Data Representation Images

Starter In your own words discuss with the person next to you a definition for the following words Pixels Resolution Colour Depth

Learning Objectives 3.2.2 Understand how bitmap images are represented in binary [pixels, resolution, colour depth]

The long and short of it. Pixels are the smallest elements of a bit-mapped image and the smallest element that can be displayed on a screen. The resolution of an image is based on the number of elements used to represent the full image. The higher the number of elements for a given size, the better the quality of the image. Colour Depth indicates the number of bits used to represent the colour of a picture element. The higher the number of bits used, the greater the range of colours.

Pixels Typical Monitor Res. 1024 x 768 pixels. You can often see these pixels on a TV screen, due to the lower resolution. Pixel are the smallest controllable element of a bitmap image At a simple level each pixel could be controlled by one bit. Meaning each pixel would be mapped to one bit in memory. The bit could be set to 1 or 0, Black or white. This however is a little dull

Pixilation The image has been “stretched” and the pixel count can not increase, it is fixed. This means that when you increase the size of the image the pixel blur and merge together.

DPI or PPI (dots per inch/pixel per inch) How many dots (pixels) there are per inch. The more there are the higher the resolution. But this will take up more space. Onscreen DPI is usually 72 DPI High quality printing is usually 300 DPI The more pixels there are, the sharper and clearer the image will be (but remember the more pixels there are, the bigger the file size too)

Example - How much memory would be required for a 300 dpi coloured image, the size of an A5 sheet, if 24-bit colour is used? A5 – 8.25 x 5.75 inches. At 300 dpi (dots per inch) we need 8.25 x 300 = 2475 Horizontally 5.75 x 300 – 1725 pixels vertically = 2475 x 1725 = 4,268,375 pixels all together.

Task Task 31.1 Perform the calculations

Memory Screen 1 1 bit colour

Graphical Methods - Colour Bit Pattern Black White 1 Colour White 1 Colour Bit Pattern R G B Black Blue 1 Green Cyan Red Magenta Yellow White 1 bit colour 3 bit colour

Black and white is boring… To create a colour display – each pixel is mapped to at least one byte (8 digits) in memory Each Pixel can now be any one of 28 or 256 different colours. Does this sound pretty resource intensive?

Memory Screen B A A 1 B

Colours Depth 1 bit gives 2 possible colours. 3 bits gives 8 different colours 4 bits gives us 24 which is 16 colours 8 bits gives 28 which is 256 colours. The more frequently used 24 bits give 224 colours. 2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2 Which is equal to 16,777,216 colours (16 million) Complete task 31.2

Size “Modern” computers allocate 24bits to each pixel – giving 224 combinations. 16,777,216 different colours. 24 bits are used as 8 bits are allocated to the three primary colours, red, green and blue (RGB) This means that a monitor of 1024 x 768 display with 24 bits per pixel you get 188,742,368 bits of memory to make one picture. Sounds a lot right?

Calculations An image of 4 inches by 3.5 inches with a 300 dpi density, and with a colour depth of 24 bits would equal 30240000 bits in total. (4 x 300) x (3.5 x 300) x 24 How would you calculate how many megabytes this would need when stored? Complete the unit measurement task and task 31.3

Task: Draw This You need to follow the instructions carefully. You have been given a set of metadata (overview of information) regarding the image. Each byte contains the height, width and colour depth of the image. You just need to draw it out in a spreadsheet. For example 00000110, 00001000, 00000010, 1. 00=White 2. 11=Blue 3. 01=Red 4. 10=Green

Plenary Explain what pixels are Explain what is meant by DPI Explain what is meant by colour depth If I had an image and it contained 3,560,455 pixels. How many megabytes would it need to be stored? (Colour depth = 24 bits)