Byte Me Article 152 – Going dotty for digital images

Going Dotty for Digital Images

The response to the new [email protected] email address that was announced in last week’s column has been great and included 3 readers who wanted to know more about pictures and pixels.  This is a good topic and one which is little understood by the average computer user.  So what is a pixel and what has this got to do with pictures?

You need to keep in mind that computers are not an analogue device, they can only deal with digital information – that is information that can be represented by a string of digits which can either have a value of one or a value of zero.  Think of a light switch as opposed to a tap – where a light switch is either on or off and nothing in between however a tap can be fully on or fully off or anything in between such as a dribble or an occasional drop.

So when it comes to pictures on a PC, the world’s cleverest software people had to invent a system of recording and displaying colours and the position of these colours to mimic real life and its infinitely variable analogue spectrum of colours and positioning.  Thus the term “pixel” was adopted to represent the smallest separable dot in a digital picture which would have its own unique location and its own singular colour.  It is millions of these dots (pixels) that make up a digital image.

2013-11-23 Byte Me Article 152- Pixels

As an example – you could set about doing a painting by using the square end of a match with only one colour at a time to place thousands of small coloured squares on a piece of paper.  It would look very rough up close but at a distance the squares would tend to blend into a ‘flowing’ picture which could look very good – if you had both the talent and patience of an artist.  The same thing happens with all digital pictures – that is the picture is divided up into a fine grid of individual pixels with their own singular colours.

So how big is a pixel and wouldn’t the size of these pixels effect the quality of the picture?  Absolutely!  As computers have become more powerful the average size of a pixel in a digital picture has become smaller (a finer grid) and thus more numerous.  Even though Australia works in a metric system there are still so many “picture related” measurements that are in imperial units of measure.  E.g. we still talk about photo frames in inches such as 6×4 and 8×10.  Such is the case also with pixel size where it is quoted in dots per inch (DPI) and also often referred to as the ‘resolution’.

From the above it can be seen that when we talk about the basic quality of a digital photo then a higher DPI photo (higher resolution) will contain more information and be more ‘true to life’ than a lower DPI photo.  On a printed sheet of paper and without a magnifying lens the human eye can start to see the individual pixels (often referred to graininess) if the resolution is less than around 300 DPI.

If we are talking about printing typical 6×4 photos at 300DPI then let’s do some math.  Every inch of the paper is divided up into a grid of 300 lines horizontally and 300 lines vertically (there is a pixel at every line intersection).  This means around 90,000 pixels in every square inch – or around 2.1 million (2.1 megapixel) on our 6×4 photo.  This sounds like a lot of pixels however modern digital cameras take a 10 megapixel photo as just an average quality shot nowadays!

Next week we will look at what resolutions to use when taking photos with a camera or when scanning a photo or a letter as well as how to email these images without running into trouble with file sizes. 

Future Byte Me topics can be emailed to [email protected] and Bruce is contactable at Kerr Solutions, 205 Musgrave Street or on 49 222 400.

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