Vector Graphics, Raster Graphics and Text

From reading the last section you might be wondering how the text of your book manages to be resolution independent. To understand this you need to understand the difference between vector graphics and raster graphics. Raster graphics explicitly specify what colour each of their pixels are e.g. pixel one is black, pixel two is black, pixel three is black etc. Whereas, vector graphics describe what colour their pixels are e.g. pixels one to three are black. Vector graphics are, therefore, much better suited to graphics that consist of geometric shapes and patterns, as these are easy to describe, and raster graphics to images that have organic contents, like photos.

Vector graphics have two very important advantages over raster graphics (don’t worry, they also have some disadvantages). Firstly: they are usually a lot smaller. Imagine if earlier the line of pixels we were describing wasn’t three long, but three million. This can be described in our vector format thusly: pixels one to three million are black; whereas, in a raster graphic, we have to have three million different entries, one for each pixel! (Though if you were to try to convert something like a photo to a vector graphic you might actually end up with a bigger file as it might not be composed of nice easy to define shapes). Secondly: vector graphics are resolution independent. That is to say you can scale the graphic as much as you like and it will be exactly the same (except for being bigger or smaller of course!)

For example, imagine our vector graphic describes a circle that has a radius of five. Now imagine we want to scale it by a factor of two. All we do is say we have circle whose radius is five*two. When our circle is drawn on the screen it will be a perfect circle (or, at least, as perfect as is possible), just twice as big as before. Now imagine we have a raster graphic that describes the same circle. The first problem we encounter is that if our circle has a radius of five pixels, we only have a 10×10 square of pixels to define it. This is not enough to create a smooth curve, so our original circle is going to be pretty jaggedy. Now, when we scale it by a factor of two, the computer has no way of knowing that this jaggedy shape is meant to be a circle, so it won’t be able to smooth out the curves even though it now has more pixels to do it. All it can do is take its best guess at figuring out what the extra pixels should be filled with by averaging the values of pixels that have been specified, a process that will actually make the circles outline worse!

Now, the text of your novel generally starts off life as a special type of vector graphic. This is how you experience it in your word-processor, where it is made up of the characters themselves, a font, which describes how the characters should be drawn on the screen, and various levels of style information, which fine tune everything. It is possible to convert this text into either a plain vector graphic or a raster graphic, though there are only a couple of special cases where you would do this, mostly when you want to edit the text in graphics application e.g if you were wanting to include a quote from the book on your book cover (though most graphics packages will allow you to use the text and graphics directly without first converting it).

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