Units and data representation - OCRImages

Computers work in binary. All data must be converted into binary in order for a computer to process it. Images are no exception.

How an image is represented as a series of pixels and represented in binary

Consider a simple black and white image. If 0 is black (or off) and 1 is white (or on), then a simple black and white picture can be created using binary.

To create the picture, a grid can be set out and the squares, known as pixels, coloured (0 - black and 1 - white):

The effect of colour depth and resolution on the quality and size of an image file

Many images need to use colours. To add colour, more bits are required for each pixel. The number of bits determines the range of colours. This is known as an image's colour depth.

For example, using a colour depth of two, ie two bits per pixel, would allow four possible colours, such as:

• 00 - black
• 01 - dark grey
• 10 - light grey
• 11 - white

Each extra bit doubles the range of colours that are available:

• one bit per pixel (0 or 1) - two possible colours
• two bits per pixel (00 to 11) - four possible colours
• three bits per pixel (000 to 111) - eight possible colours
• four bits per pixel (0000 to 1111) - 16 possible colours
• 16 bits per pixel (0000 0000 0000 0000 to 1111 1111 1111 1111) - over 65,000 possible colours

The more colours an image requires, the more bits per pixel are needed. Therefore, the more the colour depth, the larger the image file will be.

Image size

Image size is simply the number of pixels that an image contains. It is expressed as height and width. For example:

• 256 × 256
• 640 × 480
• 1024 × 764

Image file size

The size of an image file can be estimated using:

• the image height in pixels
• the image width in pixels
• the colour depth per pixel

Image file size = image height (px) x image width (px) x colour depth

Example - an image of height 200, width 400, colour depth 16 bits

200 × 400 = 80,000

80,000 × 16 = 1,280,000 bits

1,280,000 bits ÷ 8 = 160,000 bytes

160,000 ÷ 1000 = 160 kilobytes

Result: 160KB

Resolution

Image quality is affected by the resolution of the image. The resolution of an image is a way of describing how tightly packed the pixels are.

In a low-resolution image, the pixels are larger and therefore, fewer are needed to fill the space. This results in images that look blocky or pixelated. An image with a high resolution has more pixels, so it looks a lot better when it is enlarged or stretched. The higher the resolution of an image, the larger its file size will be.

Metadata

Files contain extra data called metadata. Metadata includes data about the file itself, such as:

• file type
• date created
• author

An image file also includes metadata about the image data itself, such as:

• the height and width of the image - this defines how many rows and columns the pixels are to be arranged in
• the resolution
• the colour depth

Without this metadata, the image data would not be correctly interpreted, meaning the image could not be correctly displayed.