histogram equilization alg.

This is a discussion on histogram equilization alg. within the C++ Programming forums, part of the General Programming Boards category; HELP!! Hello, I am writing a code for histogram equilization which will take a gray scale pgm image, read it ...

  1. #1
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    Unhappy histogram equilization alg.

    HELP!!
    Hello, I am writing a code for histogram equilization which will take a gray scale pgm image, read it equilize the picture then write it out. Now I am having a lot of errors specially in the read and write file, and an error in: in the type conversion of Npixels=(wXh)/256 and sum_hist = ((sum*Npixels) + .5; also in the use of buffer to output the image. can someone please help.

    Code:
    #include <iostream.h>
    #include <math.h>
    // read pgm grey scale image into memory cor[i][j]
    void read_pgm(char * file_name)
    {
      	int	 w, h;
       	char  	 buf[80];
    	FILE	 * fp;
    
    	// open file for reading 
    	if((fp = fopen(file_name, "r")) == NULL) {
    		fprintf(stderr, "readImage: Can't open %s\n", file_name);
    		exit(1);
    	}
       	// Verify that the image is in PGM format. 
       	fgets(buf, 70, fp);
       	if(strncmp(buf,"P5",2) != 0){
          		fprintf(stderr, "The file %s is not in PGM format", file_name);
          		fclose(fp);
          		exit(1);
       	}
    	// skip all comment lines 
       	do { fgets(buf, 70, fp); } while(buf[0] == '#');     
    	sscanf(buf, "%d %d", &w, &h);
       	do { fgets(buf, 70, fp); } while(buf[0] == '#');
    
            // read intensity values into memory 
    	assign_mem(w, h); // assign a 2-D array cor for the image
    	for (int i = 0; i < w; i++)
    	  for (int j = 0; j < h; j++)
    	    cor[i][j] = getc(fp);
    	if (feof(fp) != 0) {
    	  printf("something is wrong with this picture.\n");
    	  exit(1);
    	}
    	fclose(fp);
    }
    
    // writes pgm grey scale image into memory cor[i][j]
    void write_pgm(char * file_name)
    {
    	FILE	*out;
    
    	if ((out = fopen(file_name, "wb")) == NULL) {
    	  cout << "write_pgm: cannot open the file " 
      	       << file_name 
    	       << " for writing" << endl;
              exit(1);
            }
    			
    	fprintf(out,"P5\n");
    	fprintf(out,"%d %d\n", w, h);
    	fprintf(out,"255\n");
    	for (int i = 0; i < h; i++)
    	  for (int j = 0; j < w; j++)
    	    putc(int(cor[i][j]+0.5), out);
                // cor saves the image information
    
    	fclose(out);
            return;
    }
    
    int main(){
    
    	long i;
    	long w[256];
    	long h[256];
    	long sum;
    	long hist[256];
    	long sum_hist[256];
    	long Npixels = (w*h) / 256;	//error
    
    	//read image
    	 read_pgm("picture.pgm");	
    
    	//clear histogram to 0
    	 for (i=0; i < Npixels; i++)
    		hist [i] = 0;
    		 
    	//calculate the histogram sum
    	 sum = 0;
    	 for (i=0; i<Npixels; i++)
    	    hist[buffer[i]]++;		//error here 
    	
    	for (i = 0; i< 256; i++){
    		sum += hist[i];
    //error here in conversion "="
    		sum_hist [i]= (sum * Npixels) + 0.5;
    	}
    
    	//transfor image using new sum 
    	for (i = 0; i< Npixels; i++){
    		buffer[i] = sum_hist(buffer[i]);  //error
    	}
    
    
    	//write image
    	 write_pgm("picture.pgm");
    
    return 0;
    }

  2. #2
    and the hat of wrongness Salem's Avatar
    Join Date
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    Well something more useful than "error" would help.
    I mean, I assume the compiler gave a longer error message describing the problem right?

    Also,
    > long Npixels = (w*h) / 256; //error
    But you haven't read the file in yet to set w and h

    Where are the definitions of assign_mem and cor for example?

    > cor[i][j] = getc(fp);
    This will blindly store all the newlines it comes across as well.
    Use fgets() to read each line, then extract all the pixel information in that line and store it in your cor array.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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