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I'm studying the LBP algorithm and reading the paper Face Detection and Verification using Local Binary Patterns, Y Rodriguez which is a PHD thesis paper. On the page 21 (section 2.2.2 weak classifiers), the author mentioned that:

A weak classifier hp (x) consists of a look-up table of 2^9 − 1 = 511 bins

I cannot understand why there are 2^9-1 bins. What I can understand till now is that the LBP code for a specific pixel is 8 numbers either 0 or 1 calculated from its 8 surrounding pixels. However why the look-up table contains 2^9-1 bins? Any help is appreciated!

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  • 4 bits before your pixel, 1 bit for the pixel itself, 4 bits after your pixel? Commented Jul 6, 2012 at 16:18
  • hmm...what I have in mind is that the pixel is surrounded by 8 pixels in a 3 by 3 grid where the pixel itself is in the center. Then starting from the upper left corner, we compare the pixel with the center one, if greater, then it is 1 while if less than, it is 0. So we can have a serial of 8 numbers either 1 or 0. This can be turned to a decimal number (or just remain in binary number) which is the LBP code of this pixel. Commented Jul 6, 2012 at 16:41
  • However for the classifier of a particular pixel, there is a look-up table which contains 511 bins, which contains weights of 511 LBP codes. Now I'm confused, since the algorithm is doing verification in this case, so there are 2 classes, so the classifier should output a label of either class, right? Then what is the look-up table for? Well I'm confused by myself now. Commented Jul 6, 2012 at 16:43
  • scholarpedia.org/article/Local_Binary_Patterns Commented Jul 8, 2012 at 20:22

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I suggest you to read the paper "Multiresolution Gray-Scale and Rotation Invariant Texture Classification with Local Binary Patterns" written byTimo Ojala, Matti PietikaÈ inen, Senior Member, IEEE, and Topi MaÈenpaÈa. You will find the answer in chapter 2.2 For example, the pixel is surrounded by 8 pixels, the lbp bin will be 256(-1) = 1111 1111,by the same token, surrounded by 9 pixels bin 512(-1) = 1111 1111 1...you can think the pixel is surrounded by a circular chain formed by 9 pixel.

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