Question

C Programming Language

Problem Title: Introduction to Computer Vision

Jojo is an outstanding student who is currently pursuing his Computer Science degree. In one of his Computer Vision lectures, his lecturer tackled a topic about binary

thresholding.

For simplicity, binary thresholding is an image filtering method used to separate low val-
ued pixels (dark) from high valued pixels (bright). Each pixels in an image is classified

as low or high based on a certain threshold K. If a pixel Pij ≤ K, then it is classified as
low pixel, otherwise it is considered as high pixel.

The problem with binary thresholding method is that the value of K may varies from
images. If K is set too a very low value, then a lot of pixels in the image will be considered
high valued pixels, likewise the opposite case. Thus Jojo wants to find the value of K
such that the absolute difference between the amount of dark pixels and bright pixels is
minimum.

Format Input

The program is expected to read a file ”testdata.txt” which contains at most 500 test cases
of inputs, each containing 2 integers N, M which defines dimension of the image. Each
test case contains N lines of M integers, where each value represents the image pixel
value.

Format Output

Output should be expressed in format ”Case #X: Y ” - X is the number of the test
case, and followed by Y rounded to the nearest integer number, such that when binary
threshold Y is applied to the image, the absolute difference between the amount of dark
pixels and bright pixels in the filtered image is minimum..

Constraints

• 1 ≤ T ≤ 500
• 1 ≤ N , M ≤ 100
• 1 ≤ |S| ≤ 100
• 0 ≤ P_{i j} ≤ 255

Sample Input (testdata.txt)

2
19 3
1 235 160
2 159 113
162 46 106
108 216 124
22 189 16
60 119 185
156 16 239
76 123 113
83 60 25
52 3 80
184 233 53
168 162 135
6 9 214
124 250 200
195 49 48
8 13 164
91 236 108
229 166 176
58 107 43

7 11
169 127 197 173 160 172 132 202 25 5 79
111 17 106 100 120 231 32 218 43 218 88
238 80 172 106 94 204 102 143 185 160 19
32 1 215 244 178 109 246 110 54 193 89
163 212 229 95 52 7 54 146 61 105 232
44 14 229 54 2 241 22 21 43 44 39
34 235 97 138 200 195 227 106 16 79 167

Sample Output (Standard Output)

Case #1: 113

Case #2: 109

Answer 1

Program:

 /* * C Program to find optimal value for k in binary threshold */ #include<stdio.h> #include<stdlib.h> // function to sort the array in ascending order void Array_sort(int *array , int n) { // declare some local variables int i=0 , j=0 , temp=0; for(i=0 ; i<n ; i++) { for(j=0 ; j<n-1 ; j++) { if(array[j]>array[j+1]) { temp = array[j]; array[j] = array[j+1]; array[j+1] = temp; } } } } // function to calculate the median of the array float Find_median(int array[] , int n) { float median=0; // if number of elements are even if(n%2 == 0) median = (array[(n-1)/2] + array[n/2])/2.0; // if number of elements are odd else median = array[n/2]; return median; } int main(){ // intializing variable int t, n, m, i, k, num_elements, median; // file pointr FILE *fptr; // check if the file is available if ((fptr = fopen("testdata.txt","r")) == NULL){ printf("Error! opening file"); // Program exits if the file pointer returns NULL. exit(1); } // reading no of testcases fscanf(fptr, "%d", &t); // for each testcase for(k=0; k<t; k++){ // the dimension of the array fscanf(fptr, "%d %d", &n, &m); // storing number of elements num_elements = n*m; // initializing the size of array int arr[num_elements]; // storing elements into the array reading from file for(i=0;i<num_elements;i++){ fscanf(fptr, "%d", &arr[i]); } // sorting the elements of the array Array_sort(arr, num_elements); /* median will be the most optimal k value for binary threshold */ // finding median of the array median = Find_median(arr , num_elements); // printing data to stdout printf("CASE #%d: %d\n",k+1, median); } // closing the file pointer fclose(fptr); return 0; }

Explanation: The optimal value for k would be median of the pixels as a median would be int the center of the data it would result in the minimum absolute difference between the bright and dark pixels.

Input:

testdata.txt

2
19 3
1 235 160
2 159 113
162 46 106
108 216 124
22 189 16
60 119 185
156 16 239
76 123 113
83 60 25
52 3 80
184 233 53
168 162 135
6 9 214
124 250 200
195 49 48
8 13 164
91 236 108
229 166 176
58 107 43

7 11
169 127 197 173 160 172 132 202 25 5 79
111 17 106 100 120 231 32 218 43 218 88
238 80 172 106 94 204 102 143 185 160 19
32 1 215 244 178 109 246 110 54 193 89
163 212 229 95 52 7 54 146 61 105 232
44 14 229 54 2 241 22 21 43 44 39
34 235 97 138 200 195 227 106 16 79 167

output:

CASE #1: 113
CASE #2: 109