This is a glossary of terms and concepts used in creating and managing digital content.
A Bit is a basic unit of data used in computing. The name ‘bit’ comes from the term ‘binary digit’ as it can only have one of two values: 0 or 1. The number of bits per pixel will have an impact on the quality of your image.
In a 1 bit image each pixel can either be black or white, with no colour or grey scale values. For an 8 bit image, where each bit can be 1 or 0 you have 28 values = 256 possible values for grey or colour in each pixel. A group of 8 bits = byte. RGB (Red Green Blue channels) can have value of 0-255 and each channel is 1 byte.
White has an RGB value of 255, 255, 255 and black is 0, 0, 0. All other colours are made up of combinations of R, G and B. A 24 bit image has 224 = 16,777,216 possible values so if it takes 1 byte to hold each R, G and B value, you can see how file size grows.
A 4x4 pixel image has less data than a 16x16 pixel image. Here you can see the colour value for a section of an image. In the top image there is only a sandy colour with RGB values of R255, G222, B185. As the image becomes more defined in the lower image, the section has much more colour data.
TIFF (Tagged Image File Format) is a good file type to use for preservation as it is lossless. That means it is not compressed when saved and therefore does not lose any of its data. So the software used to open a TIFF will always read the same data and, as a result, the image will look the same every time it is opened. Due to the large amount of data stored, a TIFF is a large file which can take a long time to download.
JPEG is commonly used for web images because it is usually small and quick to load in a browser. The reason for this is that unlike TIFF, it does not carry as much data. It is lossy, so each time it is saved, it compresses the data used to tell the software opening it how the image should look. Some cameras will take jpegs automatically, purely to save space on a memory card. For preservation, you can convert this to TIFF or, if the camera has stored a RAW file, you can keep this version.
Video of degradation of a JPEG file compressed when saved 600 times. Generation Loss from hadto on Vimeo
The resolution of an image is how much detail is present. An image is made up from lots of pixels which contain data telling the software how that pixel should look. The more pixels you have, the more data you have. We usually refer to ppi (pixels per inch) or dpi (dots per inch). The higher the resolution, the bigger the file size.
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Above are examples showing images of different resolution. From left to right; 4x4, 8x8, 16x16 and 200 x 200 pixels.
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This project was seed funded by the Shannon Consortium's 2016 Take 1 Step initiative.
© Glucksman Library, University of Limerick
