Why Resize?

Nowadays (2007) it is common for digital photographs to be taken with a digital camera or a mobile/cell phone. The problem is that recent images are large in terms of memory required. A large image is good for medium quality printing but is bad when it comes to emailing the image, displaying it on a web-site or sending it to a mobile/cell phone.

E-mailing Images

The first problem with e-mailing an image is the time it takes to download it to a PC for viewing. A 1 Megabyte e-mail takes about 5 minutes to download for dial-up connections and about 20 seconds for a 512K ADSL connection. A 6 Megabyte e-mail - not uncommon in 2007 for sending an image - takes over 30 minutes to download for dial-up connections and 1-2 minutes for a 512K ADSL connection and about 30 seconds for a 2M ADSL connection. These figures are for the size of an e-mail - not the size of the image.

An images is a binary data file. E-mails are text data. In order to e-mail an image, it has to be encoded into a text data format. This typically increases the data size by 33%. An image of 3 Mbytes will be encoded as 4 Mbytes of e-mail!

So, for e-mailing the size needs to be kept as small as necessary. If the images is expected to be printed at a medium quality, the size will need to be large. However, if it is to be viewed on a VDU it does not have to be any larger than the size of the recipient's VDU. Many recipient's would be content with an image size of 1024x768 pixels. Reducing a modern digital camera image to 1024x768 pixels could reduce the image memory size by a whopping 90%!

Images on Websites

Images on websites are typically a lot smaller than the VDU. On the right is an image just 30 pixels wide and 20 pixels high. It only uses 718 bytes of data. That is, 10,000 of these would take the same memory as a typical image from a modern digital camera!

What is not always noticed is that an image on a web-site may be larger than it appears! When an image is placed on a web-page it is possible to set the display size. However, the actual image is downloaded before it is resized for display. It is far more efficient to resize the image on the web-server to the same size it appears on a web-page. This way, a smaller image is sent to the web-browser which means that it is seen sooner as it takes less time to send.

The actual size of images on web-pages varies a great deal. Some are very small. The image on the left is only 15x10 pixels and uses 308 bytes of memory, whereas the image on the right is 75x50 pixels and uses 2,215 bytes of memory.

Viewing Images on Mobile/Cell Phones

It is possible to view web pages on WAP phones. But, the images have to be quite small for four reasons:

  • The WAP connection may limit the size of images sent to a mobile/cell phone.
  • The mobile/cell phone may have an internal limit of the size of image it can display.
  • The download time for web access (in the UK and probably other countries) is slow. It is around 9,600bps which is slower than most dial-up modems!
  • The viewing area on a mobile/cell phone is small so there is no point sending an image at a greater resolution than can be viewed.

Mobile/Cell Phone Images

These images are generally small enough not to require resizing. The Motorola PEBL mobile phone takes photographs at a resolution of 640x480 pixels (about 0.3 MegaPixels) and to store such a photograph on the mobile phone takes around 50kBytes of phone memory. See a photograph taken with the camera in the Motorola PEBL mobile/cell phone.

Digital Camera Images

In the early days of digital cameras, the images were no bigger than 640x480 pixels - 0.3 MegaPixels. By 2001 it was not expensive to buy a digital camera that had a resolution of 1024x768 pixels - 0.8 MegaPixels. By 2003 a typical maximum resolution was 1280x960 pixels - 1.2 MegaPixels and by 2007 a resolution of 2848x2136 pixels (6.1 MegaPixels) was common.

The good news if that the cameras store their images as JPEG files and JPEG will compress an image before it is stored. As a rough guide, JPEG compression will reduce an image by around 50% and still maintain quality. It may even compress some digital images by 75% and still maintain quality. That is, a digital photograph taken on a 6 MegaPixel digital camera may only require about 1-2 Mbytes of memory when it is stored.

However, images which are over 1 Mbytes may still be too big for web-pages, mobile/cell phones or to e-mail. By resizing them to the resolution necessary for the recipient, the image may be reduced by over 90%!




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