One of the biggest hurdles for many clients is getting their
head around image resolution. It can be quite a difficult
subject to come to terms with. So it’s unfortunate that most
images supplied by clients are JPGs, the most common type
of image subject to resolution.
Images can be categorised into two groups -- raster (sometimes referred to as bitmap) and
vector. The following will help you understand the differences between the two type of
images, and how they should be used in your project.
Raster (bitmap) images
A raster image is made up of many tiny coloured squares, much like a tile mosaic. The
number of dots, or pixels, in a raster image can number in the millions. The more pixels in an
image, the more true to life the image appears, but the larger the file size becomes. If we
zoom in closer and closer on an image, using our computer, we will eventually see the
individual coloured pixels that make up the overall image.
The most common type of raster image is a photograph or scanned image. The most
common file types for raster images are JPG, TIF, PNG, GIF and BMP. The main program for
working with raster images is Photoshop, which has its own file type called PSD.
The main advantage of raster images is the rich detail they provide, as can be seen
in photographs. The more pixels there are in a given area, the richer that detail will be. The
number of pixels in an image is referred to as its resolution, and is measured in pixels per
inch (ppi). As a guide to the resolution required for different print uses:
The main disadvantage of raster images is the reduction in quality when images
are enlarged. When an image is made bigger, if the number of pixels are kept the same,
they must spread out to cover the larger area; this causes the density of pixels (the
resolution) to be reduced.
Alternatively, if the resolution is to be kept the same, the computer software must create
extra pixels to fill the gaps as the original pixels spread out. How does the computer know
what colour to make those new pixels? It guesses! Granted, the software has been written to
make very clever guesses, but either way, it results in a blurrier image than the original.
Many people understand that images can’t be enlarged without reducing the quality. But not
so many know that shrinking images can have the same effect. When drastically reducing the
size of an image, the software will either increase the resolution (density) of the pixels to a
huge degree, or it will throw pixels away -- again by guessing. You might think that keeping
the pixels and increasing the resolution is more sensible (which it usually is) but at some
stage, the image will end up being printed -- and at that point, the printing press will have to
throw away the extra pixels, because printing presses can print only so many dots of ink per
inch of paper (dpi).
The other disadvantage of raster images is the large file size. As cameras become better and
better, and can save more pixels in an image, so the file sizes increase.
A vector image is made up of lines, the shape and position of which are determined
mathematically. Vector images have a more basic appearance, and even complex gradients
of colour in a vector image appear simple compared to raster images. If we zoom in closer
and closer on a vector image, we will only ever see the lines that describe shapes. The
mathematics used to describe vector shapes allows the file size of even complex images to
be relatively small.
Vector images are commonplace, the most everyday usage being in computer fonts. Vectors
images are also commonly used to create logos and other branding elements. The most
common vector file types are EPS, PDF and AI, the latter being the file type used by
Illustrator, a common program for working with vector images.
The main advantage of vector images is that they are infinitely scalable, due to
mathematical equations describing the shapes. This explains why fonts can be increased to
any size and still have crisp edges, and allows a company logo the size of a stamp to be
scaled up to the size of a building, with no loss of quality.
The main disadvantage of vector images is the reduced richness of image that is
achievable, compared to raster images. However, vector images have a style of their own
that works well in many applications.