Introduction to digital image files
What are JPG, GIF, TIF, PNG, BMP and the many other file types you may come across? What do the file extensions mean and how do you choose?
File formats are used as a way of encoding information for storage in a computer file. Digital image formats have encoding which makes them suitable for different purposes such as for email, Web or screen presentations or for high quality print. The choices are not as complex as they may appear.
Image files can be separated into two main categories. These are VECTOR and BITMAP images.
Vector images tend to be cartoon-like images or diagrams which are drawn using computer graphics tools. Bitmap or raster files are made up of pixels and tend to be photographic images. Vector images maintain original quality when they are enlarged. However bitmap files lose quality significantly when they are enlarged. This can be seen when you zoom into a bitmap image. The individual pixels tend to blur or break up as you go in closer.
This cartoon image of a car is a good example of a vector image. A pen tool has been used to draw lines and shapes. Solid fills of colour were then added to complete the final image.
This photographic image of daisies is a good example of a bitmap (sometimes refered to as ‘raster’) graphics file. Bitmap files consist of an array of pixels of thousands of different colours, which is why they are typically used for the representation of photographic images.
In some situations vector tools and formats are more appropriate for the task in hand, while in other situations working with bitmap tools and formats is best practice. Sometimes both formats may come together.
The reason for the number of different file formats is because of the need for compression mainly due to the requirement to show images on computer screens.
Image files can be quite large, and larger files mean more disk space being used and ultimately slower downloads.
COMPRESSION is the term used to describe ways of reducing the size of a file. Compression schemes can be LOSSLESS (meaning image quality is maintained) or LOSSY (where image quality is degraded).
Digital image files vary in the number of colours they can contain. If an image has few colours, some file types can use this as a way of reducing file size.