Gamma Correction
- Control of the overall image brightness, as well as the ratio of red/green/blue.
- E.g., to fix an overbleached or too dark image (saturated).
Image after a 1.7
Gamma correction (from Stanford Graphics)
The Intensity to Voltage response curve is a power function for monitors:
- i.e., displayed_intensity = pixel_value^gamma .
 Input to monitor |
 Graph of input |
 Screen Output |
 Graph: I=V^2.5 |
(Adapted from http://www.cgsd.com/papers/gamma_intro.html
)
Gamma correction consists of applying the inverse of the above relationship
to the image before display, i.e., by computing:
- new_pixel_value = old_pixel_value^(1.0/gamma).
|
Input to monitor
|
Graph of input
|
 Gamma corrected input |
Graph: I'=I^(1/2.5) |
Screen output |
Graph of output
|
More details under :
Color Adjustment using "Curves"
This is related to Gamma correction in the sense that one uses a global 1D function to adjust image intensities.The differences are :
- the correction function is specified "by hand"
- the correction is applied to selected color channels
An example of "warming-up" an image :
Image Blurring, Sharpening
Recall the convolution by smoothing kernel such as the Gaussian kermel.
Example by using the Fourier Transform :
Blurring or smoothing filers are also known as low-pass filters.
NB: The same concept can be applied to audio files.
- A low-pass filter has the effect of removing the higher frequencies, to allow you to produce radio or telephone style effects.
- To implement this type of filter, you add a
contribution from the neighbouring pixels and reduce the contribution
from the pixel itself (think of the convolution with a Gaussian "hat")
A sharpening filter, also called a high-pass filter, emphasizes discontinuities in the pixel values by subtracting a contribution from the surrounding pixels from an increased central pixels.
This means that when the surrounding pixels are dark, the central pixel is increased in brightness which enhances the edges of an image.
Example of "sharpening" kernels: Laplacian, Mexican Hat :
Color image sharpening is typically done on three color channels separately (like RGB).