Representing Images

Bitmapped Graphics
- An image is divided into a grid of pixels.
- Each pixel in a grid cell is sampled and is then assigned a Binary Code to represent the average colour of the grid cell.
- The Binary Codes are then used to produce a copy of the original image.

Memory Bitmaps
- The Binary Codes of each pixel are stored in memory when an image is scanned.
- The image is displayed on a Visual Display Unit by transferring the Binary Codes back into the memory.
- Bitmap is used because the pixels of an image are mapped to specific positions in memory

Resolution
- The resolution of a Visual Display Unit is usually expressed by the # of pixels per row x # of pixels per column
- It is not the number of pixels that determines the sharpness of the image but the size of the pixels.
- For a sharper image the pixels need to be smaller.
- A small image size containing a high number of pixels is going to produce the sharpest image.

Colour Depth
- The number of bits used to represent the greyscale value of a pixel is called the image depth.
- The colour of a pixel can be coded using the RGB colour model.
- The RGB model mixes Red, Green and Blue to produce a specific colour.

1-Bit Colour
- A black and white image is known as a monochrome image.
- One bit is allocated to each pixel
- To get a white pixel the binary value is 1
- To get a black pixel the binary value is 0

12-Bit Colour
- 4 bits are allocated to each RGB components
- As 4 bits are allowed then 4096 different colours are available.
- This coulour depth is used most often when there is little colour needed, such as on a mobile phone

True Colour
- 24-Bit true colour images use 8 bits for each RGB component
- Having 8 bits for each component there are 256 different alternate colours
- When the colours are combined they give a total of over 16 million different colours

Vector Graphics
- Instead of dividing a graphic image into pixels a vector graphic identifies the objects that make up the image.
- Vector Graphics record information about these objects to define the image.
- Vector drawing software uses the commands that create objects as the source of information needed to describe the graphic.

Comparing Bitmaps and Vectors
- When a bitmap is scaled it is enlarged.
- As the magnification increases the vixels become more visible.
- Vector graphics avoid distortion because scaling is applied to a line's endpoint.
- Vector graphics do not deal with pixels.
- You do not lose resolution when you enlarge a true vector drawing.

Advantages and Disadvantages
- Geometric images require fewer bytes in vector graphic format than in a bitmap format.
- Images that have continuous areas of colour (such as photographs) take up fewer bytes in bitmap format than in vector.
- Geometric images load faster from secondary storage and download faster over the internet in a vectot graphic format.
- Vector graphics scale without distortion whereas bitmap does.

Compression
Run Length Encoding
-Single Compression technique that takes into account the fact that some images have long runs of pixels.
-If three or more consecutive cells have the same bit pattern then a run of cells has been found and can be encoded by two bytes.
-First byte stores the number of consecutive memory cell bytes and second byte stores the colour code.
-It is a loseless compression technique.
-Decompressing the Run Length Recording image produces an exact original image.

Lossy Compression
-Discards information that is not necessary such as a background scene.
-Decompressing an image with the Lossy technique produces a different image compared to the original.
-As the human eye is not so good at recognising exact strengths of brightness variation allowing it to be possible to highly reduce the information of the high frequency components.