Entropy change the values on every execution

Question

Here is code which i am using in my program :

calcHist( &pre_img, 1, channels, Mat(), // do not use mask
                 hist, 1, histSize, ranges,
                 true, // the histogram is uniform
                 false );

       Mat histNorm = hist / (pre_img.rows * pre_img.cols);
       double entropy = 0.0;
       for (int i=0; i<histNorm.rows; i++)
       {
          float binEntry = histNorm.at<float>(i,0);
          if (binEntry != 0.0)
          {
            entropy -= binEntry * log(binEntry);
          }
       }
       cout<<entropy<<endl;

The first thing is when ever i enter it like entropy -= binEntry * log2(binEntry); it give me error on log2 , i added the libraries of math and numeric in VS 2010 , but still getting the error and the second point in the code is whenever i run it on the same video it give me different values on every execution , like if it give me 10.0 , 2.0 , 0.05 than the same frame on the next time running the program show me 8.0 , 1.5 , 0.01 these are sample values not exact

@AlanStokes I am using three libraries for it , `#include #include #include ` — Pieter, Sep 22 '13 at 10:14
@computer I am using only `log` and its not showing me error , but the formula ths using `log2` and whenever i run the program it give me different values on every execution — Pieter, Sep 22 '13 at 10:23
@Pieter you can use log to reproduce log2. Would it be sufficient to solve your problem? — 4pie0, Sep 22 '13 at 10:29
`log2` is only defined in the C99 standard. Try to [define function `log2` yourself](http://stackoverflow.com/questions/758001/log2-not-found-in-my-math-h) — WangYudong, Sep 22 '13 at 10:38

score 1 · Answer 1 · edited Apr 13 '17 at 12:19

1

log2 is only defined in the C99 standard. A workaround for not using log2 might be to replace it with different base because the logarithm logb(x) can be computed from the logarithms of x and b with respect to an arbitrary base k using the following formula:

enter image description here

https://math.stackexchange.com/a/131719/29621

so you can replace

if (binEntry != 0.0)
          {
            entropy -= binEntry * log2(binEntry);
          }

with

if (binEntry != 0.0)
          {
            entropy -= binEntry * log(binEntry)/log(2.0);
                                                     ^
                                                also you should use `log(2.0)`
                                                because the argument should be
                                                double or float
          }

edited Apr 13 '17 at 12:19

Community

1
1

answered Sep 22 '13 at 10:42

4pie0

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You didn't get my question , i am say i have error in declaring `log2` even `log(2)` , your code still showing the error – Pieter Sep 22 '13 at 11:28
can you use other math functions? – 4pie0 Sep 22 '13 at 11:31
I am using three libraries for it , `#include #include #include ` – Pieter Sep 22 '13 at 11:36
can you use any other function from math.h, i.e. sin? – 4pie0 Sep 22 '13 at 11:42
1

Try `log(2.0)` because the argument should be `double` of `float` – WangYudong Sep 22 '13 at 11:47
@computer Your answer missing this part : `whenever i run it on the same video it give me different values on every execution , like if it give me 10.0 , 2.0 , 0.05 than the same frame on the next time running the program show me 8.0 , 1.5 , 0.01` – Pieter Sep 22 '13 at 12:17
do this happen now, when you use log? – 4pie0 Sep 22 '13 at 12:21
@computer yes it happens – Pieter Sep 22 '13 at 16:14

Entropy change the values on every execution

1 Answers1