Have you ever tried to explain to a client why the JPEG image of their logo can’t be blown up in size so that it is big enough to display on their advertising banner? Does your client develop a look of utter confusion once you start talking about raster and vector images? Well the purpose of this article is to delve into the attributes of each and simplify their differences.
Raster / Bitmap Images
Raster or Bitmap images are created from a series of tiny squares of colour known as pixels. A raster graphic can contain hundreds or thousands of individual pixels, with each pixel assigned a colour and then arranged in a pattern to form the image.
The most common type of raster image is a photograph with Adobe Photoshop the most popular software for creating and editing raster files. This is mainly because Photoshop primarily works in raster. When it comes to saving files in raster format, JPG, BMP, PSD, PNG and GIF are the preferred file format extensions used.
Advantages of Raster Images
• A high quality raster image is usually around 300 dpi. What does the dpi stand for? It stands for ‘dots per inch.’ So if an image is 300 dpi, this means there would be 300 individual dots ( or pixels) inside the 1 inch by 1 inch square. The higher the dpi, the more detailed the image will be.
• As raster images are made up of so many individual pixels, it allows the image to be modified one pixel at a time. An example of this would be when graphic designers edit or ‘photoshop’ an image of a fashion model by removing any blemishes on their face so that they look perfect.
Disadvantages of Raster Images
• When a raster image is enlarged it causes the image to become blurry. This is because there is a finite number of pixels in a raster image, so when it is enlarged the computer has to guess what specific colours should fill the gaps. Even though the computer can make a good guess at this colour it is unable to find the exact colours required to insert in the gaps left by the expanding image. It is this process that results in the image losing its clarity.
• The other main drawback when using raster graphics is that the file size tends to be large. Think of it like this, if the image quality is 300 dpi as mentioned previously, then each 1 inch by 1 inch square will have 300 individual points of colour information that the computer has to remember. So when the image is bigger, say 20 inches wide by 12 inches high, thats (20 x 12) x 300 which equals 72,000 individual points of information that the computer needs to process and load. This is a very important factor in web design as websites need to be loaded as quickly as possible. Therefore whenever you are working with images for the web it is important that the image is no larger than 72 dpi (or in some cases 150 dpi). This decreases the file size to a more manageable size for web browsers to load while also keeping the quality of the image.
The image below shows the original raster image on the left, with an enlarged version of the same image on the right. As you can see the enlargement has caused the image to have a jagged outline and blurry appearance.
Vector graphics use mathematical equations to draw up points, lines and curves. So instead of containing hundreds of individual points like a raster image, a vector contains only four points (it is possible to add more points to a vector image if necessary) to make up the four corners of the image. The computer then connected these points in a similar way to how “connect the dots” drawing books work. It does this using the mathematical equations which fills in all of the missing information between the four dots, resulting in the complete image.
The most common type of vector image are logos and fonts. Print work such as brochures and posters should also be designed as vector images using vector based drawing software such as Adobe Illustrator and saved with a file extension of either AI, EPS or PDF.
Advantages of Vector Images
• The main advantage of vector images is that they can be enlarged or shrunk to whatever size necessary without losing quality and becoming blurry. This is because the only part of the image that is expanding the the distance between the four dots. The computer the uses the mathematical equations to connect the four dots no matter how far apart they are.
• Vector images only consist of several points, compared to the hundreds or thousands of points in a raster image. Therefore the file size of vector images is considerably smaller as there is much less information to be stored. This means that vector images such as logos can be loaded very quickly with a much faster processing speed.
• When working with a vector image, the file format it is saved in allows it to be easily editable. Say for example you wanted to change a red coloured part of a vector image to blue, it can be changed with consummate ease. This is because the colours are all saved on different layers and it is simply a case of choosing the desired colour and filling the desired area with this new colour. In contrast, raster images file types are flattened when saved so any editing of colours takes more skill and time to alter the image.
Disadvantages of Vector Images
• The fact that vectors are made up of just several points connected by mathematical equations, means they are unable to be used for images that require intricate detail. In contrast, a raster image like a photograph can be a very detailed and complex image with thousands of varying colours and shades. Vector images don’t have this capability which is why they are primarily used for simple images such as logos. It is possible to add colour gradients and other effects to vector images to add more variation to the image but it is impossible for them to match the detail of raster images due to their lack of individual pixels.
As the image below illustrates, the enlarged vector image on the right still maintains its crisp edges and high quality.
So what are the key points to remember.;
Raster Images | Example: Photographs | Much greater detail but becoming blurry when enlarged
Vector Images | Example: Logos | Can be scaled to any size without losing clarrity but don’t have the same detail as raster images.
The image below shows the enlarged raster and vector images side by side so that the comparison between the two can be easily seen.
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