Understanding Image Formats Part 2

by Bruce Hitchen

File Formats and How They are Used

In my last blog post, I explained the difference between Vector and Raster images. This post is dedicated to explaining the most common file formats and how they are used and why to choose one format over another. Basically, all image formats are different ways of compressing an image so that the file size of the image is manageable. The trick is to compress the image so that its file size is low while retaining as much quality of the original image as possible.

There are two types of image compression; “lossy” and “lossless”. Lossless compression discards no information so it looks for efficient ways to represent an image without making compromises in its accuracy. Lossy compression formats accept some degradation in the image in order to achieve smaller file size. While the number of compression formats seems endless, some formats you may be familiar with are JPEG, TIFF, PNG, GIF, BMP, or RAW. Here are some of the attributes of the most common formats:

Bitmap (.bmp)

• A common image format in Windows programs
• Good for printing images
• No loss in quality
• Millions of colours
• Large File Size
• May not be recognized in other operating systems

TIFF – Tagged Image File Format (.tif)

• An image format recognized by most computer systems.
• Preferred by many professional printers due to its high quality.
• No loss in quality
• Millions of colours
• Supports transparency (Alpha Channel)
• Can be used in various operating systems: Windows, Mac, and Linux
• Large File Size
• Not compatible with all windows applications

JPEG – Joint Photographic Experts Group (.jpg .jpeg)

• The most common image format for use on the Web and in digital cameras.
• Small file sizes
• Information discarded is often not visible to the human eye.
• When a saving as a JPEG, most editing programs allow the choice of compression ratio. More compression = smaller file size but poorer image quality.
• Millions of colours
• Recognized by most computers and most applications
• Format used by many digital cameras
• Common format for pictures in websites
• Some loss in quality
• Does not support transparency
• Doesn’t look as good when used in professional quality printing

GIF – Graphic Interchange Format (.gif)

• The first image format to be used on the Web, GIFs are good for images with a limited number of colours, such as logos, graphs, and drawings, but are not as good for photos or complex images.
• No loss in quality
• Small file size due to method of storing image information
• Supports transparent areas
• Can be animated
• Limited to a maximum of 256 colours

PNG – Portable Network Graphic (.png)

• An image format created to be a license free improvement on GIF.
• Small file size due to compression
• No loss in quality
• Supports alpha channels allowing full and partial transparency
• Files with millions of colours are often larger than similar JPEG images
• Not yet supported in some programs
• Doesn’t support animation like GIF