Scor­pio News

that microfilm is difficult to tamper with. To date, as far as I’m aware, no such test test cases exist for documentation stored in an imaging system. However, the same criteria apply. Images stored in an imaging system are equally if not even more difficult to tamper with and would certainly would be accepted as the best available evidence under similar circumstances.

The Compressor.

The image is stored in the computer, which leads to the next stage. Vast though the capacity of an optical disk is, it’s not going to hold many images if each takes up in excess of 8 million dots. A typical 12″ disk would only hold perhaps a couple of thousand images at that rate. So something must be done to compress the data – to make it smaller.

Now if you tried the do-it-yourself scanner, above, using a newspaper picture, you will have noticed that in any one line there are a lot of black dots and considerably fewer white dots. Imagine, if you will, scanning a piece of paper with text on it. In this instance, the situation would be reversed, with considerably more white than black dots. Also, with typescript the number of black-white transitions and white-black transitions (that’s where one dot was white and the next was black, or vice versa) would be fewer than the number of white dots. So as the whole lot is connected to a computer let’s make the computer do some work. Make the computer count the transitions and the number of black or white dots from one transition to the next. In other words make the computer say (in computer language):

white – 10 dots, black – 3 dots, white – 6 dots .... etc.

instead of

white white white white white white white white white white black black black white white white white white white .... etc.

Even as I’ve written it, it’s, shorter. This is called compression, and although real compression systems don’t work as I’ve: just described, they follow the same sort of idea.

There’s anumber of compression algorithms, no one algorithm is perfect for all jobs (otherwise there would be no argument about which was best), each algorithm is best at one form of image or another. The main difference is the degree of compression achieved by different algorithms attempting to compress the same image.