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Question

Given a simple image of size 10 × 10 whose histogram models the symbol probabilities and is given by

p1p2p3p4
abcd

The first order estimate of image entropy is maximum when

This question was previously asked in
UGC NET 2014 Paper 1 Question Paper (28-Dec-2014)
The correct answer is

a = \(\frac{1}{4}\), b = \(\frac{1}{4}\), c = \(\frac{1}{4}\), d = \(\frac{1}{4}\)

The first order entropy estimate uses the histogram probabilities directly: \(H = -\sum_k p_k \log_2 p_k\) bits/pixel (a term with \(p_k = 0\) contributes 0). The image size 10 × 10 does not affect it.

a = 0, b = 0, c = 0, d = 1: \(H = -1\log_2 1 = 0\) bits (no uncertainty at all).

Two symbols with \(\frac{1}{2}\) each: \(H = 2 \times \frac{1}{2}\log_2 2 = 1\) bit.

Three symbols with \(\frac{1}{3}\) each: \(H = \log_2 3 \approx 1.585\) bits.

Four symbols with \(\frac{1}{4}\) each: \(H = 4 \times \frac{1}{4}\log_2 4 = 2\) bits. This is the largest value; in general entropy of K symbols is maximum, \(\log_2 K\), when all are equally likely.

Hence, the answer is a = \(\frac{1}{4}\), b = \(\frac{1}{4}\), c = \(\frac{1}{4}\), d = \(\frac{1}{4}\).

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