If you’ve been using Photoshop, Illustrator or InDesign for any length of time you’ll, no doubt, have heard the terms vectors and rasters (or pixels). And, equally likely, been told that vectors are ‘resolution independent’ whereas raster images have a fixed resolution. But what does that actually mean in terms of our artwork and how it ends up on our page?
Let’s start by examining ‘resolution independence’. Unlike a raster-based file, an artwork that is solely vector-based doesn’t contain any pixels – hence no resolution. You can think of vectors as a mathematical description of a shape based on x and y co-ordinates.
Pixels are only used to ‘output’ our vector shape. When we view the file on screen the ‘output’ is the vectors rendering to the pixels in our screen – zoom in and the vectors are re-rendered at the new scale – hence no pixellation. This is because vectors will be output at ‘device resolution’. Below we can see the same stroke as both vector and raster when zoomed to 3000% magnification in Illustrator.
Now, when we move over to a print environment the ‘device resolution’ is the maximum output resolution for the device we’re printing on. This is the resolution your vectors get converted to pixels at. Yes, vectors have to be converted to rasters in order to be printed – hence the term RIP (Raster Image Processor).
On a typical CTP Platesetter (Computer To Plate) this device resolution will be around 2400dpi – some devices go above this but 2400dpi is a pretty common resolution.
With a powerful enough eye-glass you will be able to see the pixels that make up your vector artwork on the page – but to the naked eye this is sufficient resolution to appear completely smooth without the need for anti-aliasing.
So, all the vector content on our page, which includes the fonts, get rendered at this ultra-high resolution. In fact, everything on the page is rendered at this resolution – the halftone dots that make up your raster-based images, and any other tinted content, are formed from a grid of these pixels.
So, in our raster-based image files the pixels get rendered to halftone dots and we achieve the illusion of continous tone and colour.
And this is also the reason why raster based files aren’t the best solution when we need to print artwork and retain crisp outlines. Artwork such as text and logos will want to be reproduced at the maximum possible output resolution.
Our raster-based file will typically be around 300ppi, well short of our device resolution of 2400dpi, which is fine for generating halftones but will not render a vector-like edge when it comes to text or logos.
Part of the problem is the anti-aliasing inherent in the file – this makes edge detail appear smoother but also renders to halftone dots when output via a CTP device. In the example below we can see a close-up of a greyscale artwork (left) and the grey pixels that form the anti-aliasing. On the right we can see how the grey tones render to halftone dots.
If we remove the anti-aliasing we end up with obvious aliasing (stepping) along the edges because the file doesn’t have sufficient resolution to render a smooth edge. Since the file is only 300ppi a single pixel in our image will map to 8x8 pixels on a our device (300 x 8 = 2400).
As we’ve already seen, our vector artwork will render any edge detail at the device resolution which is why it’s always preferable to use vector artwork for things like company logos, and especially so when these need to be reproduce at smaller sizes. The loss of edge detail is more pronounced on smaller artwork as it’s proportionately larger.
Below we can see how vector and raster artwork will render to colour halftone – notice the same issue with edge detail but now rendering to the cyan and magenta plates. Because the colour is contained with vectors (on the left) it is the vectors that form the outline. Once we’re dealing with raster-based content the edge detail comes from pixels.
Obviously vectors and rasters exist in different combinations on our page – if a photographic image (raster) is cropped within a graphic frame (vector) in InDesign then it’s the vector frame that controls the edge. Which is why our pictures look nice and crisply contained in our layout.
So, now that we’ve seen how vectors and rasters end up on the page you can start to make more informed choices about the best file formats for different types of graphic content – and you too will cringe the next time someone supplies their company logo as a jpeg!