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Entropy thresholding

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Entropy Thresholding is a means of thresholding an image that selects an optimum threshold value by choosing the pixel intensity from the image's histogram that exhibits the maximum entropy over the entire image.

AlgorithmEdit

C++ codeEdit

ImageSpace ImageSpace::EntropyThreshold(void)
{ 
ImageSpace *ReturnImageSpace = new ImageSpace;
BYTE *b1 = new BYTE[ XMax*YMax ];
 
int ThresholdLevel, *hist; 
double prob[256], Sum_prob_1k = 0, Sum_prob_kl = 0, 
		Sum_prob_ln_1k = 0, Sum_prob_ln_kl = 0, Entropy[256];

           //gets histogram of image
	hist = Histogram();				

           // probability distribution
	for( int i = 0; i< 256; i++)
		prob[i] =  (double) hist[i] / (XMax * YMax); 
	
           // k is trial threshold level
	for( int k = 0; k< 255; k++)		
	{
		Sum_prob_1k = 0; Sum_prob_kl = 0; 
		Sum_prob_ln_1k = 0; Sum_prob_ln_kl = 0;
		     
                   //From 1 to k
		for(i = 1; i < k; i++)			
		{	
							                                                                
                              //Sums probablities to k
			Sum_prob_1k += prob[i];		

                            //Sums prob times Log of prob to k
			if(prob[i]!=0) Sum_prob_ln_1k += (prob[i] * log(prob[i]));
		}
		
		for(i = k; i < 256; i++)		

                          //From k to end
		{								 
                           //Sums prob of k to end
			Sum_prob_kl += prob[i];		

                           //Sums prob times log of prob
		        if(prob[i]!=0) Sum_prob_ln_kl += (prob[i] * log(prob[i]));
		}
		     //Final equation of entropy for each k
		Entropy[k] = log(Sum_prob_1k) + log(Sum_prob_kl)
			- (Sum_prob_ln_1k / Sum_prob_1k)
			- (Sum_prob_ln_kl / Sum_prob_kl);
		     //protects against divide by zero
		if(Entropy[k]<0) Entropy[k]=0;
	}		

	ThresholdLevel = 0;
	for(k = 0; k<256; k++)	//Finds Maximum
		if(Entropy[k] > Entropy[ThresholdLevel]) ThresholdLevel = k;
			//Thresholds there
		cout << "Thresholded at :" << ThresholdLevel << endl;
	for( int y=0; y < YMax; y++ ) for( int x=0; x < XMax; x++ ) 
	    b1[ x + y * XMax ] = ( b0[ x + y * XMax ] > ThresholdLevel ) ? 255 : 0;
	
	ReturnImageSpace->Load(b1);
	IsThreshold = TRUE;

return *ReturnImageSpace;
}

OutputEdit

See alsoEdit

External linksEdit

link MotionDetection link 3D Entropy Bulgarian version

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