my shannon's entropy of image, please advice.

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telmo.bac..

my shannon's entropy of image, please advice.

in General Discussion • Other • 7 months ago

Hi , first i would like to thank to the forum for the help, ive made this code with the help of this forum.

Now I would like to ask for advices or recomendations.

The idea is to calculate the shannons entropy of an image.

I ve found a code in java that calculates the entropy of a string so i adapted that code:

what im doing is to load an image and then iterate over the pixels to get the hex colors values and then i join these values in one string.

At the end i end up with something like this:

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

then im calculating the entropy of that big string.

It seems it works ok, but i was thinking that is this a real way to calculate the entropy of an image?

is this in reality the entropy of an image?

Other thing i was thinking is that instead of using the hex values of each pixel , convert each hex value in just one character , for example convert "B7B4A5" into "a". What do you think? do you think that would be better?

Do you think this is a good way to calculating the entropy of an image? or can you think an improved or better way?

here is the code:

import java.util.*;
public double calculateShannonEntropy(String[] values) {
Map<String, Integer> map = new HashMap<String, Integer>();
// count the occurrences of each value
for (String sequence : values) {
if (!map.containsKey(sequence)) {
map.put(sequence, 0);
}
map.put(sequence, map.get(sequence) + 1);
}
// calculate the entropy
double result = 0.0d;
for (String sequence : map.keySet()) {
double frequency = (double) map.get(sequence) / values.length;
result -= frequency * (Math.log(frequency) / Math.log(2));
}
return result;
}
//Como convertir este codigo de arriba solo en processing?
void setup() {
/****************************************************************/
size(500, 400);
imageMode(CENTER);
/****************************************************************/
final int ww = width>>1, hh = height>>1;
PImage img;
img = loadImage("imagen.png");
final int imgW = img.width, imgH = img.height;
final int imgLen = img.pixels.length;
println(imgLen);
image(img, ww, hh);
/****************************************************************/
int col = imgW>>2, row = imgH>>1;
color colour = img.pixels[row*imgW + col];
println( hex(colour) );
/****************************************************************/
String[] pixelsStr = new String[imgLen];
for ( int i=imgLen; i!=0; pixelsStr[--i] = hex(img.pixels[i], 6) );
String colStr = pixelsStr[row*imgW + col];
println( colStr );
println( pixelsStr );
/****************************************************************/
String strings = join(pixelsStr, "");
println(strings);
String [] sa = new String[strings.length() - 1];
//println("THe collection of values we want to");
//println("calculate the entropy for");
for (int i = 0; i < sa.length; i++) {
sa[i] = strings.substring(i, i+2);
// println(sa[i]);
}
// The method returns a double need to cast to a float
float entropy = (float) calculateShannonEntropy(sa);
println("Entropy = " + entropy);
}

Replies(3)

PhiLho

Re: my shannon's entropy of image, please advice.

7 months ago

" is this in reality the entropy of an image?"
Good question, because the answer is probably "no".

I don't know Shannon entropy (beside a quick look at the Wikipedia page), but I can read code, so I understand the base idea behind the algorithm.

Here, you take the characters of the conversion of the image to string two by two, with overlapping. So not only you don't distinguish the color channels, but the bits are mixed as well.
If that's really the algorithm, taking elements of information two by two with overlapping, counting the occurrences of each pair of information, you want to keep the pairs of pixels in an appropriate structure.
The most flexible way is make a class with proper equals and hashcode methods, but here, you can just put the values in a long, they fit in.
Something like:

long[] pairs = new long[pixels.length / 2];
for (int i = 0; i < pixels.length - 1; i++)
{
pairs[i] = (pixels[i] & 0xFFFFFF) | ((pixels[i + 1] & 0xFFFFFF) << 24);
}

(totally untested)
Then, make your function to be like:

public double calculateShannonEntropy(long[] values) {
Map<Long, Integer> map = new HashMap<Long, Integer>();
// count the occurrences of each value
for (Long sequence : values) {
if (!map.containsKey(sequence)) {
map.put(sequence, 0);
}
map.put(sequence, map.get(sequence) + 1);
}
// calculate the entropy
double result = 0.0d;
for (String sequence : map.keySet()) {
double frequency = (double) map.get(sequence) / values.length;
result -= frequency * (Math.log(frequency) / Math.log(2));
}
return result;
}

Again, no guarantees on the result! But perhaps it gives you a start. To test on images of known result (or images of few colors vs. complex images).

telmo.bac..

Re: my shannon's entropy of image, please advice.

7 months ago

Hi Philho, can you please explain me what do you mean with " take the characters of the conversion of the image to string two by two, with overlapping. "

I dont understand that.

cheers

PhiLho

Re: my shannon's entropy of image, please advice.

7 months ago

sa[i] = strings.substring(i, i+2);

This line takes two characters. I don't know if it is done on purpose or if it is part of the algorithm.
The second character of this loop will also be in the string taken in the next loop.

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