Most of us don’t know how a JPEG file differs from a TIFF file and whether or not that difference is important. This presentation will provide you with a basic understanding of how images are digitized and will familiarize you with some of the fundamental characteristics of digital image files.

 

 

Tony Hanson

I have several PDF-formatted files available for this presentation:

A Digital Image is a numeric representation, usually binary, of a two-dimensional image.

Creating Digital Images

  • Scanner
  • Camera
  • “Born Digital”

Picture Element (Pixel) – Smallest element of a digital image

  • Fewer Pixels – Low Resolution Image
  • More Pixels – High Resolution Image

Aspect Ratio – Ratio of the Width versus the Height of a digital image

Image Types

  • Black and White
    • Threshold defines what is Black or White
    • Compact – 1 bit saved for each Pixel
    • OK for lines and text
  • Gray Scale
    • Palate limited to shades of Gray (equal intensives of Red, Green and Blue)
    • 8-bit color depth – Shades of Gray in 256 steps
    • 16-bit color depth – Shades of Gray in 65,536 steps
    • Good for Black and White images (and lines and text)
  • Color
    • Each pixel is broken down into three primary colors (Red, Green and Blue)
    • 24-bit color depth – each color has a 8-bit value (0 – 255)
    • 48-bit color depth – each color has a 16-bit value (0 – 65,536)
    • Good for images (and lines and text)

Image Size vs. Resolution

  • Low resolution images cannot be enlarged
  • If you plan to enlarge images you need to scan at a higher resolution
  • Effective Resolution – Pixels per inch
    • The optimal Effective Resolution for a image to be printed is about 300 Pixels per inch
  • Histogram – shows the number of pixels for each color
    • Most useful to photographers

Digital Image File Formats

  • Lossless
    • No digital data is lost or discarded when creating the file
    • Tends to create large files
    • Used by archivists and preservationists
  • Lossy – Some digital data is lost or discarded when creating the file
    • Creates more compact files
    • Degradation in image quality is acceptable for some uses
  • TIFF – Tagged Image File Format
    • Lossless
    • Creates large files
    • Many browsers will not open TIFF images
  • PNG
    • Performs lossless (i.e., reversable) file compression
    • Supports Transparent Images
  • JPEG – Joint Photographic Experts Group
    • Lossy
    • Degree of compress can be varied
      • Less Compression -> Larger files
      • More Compression -> Smaller files
    • Does not change the resolution of the image
    • Compression is performed every time the file is saved
      • Each compression further degrades the quality of the image
    • Does not support Transparent Images
  • RAW
    • A TIFF-like format created by some cameras and camera Apps
    • Need compatible software to read the files and save as TIFF, JPEG, PNG, etc..

File Conversion

  • TIFF and PNG can be converted to JPEG
  • JPEG can be converted to TIFF and PNG
    • But the damage done when the JPEG files was initially created cannot be reversed

Metadata

  • Data about the digital image
  • Saved with the image
  • Cameras add additional information
  • You can add text and tags as well
    • Windows and Mac O/S allow you to access Metadata
    • XnViewMP is a popular free software tool for managing metadata