Image Quality

Image quality determines which format you will use. The larger files use complex palettes and will render a truer image. The smaller files use fewer colors and and less image detail:
  • image.bmp---153K
  • image.gif-------53K
  • image.jpg------22K
  • image .tiff-----105K
  • image.png------141K

Bitmap images use a grid ( the bitmap or raster) of small squares known as pixels to represent images.

Each pixel is assigned a specific location and color value. When you edit an image, you are editing pixels tather than the object or shape itself.

A bitmap image is resolution-dependent: it contains a fixed number of pixels to represent its image data.Therefore, an image may become corrupted after indiscreet manipulation since each pixel will contain different or altered information.

The images to the right were scanned at 400 dpi on an HP scanner. They were saved in different formats. Images on the web can only be displayed at a resolution of 72 dpi so there is no need for a high resolution scan. The rule for scanning is

-----scan at a resolution twice the output

In this case a scan of 150 dpi would have been adequate.

Web images are designed for the web and not for print. The low 72 dpi is far below the normal 400 - 600 dpi for a printed image.

When saving an image from the web, you can crop, resize, change contrast, change color. YOU CAN NOT CHANGE THE RESOLUTION TO A HIGHER NUMBER. You can save the image at a lower resolution. Start with a good image and you will be happier with the results.

A general rule for embedded images is to keep the files as small as possible. A large file takes longer to load than a small (3K  file).  Therefore, you want to save the image in the smallest file format.

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picturebmp.bmp (156472 bytes)vista.bmp  153 KB

picturegif.gif (57646 bytes)vista.gif 53 KB

picturegif.jpg (22053 bytes)vista.jpg 22KB

picturepng.png (144419 bytes)vista.png 141 KB

picturetiff.tif (107656 bytes)vista.tiff 105 KB

picturegrey.jpg (21795 bytes)

grayscale.jpg 21 KB


These pages authored and maintained by Marcella Dawson. Revised 07/13/99. Copyright 1995 CRPC GirlTECH. All rights reserved. . Email your comments. These pages were developed through GirlTECH '96, a teacher training and student technology council program sponsored by the Center for Research on Parallel Computation (CRPC), a National Science Foundation Science and Technology Center.