Power Law Transformations (Gamma Correction) in Image Processing

Power Law Transformations (Gamma Correction) in Image Processing

Power Law Transformations (Gamma Correction) in Image Processing

The Power Low Transformations can be given by the expression:



s is the output pixels value

r is the input pixel value

c and γ are the real numbers

  • For various values of γ different levels of enhancement can be obtained.
  • This technique is quite commonly called as Gamma Correction, used in monitor displays.
  • Varying gives a whole family of curves

  • c and γ are positive constants
  • Power-law curves with fractional values of γ map a narrow range of dark input values into a wider range of output values, with the opposite being true for higher values of input levels.
  • c = γ = 1 Identity function

  • We usually set c to 1
  • Grey levels must be in the range [0.0, 1.0]

Also Read: Low Pass Filter in Image Processing

Gamma Correction

  • Cathode ray tube (CRT) devices have an intensity-to-voltage response that is a power function, with γ varying from 1.8 to 2.5
  • The picture will become darker.
  • Gamma correction is done by pre-processing the image before inputting it to the monitor with s = cr s = cr1/ 1/ γγ

Example : MRI

  1. a magnetic resonance image of an upper thoracic human spine with a fracture-dislocation and spinal cord impingement
    • The picture is predominately dark
    • An expansion of gray levels are desirable needs γ < 1
  2. result after power-law transformation with γ = 0.6, c=1
  3. transformation with γ = 0.4 (best result)
  4. transformation with γ = 0.3 (under acceptable level)

Effect of Decreasing Gamma

When the γ is reduced too much, the image begins to reduce contrast to the point where the image started to have very slight “wash-out” look, especially in the background

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes:

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>