Lesson 4: 15.3.2010 – Image Basics – Bits, Bytes and Pixels

Lesson 4: 15.3.2010 – Image Basics – Bits, Bytes and Pixels

Image Types
- Vector images – scalable, direction value, made up of shapes
- Rastor – Working with this at CIT, made up of tones, Photographic Image


Photoshop Fundamentals

· Digital images are made up of a mosaic of pixels. This means that pixel-based images will always have a fixed resolution and is said to be ‘resolution dependant’. If you enlarge such an image beyond the size at which it is meant to be printed, the pixel structure will soon become apparent.
· If the picture is not a photograph but an illustration made from a program such as Adobe Illustrator and the picture is drawn using vector paths the image will be resolution-independent. The image can then be scaled to reproduce at any size. Where the pixel image will start to break up as soon as it’s magnified the vector-drawn image will reproduce perfectly smooth.
· Megapixels to megabytes: If you multiply the ‘megapixel’ size by three you will get a rough idea of the megabyte size of the RGB image output. Eg. A 6 megapixel camera will produce 18 MB RGB images.
· The pixel dimensions of an image are an absolute value. So if you want to know how big an image can be printed, you simply divide the number of pixels by the pixel resolution you wish to print at.
· Number of pixels = physical dimension X (ppi) resolution.
· ppi: pixels per inch – This describes the digital, pixel resolution of an image.


Computer and Image Basics

In order to understand image editing programs and to perform other related tasks you will need a basic understanding of computers and the terminology associated with them.
Bit – ‘Binary Digit’ the smallest unit of a binary number having a value of 0 or 1. This is represented in a computer by an electrical current being off or on, or the direction of a magnetic polarity. It can also be represented optically on media such as CDs or DVDs.

Binary Number – The number system used by computers in which only the digits 0 or 1 are used to represent values. Other systems include decimal numbers (using 10 digits), octal numbers (using 8 digits) and hexadecimal numbers (using 16 digits).

Binary Arithmetic – Mathematical operations such as addition and multiplication can be performed on binary numbers in the same way as we do with decimal numbers.

Byte – A group of 8 bits, this can represent 256 values.

Word – The number of bits that the computer processor can manipulate or process in a single operation. Usually a multiple of 8 eg. 16, 32, 64 etc. This defines how big a number can be represented in the computer, and therefore the amount of memory that can be referenced.

Kilobyte – 1024 bytes

Megabyte – 1048576 bytes

Gigabyte – 1024 megabytes

Terabyte, Petabyte – Even bigger

File Structure – How files and folders (directories) are logically linked together by the operating systems of a computer.

File Extension – A few letter codes following a period appended to the name of a file to help identify the type and contents of a file eg. tiff, jpg etc
Server – A computer that provides services to other computers that are connected to it. Eg. file server, print server, internet server, application server.

Client – A computer that utilizes the services of a server.

Vector – A combination of a direction and a value (eg. length, intensity)

Vector Image – An image described by combining lines and shapes which are defined mathematically. Can be viewed or printed at any size with no loss of sharpness.

Raster Image – An image composed of many individual dots or ‘pixels’ arranged in a grid pattern.
NB: To display any image on a screen it must be converted to a raster image by the computer.

Pixel – ‘Picture Element’ – the smallest component of a raster image. Each pixel has a value representing a colour or a tone.

Bit Depth – The number of bits used to define the shades of colour in an image or in each channel of an image.
Higher Bit Depth = More Shades or Colours
1 bit = 2 values
8 bit = 256 values
etc

Image Size – How big the image is – it may be expressed in a number of ways; Pixel Dimensions, total number of pixels, the size of the image in inches, cm or page size.

Pixel Dimensions – The number of pixels across the width and down the height of an image.

Resolution – The number of pixels or dots per inch or cm.

File Type/ Format – The specification for how the image is described in an image file. It is usually identifiable by the file extension used. Eg. TIFF - .tif, tiff, JPEG - .jpg, .jpeg
One file type may support a number of different image description formats. Eg. TIFF or PSD can save different types of images.