represent an image on-screen at the actual size of the original scanned document. This can only be achieved when the digital image resolution equals the monitor's resolution (dpi). The Blake Archive Project has developed a Java applet, called the Image Sizer, for representing images at the actual size of the original.

Reality Check

If representation of dimensional fidelity on-screen is important, what is the likely impact on image quality?

Image quality will not be affected, only the size of the image
Image quality will often increase, as the document will be presented at its native size with details fully presented
Image quality will often decline as the screen resolution is typically lower than the digital image resolution