Images Data Representation

Objectives At the end of this lesson you should be able to describe How an image is made up of pixels How each pixel can be mapped to a bit pattern How colour displays use at least one byte (8 bits) to represent up to 255 different colours How we measure image resolution in dots per inch (dpi) What effects the file size of a given image

Bit-mapped Graphics All computer graphics are represented using a series of binary digits (bits)

Monitors Typical Monitor Resolution 1024 x 768 pixels. You can often see these pixels on a TV screen, due to the lower resolution. 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 0 or 1, black or white.

RLP Activity -> Bit Mapped Graphics Memory Screen 1 RLP Activity -> Bit Mapped Graphics

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

Colour All colour images are made up of RED, GREEN and BLUE. These can have any value from 0 to 255. Blue – R – 0 G – 0 B -255

Memory Screen B A A 1 B

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

Colours 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 which is equal to 16,777,216 colours (16 million)

Resolution The resolution of an image is measured in dots-per-inch (dpi), also known as pixels-per-inch (ppi). The dpi determines the amount of detail the image has If the image is to be displayed on a computer the resolution should be in the range 72 dpi. If image is in print, then it is usually best to set the dpi to the highest number you can - around 300 dpi

RLP Activity -> Image Rep DPI 2000px 12.6 Inches 2000px 16.2 Inches RLP Activity -> Image Rep

File Sizes An image file size is dependant on the number of pixels which in turn is dependant on Colour depth (how many bits used to describe the colours Resolution (in dpi)

File Sizes Black and white (gray scale) image 300  dpi and print size of 8 inches  x 12 inches File size is determined by the number of pixels Since the resolution is 300 dpi each inch has 300 pixels across The image is 8 inches across so there are 2,400 pixels (8 x 300) across The image is 12 inches down so there are 3,600 pixels (12 x 300) down To get the total we multiply (across x down) and the answer is 8,640,000 pixels (2400 x 3600) We now know how many pixels are in the image In a grayscale image each pixel is 8 bits (1 byte) in file size. So in total the image of 8,640,000 pixels has a file size of 8,640,000 bytes or about 8.2 MB (divide by 1024) Or if you wanted it in RGB colour multiply that figure by 3 to give a 24 MB image. File size dependant on number of pixels Resolution Colour Depth

Continue with the worksheet Activity Continue with the worksheet

In dots per inch (dpi) which equates to pixels per inch (ppi) Plenary What are digital images made up of? How many colours can be displayed using 3 bits? How do we measure the resolution of a high definition image? What affects the file size of an image? pixels 8 In dots per inch (dpi) which equates to pixels per inch (ppi) How many pixels need to be stored, which in turn is dependant on resolution and colour depth