An explanation of raster versus pixel based graphics
During the many years I have worked as a graphic designer I have encountered the following question a lot from clients – ‘Why can’t the tiny photo on my website be blown up and used on my billboard? (or some other large format application). The reason why some images can scale up in size without looking pixelated or blurry, and others cannot, comes down to file format and the application originally used to create it.
In basic terms, a raster or bitmap graphic is one which is made of thousands of tiny squares of colour information, commonly known as pixels. The best example of this is a plain-old photograph. If you increase the magnification of any photo on your computer by around one-thousand percent, you will start seeing a mosaic pattern of coloured squares (pixels).
A raster image is great in that you can control and edit pixels to create incredible levels of detail. However on the downside, raster images become pixelated when over-enlarged. This is because there is a finite amount of pixel information to work with. When you increase the size of an image in an application like Adobe Photoshop, the computer tries to fill in all the missing gaps and generate new pixels based on the colour values of its surrounding pixels. This fabrication of pixels is known as interpolation and is the cause of an image appearing increasingly blurry as you increase the size. For example, when you capture a photo which is say, 10cm x 15cm with a resolution of 300dpi (dots/pixels per inch), and try and make that photo 100cm x 150cm – the resolution or original pixel data is heavily diluted, resulting in a sub-standard looking image.
Common raster file types include jpg, tiff, psd, png, bmp & gif.
On the other side of the equation, there are vector based graphics.
The most common application used by professionals to create a vector graphic is Adobe Illustrator. As the name suggests, a vector relies on math to draw shapes featuring points, lines and curves. There are no pixels involved so vector files can be scaled up or down infinitely without compromising quality. It is for this reason that most logos are created in vector format. Fonts are another classic example of a vector based graphic.
Common vector file types include: eps, ai & svg.
So if vector graphics can scale to any size, why do we use raster?
Good question. The answer is because the way that a vector remembers colour data means they are not practical for complex images that require precise colour editing. The level of detail, or pixel data that can be achieved with raster, unfortunately, cannot be matched by a vector.
So, the bottom line is both vector and raster based graphics serve a purpose, and for the most part, they live harmoniously side-by-side. If you are looking to create a versatile graphic that can scale to any size, go with a vector. If you need precise editability and high colour detail, then raster is the right choice. Just remember, the photo you took on your mobile phone most likely won’t look great printed on the broad side of a bus!
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