Merge pull request #29 from swastijain132005/feat/edge-detection

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

Author: harshsankhla18

README.md

@@ -138,6 +138,43 @@ 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

@@ -7,7 +7,7 @@
 int main(int argc, char *argv[])
 {
     // Define allowable filters
-    char *filters = "bgrsivB:";
+    char *filters = "bgrsivtdB:";
 
 
     char filterArr[argc-3];
@@ -130,6 +130,14 @@ int main(int argc, char *argv[])
             vignette(height, width, image);
             break;
 
+        case 't':
+            threshold(height, width, image);
+            break;
+        
+        case 'd':  // Edge Detection
+            detect_edges(height, width, image);
+            break;
+            
         // Brightness Adjust
         case 'B': {
             int brightness_value = atoi(optarg);
@@ -138,7 +146,10 @@ int main(int argc, char *argv[])
         }
         default:
             printf("Unknown filter: %c\n", filterArr[i]);
-            break;
+            free(image);
+            fclose(inptr);
+            fclose(outptr);
+            return 7;
 
     }
     }
@@ -168,4 +179,4 @@ int main(int argc, char *argv[])
     fclose(inptr);
     fclose(outptr);
     return 0;
-}
+}

filter.exe

@@ -214,3 +214,73 @@ void threshold(int height, int width, RGBTRIPLE image[height][width])
         }
     }
 }
+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.c

@@ -1,3 +1,9 @@
+#ifndef HELPERS_H
+#define HELPERS_H
+
+#include <stdio.h>
+#include <math.h>
+
 #include "bmp.h"
 
 // Convert image to grayscale
@@ -15,8 +21,17 @@ void reflect(int height, int width, RGBTRIPLE image[height][width]);
 // Blur image
 void blur(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]);
+
 // Brightness adjustment filter
 void brightness(int height, int width, RGBTRIPLE image[height][width], int value);
 
 // Vignette filter
 void vignette(int height, int width, RGBTRIPLE image[height][width]);
+
+
+#endif