Point Processing (Szeliski 3.1)

cs129: Computational PhotographyJames Hays, Brown, Fall 2012Some figures from Alexei Efros,

Steve Seitz, and Gonzalez et al.

Image Processing

image filtering: change range of imageg(x) = h(f(x))

f

x

hf

x

f

x

hf

x

image warping: change domain of imageg(x) = f(h(x))

Image Processing

h

h

f

f g

g

image filtering: change range of image

g(x) = h(f(x))

image warping: change domain of imageg(x) = f(h(x))

Point Processing

The simplest kind of range transformations are these independent of position x,y:

g = t(f)

This is called point processing.

What can they do?

What’s the form of t?

Important: every pixel for himself – spatial information completely lost!

Negative

Basic Point Processing

Log

For example: Gamma “correction”

Is an instance of these power law intensity transformations.

Typically, gamma = 2.2 for a display device, and 1/2.2 for encoding.

The result is a more perceptually uniform encoding.

Gamma “correction”

Contrast Stretching

Image Histograms

Cumulative Histograms

s = T(r)

Histogram Equalization

Limitations of Point Processing

Q: What happens if I reshuffle all pixels within the image?

A: It’s histogram won’t change. No point processing will be affected…