Images, Graphics and Compression Techniques

Images and graphics on computers have traditionally been, and generally still are stored as bitmaps - therefore arrays of binary digits, which depending on the colour depth will have a certain colour mapped to each pixel in a bitmap.  A pixel is the smallest addressable area of colour in an image, and these make up bitmaps.
  Colour depth refers to the number of bits used per pixel to store the colour, and when there are a greater range of colours available for use the bitmap will allot a larger binary number to each pixel resulting in a larger number of combinations of binary digits.  For example, if there was a colour depth of 3-bit, then 8 colours would be available, due to there being 8 possible combinations (000 to 111).  The number of possible combinations can be quickly figured out by putting 2 to the power of binary digits allotted to each pixel.  Therefore if the colour depth was 8-bit then 256 colours would be available, and 2 to the power of 24 colours available for 24-bit colour depth.  The most commonly used colour depth is 24-bit colour, which is known as true colour because a colour depth of 24-bit or higher cannot be distinguished from reality by the naked eye.  When 24-bit colour is used, 8 bits are allocated for red, 8 bits for green, and 8 for blue and the combination of the three selected shades forms the outcome.
  Uncompressed bitmaps always use a large amount of memory however, as the total number of bits used equals the number of bits per pixel multiplied per number of pixels.  For example, a 24-bit (true colour) bitmap taken on a 14 megapixel camera would contain 14 million pixels.  This number is then multiplied by 24 to get 336000000 bits, which is 42000000 bytes, or 42 megabytes - the largest possible size of a 24-bit 14 megapixel image.  As such many people use compression techniques to reduce the file size of an image.  Compression may either be lossy which involves the quality of the image decreasing, or lossless which involves no data being lost.  An example of lossy compression is saving an image as a JPEG, a format which removes colour which is difficult for the human eye to see, whilst Run Length Encoding is a type of lossless compression, and involves designating blocks of the same colour within the image, so that the same colour does not need to be specified for every pixel in that area.
  Vector graphics refer to another method of storing images.  Instead of storing them as bitmaps, they store information about the shapes in and other properties of the image such as colour, so that each time they are accessed they can be created.  In order to be able to create a vector graphic however, the instructions used to create the image must have been inputted, so that they could have been recorded.  These instructions are stored within what is known as the drawing list and contain data such as the shape type, coordinates and area.  This means that images taken with a digital camera for example cannot be turned into vector graphics because these properties are not recorded.  They would also be unsuitable for images as complex as photographs due to their level of detail.
  As a result of instructions being used to create the image instead of data about each pixel being stored the image has a much lesser file size (sometimes up to a million times less), which means that they take up much less hard disk space and can be loaded more quickly.  They also do not become pixelated like bitmap images when zoomed in on, because of the way that the geometric data stored results in the scaling being more precise than a bitmap image because of the way the software can alter the graphic for that level of zoom, whilst bitmap images cannot be scaled in the same way due to containing a pre-defined amount of pixels.
  Lastly, resolution refers to an image's dimension in pixels in relation to the space it occupies.  More pixels in an image will result in a higher resolution, and as such the image will be of higher quality.  A typical resolution if 1024 x 768, but no area is specified, and as such this could appear to be pixelated (blocky and low quality) if projected onto a very large space such as the side of a building, or of very high quality if in small area such as a 19-inch visual display unit (monitor).  A better way of measuring resolution is through use of Dots Per Inch, which tells one how many pixels there are in a specified area, meaning that the quality of the image can be determined.