What are the pros and cons and which should you use?
Sounds like choosing between playing a computer game or listening to Bob Marley, sure, but ‘raster’ and ‘vector’ are actually two very different methods of reproducing graphics on screen and in a printer.
Let’s start with the most common type. ‘Raster’ simply refers to rasterized artwork, a collection of pixels – exactly like a digital photo. The image comprises a grid of bits of image data called a bitmap. Each bit is a record of 1s and 0s which tell the computer what colour the pixel is (each one is a single colour). An average photo may consist of ten million individual pixels and, because of all this information, the size of the file can be quite large.
In fact, file-size is often a good indicator of the quality of a raster image, as a larger file can contain more detail (resolution), more complex gradients or simply be printed at a larger size. This isn’t always the case, because a large, high-resolution photo can still be low quality and, conversely, a simple image—perhaps a photo of a single-coloured, smooth wall—may be fairly small in size. But it’s a good clue all the same.
Many programmes deal (almost) exclusively with raster artwork. Most photo-manipulation software, such as Photoshop, are raster-based and are designed for editing pixels. These programmes should be used for editing photos, creating digital paintings, etc. These programmes are usually fine for creating images for use on the web as one designs within a specific pixel-by-pixel area. These programmes should never be used for designing such things as logos and shouldn’t really be used for producing text, as it often comes out pixellated.
The limiting factor with raster images, along with file size, is lack of scaleability. If you increase the size of a photo, the pixels simply get larger, resulting in a pixellated image with jagged edges. The software cannot produce data which wasn’t there in the original image.
Raster file types we like at FirstpointPrint are .jpg (most common and smallest but least high quality), .tiff (uncompressed but larger), .psd (most editable, uncompressed and usually largest).
- Used for photos and digital paintings
- Potentially large files
- Usually the larger the better
- Not for logos or text
- Not scaleable without loss of quality
- .jpg, .tiff, .psd
Vector artwork is composed of a collection of mathematical equations dictating the angle and direction (vector) of digital ‘paths’, as well as any colour or stroke involved. If a path is closed, it may contain a colour or gradient. Complex artwork can be produced using vectors but this can be very time-consuming and takes a lot of skill. Vectors are usually used for simple graphics such as logos.
Programmes such as Illustrator are designed for working with vectors, although they can handle raster images as well (but can’t do a lot with them).
The key with vector graphics is scaleability. A vector logo can be blown up to any size without losing any detail or quality. The software simply recalculates it for the desired size.
Vector file types we like at FirstpointPrint are .ai (Adobe Illustrator), .eps (Encapsulated PostScript) .pdf (Acrobat, Portable Document Format), .svg (Scaleable Vector Graphics),
- Used for logos, precise graphics and text
- Usually smaller files
- .ai, .eps, .pdf, .svg
Resolution and dpi
Raster artwork must be produced at the correct resolution for its intended use. If it is reproduced at a greater size than it was created for, it will appear blurred or jagged, depending on the system producing it and the file type.
Image resizing tools will try to correct this by ‘interpolating’ data into the image to fill the gaps between the jagged pixels. However, this is only guesswork by the software and cannot replace a higher-resolution original. It will simply result in blurred, false-looking images if they’re enlarged too much.
‘dpi’ stands for dots per inch. It is the measure of how sharp an image looks when printed and has no relevance to on-screen graphics at all. The human eye can only resolve a certain amount of detail at a certain distance; 300dpi is thought to be the resolution at which two points cease to blend together when the paper is held at a usual distance from the eye and so 300dpi is considered the minimum for high-quality printing. Modern printers print images at 600dpi, so graphics are sharp even if studied closely. For large-format posters, etc., viewed at a distance, 200dpi is fine.
Vector graphics do not rely on any innate resolution or dpi and instead are created on the fly to suit the intended output system, be it on-screen or lithographic print. Thus they are as sharp as the output device can produce, at any size required. The only caveat to this is the certain effects in programmes such as Illustrator are given a ‘raster effects’ dpi setting, which may have a bearing on the output, but this is simply adjusted as needed.
- Raster artwork size and resolution set at the start
- Enlarging rasters images reduces quality
- Minimum 300dpi for high-quality print
- 200dpi for large-format
- Vector artwork fine at any size