Lecture 2Concepts, Terms and Definitions

Display Devices• They are divided into a lot of small squares called

pixels (“PICture ELements”). Each pixel can be set to black or white (mono-chrome) (i.e. turned on or off). This allows patterns of dots to be created on the screen.

Memory Mapping• Each pixel on the screen corresponds to an address

in the computers memory - this is known as memory mapping and the display is said to be a “memory mapped display.” Effectively, each pixel is numbered sequentially.

• By writing values to the correct locations in memory the appearance of the screen can be controlled by a programmer. a program can find out if a pixel is turned on or off.

• The portion of memory that is associated with the display is known as the “video memory”.

• In the PC architecture, this memory is usually physically located on the graphics card, which is why you can buy 8Mb graphics cards, 16Mb graphics cards etc.

Memory Mapping

Resolution & Screen sizeResolution:•The screen in this example is composed of 300 pixels arranged in 15 rows of 20. It is said to have a resolution of “20 by 15” (20 x 15). This is actually very small by today’s standards. Typically a display will have a resolution of 1024x768 maybe even more.

Screen Size:•Resolution is NOT the same thing as screen size. Our 20x15 display could have been 5cm across (as a mobile phone display), 40cm across (as a monitor) or 20m (as a projection system) but the resolution would always be 20x15.

Coordinates• It is much more convenient for us to think in terms of

coordinates and leave it to the computer to convert the coordinates to memory location itself.

Color• Colors can be represented in most of today’s

computers. Typically, instead of each pixel being represented by one bit (on or off) each pixel will be represented by 24 bits (3- bytes) 3 x 8 bit. Each byte represents a number between 0 and 255 and each byte is associated with one primary color - red, blue, green.

Image File• By knowing the height and depth of the image, and

values that represent the color of each pixel, a picture can be saved onto disk and this is indeed the approach taken by all the common graphical formats seen on the WWW such as .gif’s, .jpeg’s, etc.

2D drawings - Point• 2D drawing are made up of Points, Lines and shapes.

2D drawings - Line

2D drawings - Shapes• Shapes are made up of points and lines

Exercises 2

• Find out the size, resolution and color depth of the display you are using.

• Describe the difference between size and resolution?• Give an example of memory mapping for 10 X 20 resolution display

device in case of monochrome and color?• What the difference between mathematical point and computer

graphics point? And Mathematical line , computer graphics line?• What steps must a computer take to plot a point (22.25,10.4)?• What is the location in memory for the pixel with the coordinates

3 , 12 in a display device with a 10 X 20 resolution?• What is the memory size required for storing colored image of

height 7 and depth 10?