Communication, Networking, Signal and Image Processing (CS)

Question 5: Image Processing

August 2015

### Part 1

Consider the emissive display device which is accurately modeled by the equation

$\left[ {\begin{array}{*{20}{c}} X\\ Y\\ Z \end{array}} \right] = \left[ {\begin{array}{*{20}{c}} a&b&c\\ d&e&f\\ g&h&i \end{array}} \right]\left[ {\begin{array}{*{20}{c}} R\\ G\\ B \end{array}} \right]$

where R, G and B are the red, green, and blue inputs in the range 0 to 255 that are used to modulate physically realizable color primaries.

a) What is the gamma of the device?

b) What are the chromaticity components $(x_r,y_r), (x_g,y_g)$ and $(x_b,y_b)$ of the device's three primaries.

c) What are the chromaticity components $(x_w,y_w)$ of the device's white point.

d) Sketch a chromaticity diagram and plot and label the following on it:

$\\ 1. (x,y)=(1,0)\\ 2. (x, y) = (0,1)\\ 3. (x, y) = (0,0)\\ 4. (R,G,B) = (255, 0 , 0)\\ 5. (R, G, B) = (0, 255,0)\\ 6. (R,G,B) = (0, 0, 255)$

e) Imagine that the values of $(R,G,B)$ are quantized to 8 bits, and that you view a smooth gradient from black to white on this device. What artifact are you likely to see, and where in the gradient will you see it?

### Part 2

Consider an X-ray imaging system shown in the figure below.

Photons are emitted from an X-ray source and columnated by a pin hole in a lead shield. The columnated X-rays then pass in a straight line through an object of length T with density u(x) where x is the depth into the object. The number of photons in the beam at depth $x$ is denoted by the random variable $Y_x$ with Poisson density given by

$P\{Y_x = k\} = \frac{e^{-\lambda_x}{\lambda}^{k}_{x}}{k!} .$

Where x is measured in the units of $cm$ and $\mu(x)$ is measured in units of $cm^{-1}$.

a) Calculate the mean of $Y_x$, i.e. $E[Y_x]$.

b) Calculate the variance of $Y_x$, i.e. $E[(Y_x-E[Y_x])^2]$.

c) Write a differential equation which describes the behavior of $\lambda_x$ as a function of $x$.

d) Solve the differential equation to form an expression for $\lambda_x$ in terms of $\mu(x)$ and $\lambda_0$.

e) Calculate an expression for the integral of the density, $\int_{0}^{T} \mu dx$, in terms of $\lambda_0$ and $\lambda_T$