For example, for 16-bit data representation there are 16 bit planes: the first bit plane contains the set of the most significant bit, and the 16th contains the least significant bit.
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 a bit plane gives a better approximation.
If a bit on the nth bit plane on an m-bit dataset is set to 1, it contributes a value of 2(m-n), otherwise it contributes nothing. Therefore, bit planes can contribute half of the value of the previous bit plane. For example, in the 8-bit value 10110101 (181 in decimal) the bit planes work as follows:
|Bit Plane||Value||Contribution||Running Total|
|1st||1||1 * 2^7 = 128||128|
|2nd||0||0 * 2^6 =0||128|
|3rd||1||1 * 2^5 = 32||160|
|4th||1||1 * 2^4 = 16||176|
|5th||0||0 * 2^3 = 0||176|
|6th||1||1 * 2^2 = 4||180|
|7th||0||0 * 2^1 = 0||180|
|8th||1||1 * 2^0 = 1||181|
One aspect of using bit-planes is determining whether a bit-plane is random noise or contains significant information.
One method for calculating this is compare each pixel (X,Y) to three adjacent pixels (X-1,Y), (X,Y-1) and (X-1,Y-1). If the pixel is the same as at least two of the three adjacent pixels, it is not noise. A noisy bit-plane will have 49% to 51% pixels that are noise.
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.
As illustrated in the image above, the early bitplanes, particularly the first, may have constant runs of bits, and thus can be efficiently encoded by run-length encoding. This is done (in the transform domain) in the Progressive Graphics File image format, for instance.
- "Bit Plane". PC Magazine. Retrieved 2007-05-02.
- "Bit Plane". FOLDOC. Retrieved 2007-05-02.
- Strutz, Tilo (2001). "Fast Noise Suppression for Lossless Image Coding". Proceedings of Picture Coding Symposium (PCS'2001), Seoul, Korea. Retrieved 2008-01-15.
- Cho, Chuan-Yu; Chen, Hong-Sheng; Wang, Jia-Shung (2005-07). "Smooth Quality Streaming With Bit-Plane Labelling". Visual Communications and Image Processing (abstractBibcode:2005SPIE.5960.2184C. doi:10.1117/12.633501.) (The International Society for Optical Engineering) 5690: 2184–2195.