The purpose of this letter is to point out that the Udpikar and Raina (1987) modified block truncation coding (BTC) has been misleading researchers as a different BTC scheme in multiple publications for years. In fact, their algorithm is exactly identical to the second version of the Lema and Mitchell (1984) absolute moment block truncation coding (AMBTC). Mathematical proof is provided.
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Kai‐Kuang Ma (1997) studied this question.
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