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The nature of digital images

Digital images are formed from small blocks of colour called pixels. Each pixel can take one of 16.7 million colours, and in a typical image there will be 4 or 5 million individual pixels, sometimes more, sometimes less depending on grade of camera. It is this number of pixels (called mega-pixels, usually abbreviated to "mp") that is used in camera marketing. Generally, the more mega-pixels in an image the better it's quality will be, although there is a law of diminishing returns.

Pixels picture

The computer stores each pixel in an electronic file, made up of 8 bits of information per pixel. The amount of information stored is therefore colossal, for example a typical 5mp picture actually has 668 million individual bits of information!


An example is shown to the right of this paragraph, taken from the thumbnail image of the Ferrari on the Legal Matters page and magnified many times to show the pixel structure.

The computer carries out all the necessary calculations to display or print the image as required. Pixels are individually accessible within the file, and therefore image editing software can relatively easily alter them. This software ranges from free to expensive, with many features and abilities to alter the file. It is a very simple matter to remove items from the picture, add different items or to change the overall picture.

An example is shown below, with the flue taken away and replaced with the background. This process is known as cloning and for this image it took 5 minutes. The result is not perfect, but with more time spent the result will be indistinguishable.

Factory roof original, with flue Factory roof, copy without flue

Users of digital images used for evidential and record purposes need to know that the picture has not been altered, even subtly. This is extraordinarily difficult to achieve normally with current technology, but EviPix have developed a combination of procedures and software techniques that enable a user to not only know that one of our images has not been altered, but also to prove it. Conversely if one of our images has been altered we can show where the changes are, down to pixel level.

That is why we say EviPix - Digital pictures with provenance.