What is a pixel?
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A pixel (or picture element) is the smallest complete sample of an image, however this is about as informative as saying that an atom is the smallest complete unit of matter — sort of true, but a long way short of understanding atomic physics. Luckily pixels are a little less complicated than atoms.
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Lets follow the atomic analogy and explore pixels.
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We can consider pixels, like atoms, to be the fundamental building blocks. The scientists in the audience are probably saying that atoms are neither the fundamental building block, or the smallest component of matter! True, but then neither is a pixel - in fact on a computer screen (LCD), a single pixel is made up of triads (three separate components; one red, another green and the third, blue). These three colors, often called RGB (red, green, blue) make up the full available color range (gamut). Each triad has dynamics in terms of range and intensity and we can get 'bogged down' in detail very quickly - just as we could in the subatomic world of protons, neutrons, electrons, neutrinos, quarks, and so on. So accepting that we are taking an arbitrary starting point, lets all agree for the sake of the argument, that the pixel is 'the smallest complete sample of an image'.
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All web publishing is pixels.
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If you read the section on Media Dynamics you will remember that we made the point that web publishing was about publishing data, not a physical object. Well once this data is rendered by the device a consumer is using, the data is displayed by the 'pixels' on the screen. In essence the 'pixels' are the physical outcome of your published data. Although it is all created by 'pixels', the outcomes vary enormously, just as the combination of different atoms in different forms creates infinite variation and complexity in our physical world. Yet we are subject to the physical laws that govern atoms.
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The 'pixel' laws.
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Elsewhere, we are going to cover the pixel's impact on type, color, images and other aspects of web communication. For now, we are only going to cover a single concept — the pixel as a measure.
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The pixel as a measure.
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Some measurements are more or less absolute. A meter for example is an exact length, the distance traveled by light in a vacuum during a time interval of 1/299 792 458 of a second, or to most of us, the length shown to us on a physical ruler or other measuring device. In web publishing, we still refer to absolute measurements such as inches and centimeters, however the 'pixel' as a measurement is both common and important.
Measuring using pixels is a little like counting bricks in a wall. I can tell you that a wall is 100 bricks long by 100 bricks high, but how big is it really. You know one thing from my description — there are 10,000 bricks in the wall (100 x 100). You may also be able to picture the size of the wall in your mind, after all, bricks are often a standard size and you probably expect that the gaps between them are relatively small.
Pixels are the same as bricks! Some pixels are square, some are rectangular (aspect ratio). Pixels have gaps between them (usually small). Pixels are roughly similar in size (most of the time). So as you can see, the accuracy of counting pixels gives you a quantity - for example a screen that has 1024 actual horizontal pixels, by 768 actual vertical pixels, has 786,432 actual pixels in the screen (know as native resolution). This is nearly 0.8 mega-pixels (terminology you have probably come across when buying a digital camera).
In web publishing, we generally accept that most computer displays have 72 pixels per inch (horizontal and vertical). This means that we can consider that a 72 pixel by 72 pixel image will consume one-square-inch of actual on-screen 'real estate' (roughly 6 square centimeters). There are at least two problems with this, the first is the fact that the 'real' physical dimension relies on there being 72 pixels per inch. This rule of thumb comes from the display capability of early computer monitors, most modern displays have more than 72 pixels per inch (higher pixel density). The second problem is that most modern displays do not need to use their 'native resolution', they can display content in other selected 'artificial' resolutions that may be bigger or smaller (less or more condensed), something known as 'logical pixels'.
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Our rule of thumb ...
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All of this knowledge will help you understand what happens to your published content, why certain problems occur and certain things happen. It also reinforces that you only have limited control over what the consumer actually sees. To publish effectively however we need to make assumptions and apply a 'standard', so that we are all working in the same manner.
The generally accepted 'standard' for web publishing is ... there are 72 square pixels per inch and they produce their output using the RGB (red, green and blue) color triads. This means that with some assurance, you can refer to pixels as a measurement and you can 'convert' this measurement back into absolute measures (centimeters or inches) - at least as an approximation.
If this is all new, you may need to come back to this page a few times, because as mentioned elsewhere, you are publishing pixels and pixels are important!
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