Presentation Format for Digitized Images

Page Index

• Introduction
• Compression
• JPEG or GIF
• Thumbnails
• Examples
• Notecard Presentation Image 1
• Notecard Presentation Image 2

Introduction

Presentation images can be derived from the archival formats. JPEG format is generally recommended for online presentation due to its platform independence and small file size. GIF is best used for thumbnails. In some cases, GIF may also be suitable for online presentation of digitized images of bitonal (black and white) documents with nothing but discrete-edged print, such as published text, but wherever continuous tone (shading) is present, JPEG is preferred. Presentation images can easily be stored online as their size is considerably smaller and they are fast and easily accessible to users.

Compression

Creating smaller files is done by compressing the image files through the use of "lossy compression". Depending on the degree of compression, some information is lost, and the image cannot be perfectly reconstructed. The greater the compression, the greater the information loss. But when the purpose is only for viewing the image on a computer screen, this would be not be a strong constraint, owing to the limitations of the human eye. For instance, small color changes are perceived less accurately than small changes in brightness. For information on working with JPEG and GIF files, visit the Georgetown University Library's page on Optimizing Graphics for the Web.

JPEG or GIF

JPEG and GIF both provide smaller file size for presentation of images, but they cannot be used interchangeably. JPEG is recommended when the image is photographic, whereas GIF is recommended when the image is a line drawing. GIF files are reduced by only rendering up to 8-bit colour (256 colours) - but the image is not compressed. On the other hand, JPEG preserves colors, but the image is compressed and suffers significant distortion at high compression ratios. For a good illustration of the differences between GIF and JPEG files, visit University of Illinois at Urbana-Champaign's website What's the difference between a GIF and a JPEG?

Thumbnails

A thumbnail image is a very small image that is often presented bibliographic record or other information. It is designed to be an online form that displays quickly and allows users to determine whether they want to view a larger image. Thumbnails are usually in GIF or JPEG file formats with a resolution of 72dpi and a bit-depth of 4-bit grayscale or 8-bit color.

Examples

Shown below are two Biao Min notecards that have been saved in GIF format. These images were derived from the archival image files which were inititally scanned at a resoulution of 300dpi and saved in TIFF format. The GIF images are significantly smaller than the TIFF images in file size. Hence, even at slower net speeds, web pages can load images faster (when compared to TIFFs) and no image software is required (the images can be viewed in the browser itself).
Click on the thumbnails to view the GIF format of the digital images (the thumbnails are also in GIF format).

Notecard Presentation Image 1

Image Details: Width: 1484 pixels; Height: 878 pixels; Bit Depth: 8 bits per pixel; Color Representation: Palettized; Compression: Lempel-Ziv; Size: 580 KB

Notecard Presentation Image 2

Image Details: Width: 1480 pixels; Height: 876 pixels; Bit Depth: 8 bits per pixel; Color Representation: Palettized; Compression: Lempel-Ziv; Size: 614 KB