3.3.6 Representing Images

Bitmap Images

• An image is divided into pixels (Short for Picture Element), each pixel represents the smallest single point on the screen.
• A pixel can only be one colour at a time, and when a picture is saved, the colour of each individual pixel must be stored in binary.
• The more bits that are used to store each pixel, the more colours are potentially available. VDUs display output using millions of pixels.
• Each pixel is represented by a number of bits and programs can specify where each pixel is placed on the screen and what colour it is going to be.
• An image represented this way is called a Bitmap.
• The following 1-bit image shows how pixels are made up of binary code.
• The digit 0 is used for Black and the digit 1 is used for White.
• In reality, images are far more complexed than the image shown, with colour depth and image resolution having a big impact on the quality and file size of an image.

• The image above (Which has been enlarged) is 8 pixels by 8, so 64 pixels in total.
• Only black and white are being used, so a single bit would be enough to store each pixel.
• Using 0 for black and 1 for white we would use 64-bits or 8-bytes to store the pixels in this image.

• The number of bits per pixel is referred to as the Colour Depth. It is important for the quality of the colours in an image.
• If 1-bit is used to encode each pixel, then only two colours can be used (2¹). As we mentioned before, the 0 represents black and the 1 represents white.
• To be able to represent more than two colours in an image, each pixel requires more bits.
• To work out the minimum required colour depth from the number of colours in the image, convert the number of colours to a power of 2.
• For Example:
  • A 1-bit image can display 2¹, or 2 colours.
  • A 2-bit image can display 2², or 4 colours.
  • A 3-bit image can display 2³, or 8 colours.
  • A 4-bit image can display 2⁴, or 16 colours.
  • A 5-bit image can display 2⁵, or 32 colours.
  • A 6-bit image can display 2⁶, or 64 colours.
  • A 7-bit image can display 2⁷, or 128 colours.
  • A 8-bit image can display 2⁸, or 256 colours.
  • A 16-bit image can display 2¹⁶, or 65,536 colours.
• If the colour depth is increased, more bits are used to represent each pixel, and the overall size of the file will increase.
• The following image demonstrates how the quality of an image changes depending on the number of bits used.

Image Resolution- Also known as Image Size

• Resolution is the number of pixels (Picture Elements) or dots (Dots per inch - dpi) that make up an image.
• The greater the number of pixels per inch —> the sharper the image will be, the greater the quality and the larger the files size of the image.
• Pixel density is measured in pixels per inch (PPI) and is used to describe the resolution of a computer screen, camera or scanner.
• An image on the Internet is typically 72PPI, which is a low resolution. If you try to enlarge the image on the screen, the image will become pixelated and blurry. See example below showing how an image becomes blurred if you increase its size on the screen:

The size of an image file depends on the colour depth and dimensions. The size of an audio
file depends on the sample rate and bit depth. The size of an image file and an audio file
can be very large.

• The file size of an image depends on the colour depth and the number of pixels in the image.
  • Size in bits = image width x image height x colour depth
  • Size in bytes = (image width x image height x colour depth)/8-bits

W ---> The width of an image, measured in pixels.
H ---> The height of an image, measured in pixels.
D ---> The colour depth; the number of bits used to store each pixel.
To convert file size from *bytes* to *kilobytes*, divide the number of bytes by 1,000.
To convert from *bytes* to *megabytes*, divide the number of bytes by 1,000,000.