Gamma correction
From DmWiki
Gamma correction controls the overall brightness of the image. The problem with displaying images on physical devices (such as monitors and printers) is that the human eye responds to relative color differences rather than absolute. A real-life example of this phenomenon can be seen by considering three light bulbs with 50W, 100W and 150W each. Although the brightness difference between them is the same (Δ = 50W), the difference between the 50W and 100W bulb will appear to the human eye larger than the difference between the 100W and the 150W bulb. This is simply due to the fact that the relative differences are not the same: in the first case the power doubles, whereas in the second case it increases by only 50%.
This means that there is a visual difference between the actual intensity of an image and what the human eye perceives. Images which are not gamma-corrected will generally appear darker. The solution to this problem is to use a logarithmic scale for color intensities. The intensities of the colors of the image are re-computed such that the new values match our perception. This process is referred to as gamma correction and is typically performed on hardware.
How Gamma Correction is Implemented
In computer displays, the relationship between the displayed pixel intensity (I) and the actual pixel value of an image (Po) is non-linear:
The value of γ (gamma) is display-dependent. Typically, in CRT displays, γ is around 2.5. This means that if a pixel on an image has a pixel with an intensity of 0.5 (representing a medium-grey color), the displayed brightness of that pixel on the display will be approximately (0.5)2.5 = 0.18 (i.e. the user would see a much darker grey than what the image describes).
To solve this problem, the input signal is gamma corrected; the pixel values of the image are modified such that the intensity distortion the monitor exhibits is compensated. The following relationship is used to correct the image:
where Pc is the corrected pixel value and Po is the original pixel value. Using the above example, to display a pixel value of 0.5 on a monitor with γ = 2.5, it has to be first gamma corrected using the second relationship: Po = (0.5)1/2.5 = 0.759, which is then fed into the display. Because of the non-linear behaviour of the display as described in the first relationship, the pixel value 0.759 will appear on the screen as 0.5, which is the actual intensity of the image.
