Exam 7: Multimedia Network Communications and Applications, Wireless Networks and Content-Based Retrieval in Digital Libraries

Assume an analog halftoning process uses a screen size of 200 "dots" (disks) per inch, with any size available. Suppose we wish to approximate this digitally, with about 100 intensity levels, not by using an ordered dither but by using an $n \times n$ pattern for each pixel. How many bilevel dots per inch must our printer be capable of producing?

Suppose an $8 \times 8$ image block happens to have the following entries: 183 160 94 0 0 0 0 0 183 153 0 0 0 0 0 0 179 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 (Note that this is a greylevel, 8-bit image, not DCT output). Now suppose we decide to encode this image into the frequency domain as follows: • First we go down each column, and carry out a 1-dimensional $D C T$ transform, replacing each column by its set of DCT coefficients. • However, for the first column we use only a length-3 DCT (i.e., $N=3$ ); for the second column we use a length-2 DCT, and for the third column we use a length-1 DCT, always leaving zeros in the transform domain just where they appeared in the original, image domain. • We leave the DC coefficient always at the top of each column processed. • Then we use the output from the above stage and go on to do the same procedure for rows 1 to 3. Question: (a) Which takes more calculations, the above procedure, or the ordinary 2-D DCT transform? Explain. (b) Broadly, what is the difference, if any, in the output DCT Image between the new transform and the standard one, for this particular image? Note: One need not do any calculations for this question but, for reference, recall that the 2-D DCT for an $M \times N$ block size is defined as $F(u, v)=\frac{2 C(u) C(v)}{\sqrt{M N}} \sum_{i=0}^{M-1} \sum_{j=0}^{N-1} \cos \frac{(2 i+1) \cdot u \pi}{2 M} \cos \frac{(2 j+1) \cdot v \pi}{2 N} f(i, j),$ where $i, u \in[0, M-1], j, v \in[0, N-1]$ , and the constants $C(u)$ and $C(v)$ are determined by $C(\xi)=\left\{\begin{array}{cl}\frac{\sqrt{2}}{2} & \text { if } \xi=0 \\1 & \text { otherwise. }\end{array}\right.$ The 1-D DCT is given by $F(u)=\frac{2 C(u)}{\sqrt{N}} \sum_{i=0}^{N-1} \cos \frac{(2 i+1) u \pi}{2 N} f(i)$

Briefly, for grey input images explain what half-toning and dithering are. How are they related to each other? What is ordered dithering?

What constitutes "interactivity" in a multimedia project? Please discuss briefly the levels of interactivity possible, from least interactive to most interactive.

Consider an alphabet with two symbols $A, B$ , with probability $P(A)=x$ and $P(B)=1-x$ . Plot the entropy as a function of $x$ . Note: you might want to use $\log _{2}(3)=1.6, \log _{2}(7)=2.8$ .

Dissolve: Suppose we have video1 dissolving into video2, over a time $t$ from 0 to $t_{\max }$ (video1 gradually disappears, and video2 gradually appears). There are 2 ways that this task is commonly carried out: "Ordinary Dissolve" and "Dither Dissolve". In Ordinary Dissolve, every pixel value is changed, over time, so that it contains partly the contents of video1 and partly the contents of video2, summed additively. In Dither Dissolve, pixels are either all-video1 or all-video2, not a mix; the decision of which video to take pixel values from is based on a random number generator. Write pseudocode solutions for accomplishing these two kinds of gradual video transition. For each type, just show the algorithm for filling up R (Red) values - Green and Blue will be similar.

Why do we use CMY color primaries for printing, instead of RGB ones? Hint: paper is white, not black. (b) What colour is Yellow and Cyan, printed together? Why?

In MPEG, what are all the different kinds of frames? (b) What are they used for? (c) Is the method that is used for motion compensation in MPEG based on x, y translation the most complicated method for motion compensation in use now in any standard? If so, explain why; if not, explain what other method is used. (d) Does MPEG video compression require a higher bitrate for video clips that have more action in them? Explain.

How would you create your own video wipe transition from the top-left corner of the viewport down to the bottom-right corner - a diagonal transition? Fig. 1 shows such a video transition. Figure 1: Wipe transition, at $\mathrm{t} / \mathrm{tmax}=0.66$ Here, we wish to simply take pixels from either the first or the second video, depending on whether they are above or below the moving diagonal line. Write some pseudo-C or pseudo-Premiere pseudocode to produce correct pixel values during the transition. Hint: for any $x$ and $y$ position, we can determine where the line that $\{x, y\}$ is on, which is parallel to the wipe, cuts the main diagonal from top-left to bottom-right of the frame, simply using similar triangles. To do so, it's easiest to calculate the y-intersept of that line (where it hits the y-axis). Figure 2: Wipe transition geometry.

What is texture? Explain in detail what is meant by texture descriptors, for use in content-based image and video search.

Describe the most important features for video search based on content descriptors. If you had to find a single frame to describe an entire video, what features would you use to drive the search?

Among FDMA, TDMA or CDMA, which one provides the most efficient use of the allocated spectrum for multiple access? Justify your choice.