topic 2 - image formation point processing

Question 1

what is spatial sampling - digital image formation?

Answer 1

Projection onto discrete sensor array.

Spatially continuous light intensity function.

Sensor photosites register average colour.

Spatially discrete light intensity function.

Question 2

what is light intensity sampling - digital image formation?

Answer 2

Continuous colours mapped to a finite, discrete set of colours. (a.k.a. quantisation).

Question 3

what is spatial and intensity sampling - digital image formation?

Answer 3

continuous image.

sampled.

quantised.

Digital image: sampled & quantised

Question 4

what is greyscale and colour in digital image formation?

Answer 4

Colour images have 3 values per pixel; monochrome (grey-scale) images have 1 value per pixel.

A Digital image is a grid of squares, each of which contains a single color.

Each square is called a pixel (for picture element).

Question 5

what is the human visual system?

Answer 5

The eye has 3 types of cone photoreceptors:

sensitive to red, green, or blue light.

The brain transforms RGB into separate brightness and colour channels (e.g., LHS).

Question 6

what does point processing do?

Answer 6

Here the term point relates to pixel

Point processing adjusts a pixel’s value according to a transformation function usually for the purpose of improving the perceived image quality

The transformation is independent of the values of the neighbouring pixels.

Question 7

what are typical adjustments of point processing?

Answer 7

Brightness and contrast adjustment

Gamma correction

Histogram equalization

Colour correction.

To determine the ‘quality’ of an image it is useful to consider its image histogram. The histogram is simply records the number of pixels within an image at each light intensity value and is displayed as a bar chart.

Question 8

what is point processing - image histogram?

Answer 8

Let I be a single band (greyscale) image.

I(r,c) is an 8-bit integer between 0 and 255 representing the light intensity of a pixel located at row r and column c in within an image.

Histogram, hI, of I:

hI a 256-element array, that can be displayed as a bar chart.

hI (g), for g = 0, 1, 2, 3, …, 255, is an integer.

hI (g) = number of pixels in I that have value g.

Question 9

what does a look up table do? (LUT)

Answer 9

A look-up table (LUT) implements a functional mapping of pixel intensity values.

Question 10

how do you remap the intensity values? (LUT)

Answer 10

To remap the intensity values of an input image I, to the output image, J.