MATLAB: How to calculate enrgy of a matrix

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I am having a 2*2 matrix. say it is A=[1 2; 3 4]; how to calculate energy with this.

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What is your definition of energy? You could say that if the array is an image, the pixel values are energy already so you can just sum the array

energy = sum(A(:));

Or you can use the energy as defined from the gray level co-occurrence matrix of the array. From the help for graycoprops():

Calculate statistical properties of the GLCM.
stats = graycoprops(glcm)
stats = 
         Contrast: 2.8947
      Correlation: 0.0783
           Energy: 0.1191
      Homogeneity: 0.5658

Perhaps you have some other definition - I don't know. After you read this, please clarify your definition of energy.

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MATLAB: Calculate signal power of image

vipul, there is no power in an image, by the strict definition, but there is energy. You may have some other layman's definition. If so, let's hear it. Let's look at the units. I'll use SI units here. Power is in watts, or joules per second. A light source has power and it emits it, and it lands on your scene which then reflects it onto your camera sensor. So what hits a given pixel? It's called irradiance and its units are power per unit area or watts/m^2 or joules/(seconds*m^2). By the way, you can look up any of these radiometric definitions in Wikipedia or here

So what does a gray level really mean? The gray level is a measure of energy? Why energy? Exposure is the irradiance integrated over time and has units of joules per square meter (the "per second" units got integrated away). It's the measure of the energy of all the photons that hit the area of sensor. When you look at exposure for one single pixel, then you sum up (integrate) the exposure over the area of that one pixel, so the meters squared go away and you are left with joules. So gray level is proportional to joules, or energy. It's also proportional to power since it's power integrated over the area of a pixel and over the time of exposure, but both of those are constants for a given snapshot. So the pixel value is proportional to power by a conversion factor (that has units), but it really has units of energy.

So when you see SNR in the Wikipedia article, the gray level goes in place of the P (Power) in the equation, not in place of the A (amplitude). And when you see formulas with 10Log or 20Log, you use the 10Log formula, not the 20Log formula.

I hope that explains it better.

MATLAB: How to obtain entropy image, inverse difference image ,contrast image, energy image, etc from the GLCM features ..

glcm=graycomatrix(I);
F=graycoprops(glcm,{'Contrast','Homogeneity','Correlation','Energy'}); 
contrast=F.Contrast;
homogeneity=F.Homogeneity;
correlation=F.Correlation;
energy=F.Energy;
figure
subplot(2,2,1); imshow(contrast)
subplot(2,2,2); imshow(homogeneity)
subplot(2,2,3); imshow(correlation)
subplot(2,2,4); imshow(energy)

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MATLAB: How to obtain entropy image, inverse difference image ,contrast image, energy image, etc from the GLCM features ..

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