Point Operations – Chapter 5

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