Types and Uses of File Formats

Bitmap

Features

Bitmap graphics are also known as raster graphics. This file format is most commonly used on the web and on computers. Bitmap graphics are made up of pixels which specify what colours will be created. An image could contain hundreds of thousands of pixels. An advantage of a bitmap graphic would be that the graphic is displayed realistically. Another advantage would be that bitmap images are very easy to modify.

Limitations

One of the main disadvantages of bitmap graphics is that they don’t scale very well and the picture quality decreases once the image is enlarged. Bitmap files are also a large file type when saved to the disk and the higher the quality of image the larger the file size.

This is an example of a bitmap image that has been enlarged.

Vector

Features

A vector graphic is made up of lines, points and curves to create complex objects. Vector graphics are created through a series of mathematical calculations and this allows the graphic to be re-sized without losing any of its quality. Another positive feature of vector graphics is that they don’t take up much disk space when saved to a hard disk.

Limitations

Vector graphics use up a lot of the computer’s memory and this means that it could take longer to show up on the screen. Another limitation of vector graphics would be that they don’t appear to look as realistic as bitmap images and that’s why they are not a good choice for displaying photographs.

This is an example of a vector image and as you can see the image does not look very realistic.

JPEG

Features

JPEG (Joint Photographic Experts Group) is a method of compressing photograph images. JPEG is used to compress full-colour (24 bit) or grey-scale digital images of “neutral” (real world) scenes. JPEG is most commonly used to compress and store images on a web page. An advantage of a JPEG file is that it can manage many colours aswell as keeping the file size as small as possible. Most good websites support JPEG.

Limitations

Due to the lossy algorithm that JPEG files use you will lose some of the detail when converting different formats into JPEG. The edges of lines of a vector image may get blurred out when using JPEG.

GIF

Features

GIF (Graphics Interchange Format) is a bitmap image format. This format is one of most widely used on the web. GIF supports up to 8 bits per pixel which allows a single image to reference a palette of up to 256 different distinct colours. GIF images can have transparency for use over a website. Their file size in generally small and you would be able to view and download them quite quickly.

Limitations

The quality of the image may decrease if you are using too many colours as GIF files have a 256 colour limit.

Below is the same image saved in different file formats and this has resulted in the size of the files varying.

BMP – 29,404 KB

This image has been saved through Adobe Photoshop and has a relatively large file size.

JPEG – 561 KB

This file size is smaller than the others which means that it could be downloaded and viewed quicker.

PNG – 5695 KB

This file type was designed for transferring images on to the web so the file type would be quite small for quick transfer rates.

This file format uses no compression at all therefore resulting in a large file size.

File Format Compression Techniques

File compression is reducing the size of a file in order to save disk space or for quicker transfer rates. There are a few different file format compression techniques.

Lossy Compression

Lossy Compression is when some data is lost to enable the file size to be reduced dramatically. Lossy compression is most commonly used to compress multimedia data such as video, audio and images. Lossy compression reduces the file size a lot more than lossless compression would.

Lossless Compression

Lossless compression is when you extract the exact original data from the compressed file without losing any of its contents. Lossless compression is very popular in zip files and you would have to use a program such as WinZip to extract the data from the compressed file.

This image is comparing the difference between lossy compression and lossless compression. It shows us that the lossy compressed file is smaller than the lossless compressed file but when they are both restored the lossy compressed file loses some of its data whereas the lossless compressed file doesn't.

This image is what the program WinZip looks like. Using this program you select which file you would like to extract on to your computer and where you would like to save it.

Resolution and Colour Depth

Resolution

Image resolution can also be known as dpi (dots per inch) or ppi (pixels per inch). Image resolution describes the detail of an image and the number of pixels that it can contain. The more pixels in an image, the better the quality but it might also increase the file size. An image that is enlarged will lose its quality if it is created by pixels.

This is what happens when a graphic that is created by pixels is enlarged. It becomes pixelated and loses its quality.

Colour Depth

Colour depth or bit depth is used to describe the maximum amount of colours that are used within an image. Bit depth is used to describe the colour of a single pixel and each pixel is represented by one 8-bit byte. The 8 bits are divided up between red, green and blue, normally three bits for red, three bits for green and two bits for blue.