Copyright Michael Karbo, Denmark, Europe.

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    Chapter 10. Digital colour images

    If we want to understand how a digital camera works, then it is difficult to avoid terms like bytes and ram, RGB colours and JPEG compression. You don’t have to be a professor in these areas to be a good photographer but it really is an advantage if you have a feeling of what all these things are about. So even though the following description is rather theoretical, keep going!

    Figur 34. The colour system RGB consists of the three primary colours red, green and blue. By mixing them with different luminosities you get all the other colour nuances.

    If the three primary colours are blended with maximum luminosity you get the colour white, which can be seen in the middle of the image. If none of the colours are “switched on”, then the pixel is black. A screen’s background colour is already black.

    Figur 35. RGB colour blending is the result of all three primary colours ”switching on” to different degrees.

    Normally, there is a scale from 0 to 255 for every colour. The value 0 tells you, that the colour isn’t switched on and the value 255 that it is giving out light at full strength.

    RGB colours

    A camera’s colour photographs are RGB images. This means that all colours are blends of the three primary colours.

    The individual pixel’s colour comes into existence as a does of red, a dose of green and a dose of blue light. The three colours “melt” together into the colour nuance of the pixel in question.

    Let’s take a look at a camera’s LCD screen. When it isn’t displaying images, then it is black. Black is, therefore, the background colour; it is the colour, which is seen when there are no other colours to be seen (contrary to this, the background colour of paper is typically white).

    When we can see an image on a LCD screen, it is because the screen’s pixels lighten up. And every pixel, in fact, consists of three small ”lamps” – a red, a green and a blue. By switching these lamps on one by one, millions of different colour nuances can be achieved.

    Data is calculated in bytes

    The colour blends are mathematically described, i.e. with number values. This is done with the help of bytes, which are the values of computers. Three numbers are necessary to describe one colour blend, each with a value between 0 and 255.

    This gives an amount of data, which is 3 bytes, because a byte is a number between 0 and 255. We arrive at exactly three bytes because there are three primary colours in every digital colour image.

    An image, which is built up of pixels with these sorts of colours has 24 bit colour or just RGB colours.

    The lowest number value, the computer can calculate with is a bit. This is a number, which can have a value of 0 or 1. Electronically this corresponds to a little switch, which is either switched on or switched off. When we connect eight of these bits together, we get one byte.

    1 byte

    8 bit

    1 kilobyte (KB)

    1.000 bytes

    1 Megabyte (MB)

    1000 KB
    1.000.000 bytes

    1 Gigabyte (GB)

    1000 MB bytes

    Figur 36. In some cases 1024 is used instead of 1000, so that 1024 bytes = 1 KB, etc. In practice, it doesn’t mean too much whether we calculated with 1000 or 1024, the first is just the easiest.

    These numbers have to be remembered in computers and cameras. So bytes are also used as the term for memory in a computer or a camera – i.e. ram and hard disk storage.

    The more bytes there are room for in storage, the more numbers (i.e. photographs) can be stored.

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