2D Image Input

The basic two-dimensional image is a monochrome (greyscale) image which has been digitised.

Describe image as a two-dimensional light intensity function f(x,y) where x and y are spatial coordinates and the value of f at any point (x, y) is proportional to the brightness or grey value of the image at that point.

A digitised image is one where

• spatial and greyscale values have been made discrete.
• intensity measured across a regularly spaced grid in x and y directions
• intensities sampled to 8 bits (256 values).

For computational purposes, we may think of a digital image as a two-dimensional array where x and y index an image point. Each element in the array is called a pixel (picture element). See Figs. 30 and 31.

 

Fig.  Greyscale image and highlighted region

  
Figure: Pixel values in highlighted region

2D Input Devices

TV Camera or Vidicon Tube

A first choice for a two-dimensional image input device may be a television camera -- output is a video signal:

• Image focused onto a photoconductive target.
• Target scanned line by line horizontally by an electron beam
• Electric current produces as the beam passes over target.
• Current proportional to the intensity of light at each point.
• Tap current to give a video signal.

This form of device has several disadvantages.

Limited resolution

-- finite number of scan lines (about 625) and frame rate (30 or 60 frames per second)

Distortion

--

• unwanted persistence between one frame and the next
• Non-linear video output with respect to light intensity.
• Non-flat target on tube.

CCD Camera

By far the most popular two-dimensional imaging device is the charge-coupled device (CCD) camera.

• Single IC device
• Consists of an array of photosensitive cells
• each cell produces an electric current dependent on the incident light falling on it.
• Video Signal Output
• Less geometric distortion
• More linear Video output.

Frame Stores

Video Signal must be digitised.

A device known as a frame storeor frame grabber usually performs this task. It:

• Digitises the incoming video signal
• Samples signal into discrete pixels at appropriate intervals -- line by line.
• Samples signal into a (8 bit) digital value.
• Stores sample frame own memory.
• Frame easily transferred to computer memory or a file.