1. Point Operations – Chapter 5

2. Gamma Gamma is a form of point operation
It’s intended purpose is to compensate for the non-linear relationship between the image capture device and the image display and (ultimately) the human viewer
That’s the technical low down – realistically it can be viewed as nothing more than a way to enhance image intensities in a non-linear fashion

3. Gamma
The function is:
is called the “gamma value”

4. Gamma
Various “gamma curves”

5. Gamma So, what does it really do? In “words”, what does this mean?
Dark input pixel values are amplified a lot
Bright input pixel values are amplified very little

6. Gamma
The steepness of the curve in the dark range can often be a problem
Low intensities is where most image sensors experience a great deal of noise
Over amplifying causes bad artifacts to appear
The solution is to split the curve into two sections, a linear portion and an exponential portion

7. Modified gamma The point where the curve
transitions from linear to exponential

8. Modified gamma

9. Gamma correction

10. Implementation
Applying an exponential function to every pixel in a large image is very time consuming so we don’t do it
Instead, we create a look-up table (LUT) of all possible pixel input/output pairs
For an 8-bit image this is only 256 calculations and memory locations
Image pixels are used as indexes into the table
This technique can be used for any function, not just gamma

11. Alpha blending Sometimes we want to overlay one image onto another
The technique used is called alpha-blending
Realistically it’s nothing more than a pixel-by-pixel weighted average
Graphics processing units have this as a built in function since graphics systems (e.g. games) do this often

12. Alpha blending
Setting alpha to 1 makes the foreground image completely transparent
Setting alpha to 0 makes the foreground image complete opaque
Setting it between 0 and 1 allows you to “see through” the foreground image into the background image

13. Alpha blending

14. Homework
See handout