The third post in the series deals with file types and why you should create and use files in specific formats for certain tasks. As well as helping self-printers/designers, this article will hopefully help clients understand the limitations designers have to contend with when using differing file types and the restrictions these impose.
Raster or Bit-map Images
When you take a photo on your phone or camera, the image resolution is often referred to as a certain number of megapixels and the more megapixels, the better the image quality (generally). For each image captured, there is a corresponding, finite number of pixels. This is a raster or bit-map image.
The image above, shows a typical raster image on the left and a simulation of the pixel grid (right), that also shows what would happen if the image had been enlarged from a smaller size (the image starts to break up).
These pixels – or dots – form images by arranging them in a grid. each one of which is ‘filled’ with a dot of a specific colour. When you’re looking at these images on your screen, these dots are referred to as pixels whereas in print they are still spoken of as dots (for more information, also see here).
Raster images are the most commonly used file type; they are used extensively by both web designers and photographers alike, and seen as pictures in books, magazines and other printed media everywhere.
- They are used where a wide range of colours and gradients are required, often with subtle variations and a high level of detail
- They offer more control when modifying in an editing program – such as Adobe Photoshop – because each pixel can be changed individually
- They are ideal for most web images, photographs and images – including logos – that require a great deal of detail that is not possible in a vector file
- They cannot enlarged without loss of picture quality
- File size can be huge; very large images require powerful computers to edit them and they soon take up storage space
- Layers sometimes have to merged to reduce the file size but the image then becomes more difficult to edit
A vector image is formed in an entirely different way to a raster file. Rather than consisting of a finite number of pixels, a vector graphic uses a system of points, each of which has a definite position on something similar to a graph; x and y axes co-ordinates that record positions and properties that can be specified, such as values for stroke and fill colours, thickness etc.
When a vector image is enlarged, the computer knows how to scale the co-ordinate positions, still fill with the colour specified, the stroke size etc., and creates a version where no quality is lost (see above image simulation).
With a raster image, the computer has to ‘guess’ how to make the image bigger; how to fill the ‘holes’ that enlarging creates usually leads to a loss in detail and quality.
- Vector files can be enlarged infinitely, with no loss in quality. This is particularly important in logos which may be used on a website to the size of a lorry or bigger.
- Everything contained within a vector file is ‘recorded’ mathematically, so the file size is (usually) low, certainly when compared to a corresponding raster image
- Extra layers created in a vector file can be kept rather than merged, retaining their editability.
- They do not have the same range of customisable, special effects and features that raster files have (realistic dropped shadows, photo-realistic effects etc.) and though these can be simulated, they take a great deal longer to reproduce as a vector
- They tend to be more used for images that incorporate restricted detail (more simplified, ‘poster’ effects) and not photographs that include detail or subtle shading
So which should you chose; raster or vector?
Generally, raster images should be used when you require an image to have a high level of detail – such as a photograph – and you’re either convinced it won’t be enlarged greatly and will accept the loss in quality if it is.
Vector images, however, should be used for graphics that require very much less detail – usually logos – that may need to be enlarged at some point, but need to retain their quality and clarity.
The vast majority of our logos, for instance, are generated as vector graphics so the client has the flexibility to use them as they need. We can still include a high level of detail within a vector file (see below) but this is quite a specialised process, so seek the help of a designer, especially if you’re looking for a high level of detail but in a vector format.
Finally, if you plan on creating a vector image yourself – and don’t have the necessary expertise to produce a highly detailed version – the design of the finished image needs to be kept as simple as possible.
For any more information on anything print or design related, please email us at firstname.lastname@example.org or using the comments box below.