Taking a closer look at GIFs, JPEGs, and PICTs

October 15, 1995,
Maia Jin
ILS 603, Image Databases, Professor Besser
Individual Paper

Introduction

The idea for this paper stemmed from two imaging projects I am doing for Professor Besser's T-shirt Database Project and the second is my project with the Museum Informatics Project (MIP) at UC Berkeley.

Our directions for the T-shirt Database assigment asked us to scan 30 t-shirts, saving them first as PICT files, and then first converting these files to GIF thumbnails and also converting the PICT files to JPEGs (at 72 ppi). While I was in the midst of this project, I began working with Natalie Munn at MIP. For my project there, Natalie needed to send me image files in order for me to make GIF thumbnails and JPEG in-line images. I needed to determine the best way for Natalie to send the original images to me and my initial impression was that PICT files were the only way to go, but Natalie assured me that JPEGs would be just as good. This was confusing to me, as I had assumed that GIFs should be made out of PICT files and not JPEG files for the best quality, since JPEG uses a Lossy Compression algorithum (more on this later). To clear up this confusion, I prepared three studies to try and determine if it made a difference whether color files are converted into GIFs from JPEGs or from PICTs. I also compared GIF thumbnails with JPEG thumbnails in terms of the quality of the color images. Finally, I looked at the best way to make thumbnail images of scanned text, as some of the documents from Natalie were going to be text on paper.

  1. Study One: Conversion of JPEGs or PICTs to GIF format - is there a difference to your eye? Does the level of compression make a difference?

  2. Study Two: GIF vs. JPEG, which should you choose for full size color pictures?

  3. Study Three: Bitmapped vs. Grayscale, which should you choose for Text Documents?

Standards for these Studies

The images used in this paper were scanned on the HP ScanJet Digitizer in the SILS lab and I used Photoshop to convert the files into different formats.[1] All images were scanned at either 150 ppi or 72 ppi, and all thumbnails were given a standard height of 150 pixels.

Brief Overview on a few Digital Image Formats [2]

The factors that affect the quality for a digital image include resolution, size, color depth, compression, contrast, brightness, and color range. One must always consider the quality of the original image, and the end use of the image which affects the format that is chosen to store the image. This project was designed to look at PICTs, GIFs, and JPEGs (which isn't really a format, see below), but there are many others, such as TIFF (the optimum file format for almost any project, according to Chamberlain, (which is good for layers and channels, but generally needs to be converted into another format for use in desktop publishing projects), and PCX(first used only by PCs, but now available on cross-platform programs such as Photoshop and PageMaker).

PICT is the default file format for almost any Macintosh application, but is not necessarily the best option. PICT files can not be read in HTML documents, and "are prone to corruption" according to Chamberlain. GIF files are the standard file format for storing 1- to 8-bit images, and can be read on Macs and PCs.

Loss Less vs. Lossy Compression

In order to make the size of image files smaller, there are two types of compression: Lossy and Loss Less. A Loss Less compression algorithum does not remove any of the data when it compresses the image data, whereas a Lossy compression algorithum does discard some of this data in order to compress at a higher ratio. JPEG (Joint Photographic Experts Group) is actually not a file format, but a compression technique, which is very popular for its good image quality. JPEG uses a Lossy compression scheme, however, the human eye can not generally detect the kinds of color data that the JPEG images are missing. A JPEG compressed image can be stored as a variety of formats, but you can also store files as "JPEGs" and that is what we are looking at in this project, I am considering JPEGs, GIFs, and PICTs, "types" of files.

[1] For more information on how to convert your own images from PICT to GIF thumbnails, see the T-shirt Image Database Thumbnails documentation

[2] Most of the information in this section was gathered from John Weise's guide, Preparing Quality Images for Computer Networks and The MacAuthority Issue on Understanding image file formats by Bryan Chamberlain. For a discussion of data transfer time and digital images, see Weiss' guide and for a description of the various types of file formats and explanation of bit depth, see Chamberlain's article.


Forward to see Study One

Fast forward to the Conclusion

Maia Jin, mjin@umich.edu, October 1995