feat(filter): add edge detection (-D flag) using Sobel operator

Author: swastijain132005

README.md

@@ -144,6 +144,43 @@ Otherwise, it is set to **black** (`R = G = B = 0`).
 This results in a high-contrast, two-tone image where all intermediate shades are eliminated — essentially a hard binary “black-and-white” conversion.
 
 
+### 7.) The Edge Detection (Sobel) Algorithm
+
+The “edge detection” filter highlights sharp changes in pixel intensity, producing a sketch-like outline of the image.
+
+For each pixel, the horizontal and vertical gradients are calculated using a 3×3 Sobel kernel applied to the surrounding pixels:
+
+Horizontal (Gx):
+-1 0 1
+-2 0 2
+-1 0 1
+
+Vertical (Gy):
+-1 -2 -1
+0 0 0
+1 2 1
+
+The gradient magnitude for each color channel is then computed as:
+
+value
+=
+𝐺
+𝑥
+2
++
+𝐺
+𝑦
+2
+value=
+Gx
+2
++Gy
+2
+	​
+
+
+The result is clamped between 0 and 255 and replaces the original pixel value. This produces a monochrome image where edges are highlighted, giving a pencil-sketch effect.
+
 ---
 
 ### Usage

filter.c

@@ -7,29 +7,27 @@
 int main(int argc, char *argv[])
 {
     // Define allowable filters
-    char *filters = "bgrsivt";
+    char *filters = "bgrsivtd";
+
+   
 
 
-    char filterArr[argc-3];
-    
-    // gets all filter flags and checks validity
-    for(int i=0; i<argc; i++){
-        char temp = getopt(argc,argv,filters);
-        if(temp == -1) break;
-        filterArr[i]= temp;
-        if(filterArr[i] == '?') {
-            printf("Invalid filter option");
-            return 1;
-        }
-    }
-    
+    char filterArr[argc - 3];
+int filterCount = 0;
 
-    // Ensure proper usage
-    if (argc < optind + 2)
+while ((opt = getopt(argc, argv, filters)) != -1)
+{
+    if (opt == '?')
     {
-        printf("Usage: ./filter [flag] infile outfile\n");
-        return 3;
+        printf("Invalid filter option\n");
+        return 2;
     }
+    filterArr[filterCount++] = (char)opt;
+}
+
+    
+
+    
 
     // Remember filenames
     char *infile = argv[optind];
@@ -131,10 +129,18 @@ int main(int argc, char *argv[])
         case 't':
             threshold(height, width, image);
             break;
-        default:
-            printf("%c", &filterArr[i]);
-            break;
-        
+
+            case 'd':  // Edge Detection
+    detect_edges(height, width, image);
+    break;
+
+      default:
+    printf("Unknown filter: %c\n", filterArr[i]);
+    free(image);
+    fclose(inptr);
+    fclose(outptr);
+    return 7;
+
     }
     }
     // Write outfile's BITMAPFILEHEADER

helpers.c

@@ -1,6 +1,8 @@
 #include "helpers.h"
 #include <stdint.h>
 #include <stdlib.h>
+#include <math.h>
+#include <stdio.h>
 #include "bmp.h"
 int min(int a,int b){
     if(a<b) return a;
@@ -198,3 +200,78 @@ void threshold(int height, int width, RGBTRIPLE image[height][width])
         }
     }
 }
+
+
+// ----------------------
+// EDGE DETECTION (SOBEL)
+// ----------------------
+void detect_edges(int height, int width, RGBTRIPLE image[height][width])
+{
+    // Temporary copy of the image
+    RGBTRIPLE **copy = malloc(height * sizeof(RGBTRIPLE *));
+    for (int i = 0; i < height; i++)
+        copy[i] = malloc(width * sizeof(RGBTRIPLE));
+
+    for (int i = 0; i < height; i++)
+        for (int j = 0; j < width; j++)
+            copy[i][j] = image[i][j];
+
+    // Sobel kernels
+    int Gx[3][3] = {
+        {-1, 0, 1},
+        {-2, 0, 2},
+        {-1, 0, 1}
+    };
+    int Gy[3][3] = {
+        {-1, -2, -1},
+        { 0,  0,  0},
+        { 1,  2,  1}
+    };
+
+    for (int i = 0; i < height; i++)
+    {
+        for (int j = 0; j < width; j++)
+        {
+            int sumRx = 0, sumGx = 0, sumBx = 0;
+            int sumRy = 0, sumGy = 0, sumBy = 0;
+
+            for (int di = -1; di <= 1; di++)
+            {
+                for (int dj = -1; dj <= 1; dj++)
+                {
+                    int ni = i + di;
+                    int nj = j + dj;
+
+                    if (ni >= 0 && ni < height && nj >= 0 && nj < width)
+                    {
+                        RGBTRIPLE pixel = copy[ni][nj];
+                        int kx = Gx[di + 1][dj + 1];
+                        int ky = Gy[di + 1][dj + 1];
+
+                        sumRx += pixel.rgbtRed * kx;
+                        sumGx += pixel.rgbtGreen * kx;
+                        sumBx += pixel.rgbtBlue * kx;
+
+                        sumRy += pixel.rgbtRed * ky;
+                        sumGy += pixel.rgbtGreen * ky;
+                        sumBy += pixel.rgbtBlue * ky;
+                    }
+                }
+            }
+
+            // Calculate gradient magnitude and clamp
+            int red   = min(max((int)round(sqrt(sumRx*sumRx + sumRy*sumRy)), 0), 255);
+            int green = min(max((int)round(sqrt(sumGx*sumGx + sumGy*sumGy)), 0), 255);
+            int blue  = min(max((int)round(sqrt(sumBx*sumBx + sumBy*sumBy)), 0), 255);
+
+            image[i][j].rgbtRed   = red;
+            image[i][j].rgbtGreen = green;
+            image[i][j].rgbtBlue  = blue;
+        }
+    }
+
+    // Free temporary array
+    for (int i = 0; i < height; i++)
+        free(copy[i]);
+    free(copy);
+}

helpers.h

@@ -1,3 +1,9 @@
+#ifndef HELPERS_H
+#define HELPERS_H
+
+#include <stdio.h>
+#include <math.h>
+
 #include "bmp.h"
 
 // Convert image to grayscale
@@ -19,3 +25,8 @@ void vignette(int height, int width, RGBTRIPLE image[height][width]);
 
 //Threshold Filter(Black & White)
 void threshold(int height, int width, RGBTRIPLE image[height][width]);
+
+// **New: Edge Detection filter**
+void detect_edges(int height, int width, RGBTRIPLE image[height][width]);
+
+#endif