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A bit-plane of a digital medium (such as image or sound) is a set of bits having the same position in the respective binary numbers. For example, for 16-bit data representation there are 16 bit-planes: the first bit-plane contains the set of the most significant bits and the 16th contains the least significant bits.
It is possible to see that the first bit-plane gives the roughest but the most critical approximation of values of a medium, and the higher the number of the bit plane, the less is its contribution to the final stage. Thus, adding bit-plane gives a better approximation.
Incrementing a bit-plane by 1 gives the final result half of a value of a previous bit-plane. If a bit is set to 1, the half value of a previous bit-plane is added, otherwise it does not, defining the final value.
As an example, in PCM sound encoding the first bit in sample denotes the sign of the function, or in the other words defines the half of the whole amplitude values range, and the last bit defines the precise value. Replacement of more significant bits result in more distortion than replacement of less significant bits. In lossy media compression that uses bit-planes it gives more freedom to encode less significant bit-planes and it is more critical to preserve the more significant ones.