The File Contents Id

While PMIO is scanning a location for images and building thumbnails, it also performs a procedure called SHA-1 on the image data. This procedure is designed to produce a number (called a hash) which is always the same for the same data, but which should never be the same for different data. In other words, no two images should ever have the same hash code; the code is unique for each image. This code is 160 bits long and can be represented as a 40 character string of letters and numbers. In PMIO, this string is called the File Contents Id.

Creating the File Contents Id

So, a file contents id is a string which uniquely identifies the contents of a file. PMIO uses these to keep track of your images in a way that does not rely on the image's file name. It also uses them to keep track of information that has been associated with each image, for example the image memos and rotations.

You can use the contents id as an advanced way of referencing images without relying on the file name. If you use the Edit/Copy Special/Contents Id menu option, PMIO will copy the file contents id(s) of the selected image(s) to the Clipboard. You can paste these into Notepad, for example, and later, use PMIO's Power Filter to find the images again. The advantage of this approach is that you will be able to find the images again even if their file names have changed, or they have been moved to offline storage.

The disadvantage of this system is that it relies on the file contents never changing. If you edit a picture and change just one pixel to a shade brighter, the resulting contents id will be completely different. This means that you should only assign memos or rotations to an image, or make a note of the contents id when you are sure the image will not be edited (or even simply re-saved) again in the future.

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