diff --git a/cs677/pa3/edge-detect.cc b/cs677/pa3/edge-detect.cc
index 98c7706..b8cb494 100644
--- a/cs677/pa3/edge-detect.cc
+++ b/cs677/pa3/edge-detect.cc
@@ -8,12 +8,14 @@ using namespace std;
 void usage(const char * progName);
 void squashAndPadData(unsigned char * raw_data, unsigned char * padded_data,
                       int width, int height);
+void applyEdgeDetection(unsigned char * padded_data, unsigned char * edge_data,
+                        int width, int height, int threshold);
 
 void usage(const char * progName)
 {
     cout << "Usage: " << progName << " [options] <input-BMP-file>" << endl;
     cout << "  Options:" << endl;
-    cout << "    -l threshold_level : set threshold level" << endl;
+    cout << "    -l level : apply threshold at level" << endl;
     exit(42);
 }
 
@@ -22,7 +24,7 @@ int main(int argc, char * argv[])
     int argi;
     string inputFileName;
     string outputFileName;
-    unsigned int threshold_level = 60;
+    int threshold_level = -1;
 
     for (argi = 0; argi < argc; argi++)
     {
@@ -57,7 +59,15 @@ int main(int argc, char * argv[])
     squashAndPadData(bmp_data, padded_data, width, height);
     delete[] bmp_data;
 
+    unsigned char * edge_data = new unsigned char[width * height * 3];
+    applyEdgeDetection(padded_data, edge_data, width, height, threshold_level);
+    BMP outputImage(outputFileName.c_str(),
+                    width,
+                    height,
+                    edge_data);
+
     delete[] padded_data;
+    delete[] edge_data;
 }
 
 void squashAndPadData(unsigned char * raw_data, unsigned char * padded_data,
@@ -79,3 +89,33 @@ void squashAndPadData(unsigned char * raw_data, unsigned char * padded_data,
         *padded_data++ = 0;
 }
 
+void applyEdgeDetection(unsigned char * padded_data, unsigned char * edge_data,
+                        int width, int height, int threshold)
+{
+    unsigned char (*in)[height+2][width+2] =
+        (unsigned char (*)[height+2][width+2]) padded_data;
+    unsigned char (*out)[height][width][3] =
+        (unsigned char (*)[height][width][3]) edge_data;
+    for (int y = 0; y < height; y++)
+    {
+        for (int x = 0; x < width; x++)
+        {
+            /* Apply the Sobel operator on the in to
+             * produce an out value. */
+            int gradY = (*in)[y][x] + 2 * (*in)[y][x+1] + (*in)[y][x+2]
+                      - (*in)[y+2][x] - 2 * (*in)[y+2][x+1] - (*in)[y+2][x+2];
+            int gradX = (*in)[y][x+2] + 2 * (*in)[y+1][x+2] + (*in)[y+2][x+2]
+                      - (*in)[y][x] - 2 * (*in)[y+1][x] - (*in)[y+2][x];
+            int grad = gradX + gradY;
+            if (grad < 0)
+                grad = 0;
+            else if (grad > 255)
+                grad = 255;
+            if (threshold >= 0)
+                grad = grad < threshold ? 0 : 255;
+            (*out)[y][x][0] = grad;
+            (*out)[y][x][1] = grad;
+            (*out)[y][x][2] = grad;
+        }
+    }
+}