diff --git a/alg/libqr.cpp b/alg/libqr.cpp
index a268d12..16f1618 100644
--- a/alg/libqr.cpp
+++ b/alg/libqr.cpp
@@ -240,13 +240,38 @@ bool check_blur_by_laplacian(ProcessState &ps, Mat &gray, string &err)
 }
 
 static
-bool check_blur(ProcessState &ps, Mat &gray, string &err)
+bool check_blur_by_sobel(ProcessState &ps, Mat &gray, string &err)
 {
-    bool use_energy_gradient = false;
-    if (use_energy_gradient) {
-        return check_blur_by_energy_gradient(gray, err);
+    const float thres = 15;
+    Mat sobel_x, sobel_y;
+    Sobel(gray, sobel_x, CV_64F, 1, 0, 3);
+    Sobel(gray, sobel_y, CV_64F, 0, 1, 3);
+
+    Mat magnitude;
+    magnitude = sobel_x.mul(sobel_x) + sobel_y.mul(sobel_y); // Gx^2 + Gy^2
+    sqrt(magnitude, magnitude); // sqrt(Gx^2 + Gy^2)
+
+    Scalar meanMagnitude = mean(magnitude);
+    double sharpness = meanMagnitude[0] / (gray.rows * gray.cols) * 1000;
+    if (sharpness < thres) {
+        err = string_format("image too blurry: %lf < %lf", sharpness, thres);
+        return false;
     }
-    return check_blur_by_laplacian(ps, gray, err);
+    return true;
+}
+
+static
+bool check_sharpness(ProcessState &ps, Mat &gray, int method, string &err)
+{
+    if (method == 0) {
+        return check_blur_by_laplacian(ps, gray, err);
+    } else if (method == 1) {
+        return check_blur_by_energy_gradient(gray, err);
+    } else if (method == 2) {
+        return check_blur_by_sobel(ps, gray, err);
+    }
+    err = "unknown sharpness method: " + to_string(method);
+    return false;
 }
 
 #define COUNT_COMPONENTS 0
@@ -429,7 +454,7 @@ bool emblem_dot_angle(ProcessState &ps, InputArray in, float &angle, string &qrc
 
         qrcode = ps.qrcode;
 
-        if (!check_blur(ps, ps.dot_area_gray, err)) {
+        if (!check_sharpness(ps, ps.dot_area_gray, ps.sharpness_method, err)) {
             return false;
         }
 
diff --git a/alg/libqr.h b/alg/libqr.h
index cd2c4e0..ffc06c1 100644
--- a/alg/libqr.h
+++ b/alg/libqr.h
@@ -25,6 +25,7 @@ struct ProcessState {
     string qrcode = "";
     double clarity;
     float laplacian_thres = 0.1;
+    int sharpness_method = 2;
 };
 
 bool preprocess(ProcessState &ps);
diff --git a/alg/qrtool_wasm.cpp b/alg/qrtool_wasm.cpp
index fd8567e..6cfd914 100644
--- a/alg/qrtool_wasm.cpp
+++ b/alg/qrtool_wasm.cpp
@@ -23,10 +23,12 @@ std::string make_resp(bool ok, string err, int angle = -1, string qrcode = "", d
 
 
 extern "C" {
-const char *qrtool_angle(uint8_t *data, int width, int height, uint8_t *dot_area, float camera_sensitivity) {
+const char *qrtool_angle(uint8_t *data,
+                         int width, int height,
+                         uint8_t *dot_area,
+                         float camera_sensitivity) {
     ProcessState ps;
     ps.laplacian_thres = camera_sensitivity / 10.0;
-
     auto start = std::chrono::system_clock::now();
     static char ret[512];
     printf("qrtool_angle, width: %d height %d\n", width, height);