diff --git a/configs/test_cam1_face_det_rtsp_server.json b/configs/test_cam1_face_det_rtsp_server.json
index c425c00..f41ba1f 100644
--- a/configs/test_cam1_face_det_rtsp_server.json
+++ b/configs/test_cam1_face_det_rtsp_server.json
@@ -24,8 +24,8 @@
           "type": "preprocess",
           "role": "filter",
           "enable": true,
-          "dst_w": 640,
-          "dst_h": 640,
+          "dst_w": 1280,
+          "dst_h": 720,
           "dst_format": "rgb",
           "keep_ratio": false,
           "use_rga": true
@@ -36,12 +36,13 @@
           "role": "filter",
           "enable": true,
           "model_path": "./models/RetinaFace_mobile320.rknn",
-          "conf": 0.2,
+          "conf": 0.3,
           "nms": 0.4,
           "max_faces": 10,
           "output_landmarks": true,
           "input_format": "bgr",
-          "face_class_index": -1
+          "face_class_index": 1,
+          "letterbox": true
         },
         {
           "id": "osd_cam1",
diff --git a/plugins/ai_face_det/ai_face_det_node.cpp b/plugins/ai_face_det/ai_face_det_node.cpp
index 9d6b73b..2b15453 100644
--- a/plugins/ai_face_det/ai_face_det_node.cpp
+++ b/plugins/ai_face_det/ai_face_det_node.cpp
@@ -368,6 +368,42 @@ void ResizeRgbBilinear(const uint8_t* src, int src_w, int src_h, int src_stride,
     }
 }
 
+struct LetterboxInfo {
+    float scale = 1.0f;
+    int pad_x = 0;
+    int pad_y = 0;
+};
+
+void LetterboxRgb(const uint8_t* src, int src_w, int src_h, int src_stride,
+                  uint8_t* dst, int dst_w, int dst_h, bool swap_rb,
+                  uint8_t pad_value, LetterboxInfo& info) {
+    info = {};
+    if (!src || !dst || src_w <= 0 || src_h <= 0 || dst_w <= 0 || dst_h <= 0 || src_stride <= 0) return;
+
+    const float s = std::min(static_cast<float>(dst_w) / static_cast<float>(src_w),
+                             static_cast<float>(dst_h) / static_cast<float>(src_h));
+    const int rw = std::max(1, static_cast<int>(std::round(static_cast<float>(src_w) * s)));
+    const int rh = std::max(1, static_cast<int>(std::round(static_cast<float>(src_h) * s)));
+    const int px = (dst_w - rw) / 2;
+    const int py = (dst_h - rh) / 2;
+
+    info.scale = s;
+    info.pad_x = px;
+    info.pad_y = py;
+
+    // fill background
+    std::fill(dst, dst + static_cast<size_t>(dst_w) * static_cast<size_t>(dst_h) * 3, pad_value);
+
+    std::vector<uint8_t> tmp(static_cast<size_t>(rw) * static_cast<size_t>(rh) * 3);
+    ResizeRgbBilinear(src, src_w, src_h, src_stride, tmp.data(), rw, rh, swap_rb);
+
+    for (int y = 0; y < rh; ++y) {
+        uint8_t* drow = dst + (static_cast<size_t>(py + y) * static_cast<size_t>(dst_w) + static_cast<size_t>(px)) * 3;
+        const uint8_t* srow = tmp.data() + static_cast<size_t>(y) * static_cast<size_t>(rw) * 3;
+        memcpy(drow, srow, static_cast<size_t>(rw) * 3);
+    }
+}
+
 }  // namespace
 
 class AiFaceDetNode : public INode {
@@ -382,6 +418,7 @@ public:
         nms_thresh_ = config.ValueOr<float>("nms", 0.4f);
         max_faces_ = std::max(1, config.ValueOr<int>("max_faces", 10));
         output_landmarks_ = config.ValueOr<bool>("output_landmarks", true);
+        letterbox_ = config.ValueOr<bool>("letterbox", letterbox_);
         face_class_index_ = config.ValueOr<int>("face_class_index", face_class_index_);
         if (face_class_index_ != 0 && face_class_index_ != 1) {
             face_class_index_ = -1;  // auto
@@ -541,22 +578,30 @@ private:
 
         const uint8_t* input_ptr = nullptr;
 
-        // Fast path: already packed, correct size, no channel swap.
-        if (!need_swap && src_w == in_w && src_h == in_h &&
-            static_cast<size_t>(src_stride) == src_row && frame->data_size >= src_row * static_cast<size_t>(src_h)) {
-            input_ptr = src;
+        LetterboxInfo lb;
+        if (!letterbox_) {
+            // Fast path: already packed, correct size, no channel swap.
+            if (!need_swap && src_w == in_w && src_h == in_h &&
+                static_cast<size_t>(src_stride) == src_row && frame->data_size >= src_row * static_cast<size_t>(src_h)) {
+                input_ptr = src;
+            } else {
+                input_buf_.resize(in_size);
+                if (src_w == in_w && src_h == in_h && static_cast<size_t>(src_stride) == src_row) {
+                    memcpy(input_buf_.data(), src, in_size);
+                    if (need_swap) {
+                        for (size_t i = 0; i < in_size; i += 3) {
+                            std::swap(input_buf_[i], input_buf_[i + 2]);
+                        }
+                    }
+                } else {
+                    ResizeRgbBilinear(src, src_w, src_h, src_stride, input_buf_.data(), in_w, in_h, need_swap);
+                }
+                input_ptr = input_buf_.data();
+            }
         } else {
             input_buf_.resize(in_size);
-            if (src_w == in_w && src_h == in_h && static_cast<size_t>(src_stride) == src_row) {
-                memcpy(input_buf_.data(), src, in_size);
-                if (need_swap) {
-                    for (size_t i = 0; i < in_size; i += 3) {
-                        std::swap(input_buf_[i], input_buf_[i + 2]);
-                    }
-                }
-            } else {
-                ResizeRgbBilinear(src, src_w, src_h, src_stride, input_buf_.data(), in_w, in_h, need_swap);
-            }
+            LetterboxRgb(src, src_w, src_h, src_stride, input_buf_.data(), in_w, in_h,
+                         need_swap, 0, lb);
             input_ptr = input_buf_.data();
         }
 
@@ -609,7 +654,7 @@ private:
         det.img_h = src_h;
         det.model_name = "retinaface";
 
-        DecodeRetinaFace(tensors, src_w, src_h, in_w, in_h, det);
+        DecodeRetinaFace(tensors, src_w, src_h, in_w, in_h, lb, det);
         if (!logged_decode_.exchange(true)) {
             LogInfo("[ai_face_det] decoded faces=" + std::to_string(det.faces.size()));
         }
@@ -619,6 +664,7 @@ private:
     void DecodeRetinaFace(const std::vector<Tensor>& outs,
                           int orig_w, int orig_h,
                           int in_w, int in_h,
+                          const LetterboxInfo& lb,
                           FaceDetResult& out) {
         // Find loc/conf/landms tensors.
         std::vector<NcTensor> locs;
@@ -681,6 +727,8 @@ private:
         int face_idx = face_class_index_;
         float max_p0 = 0.0f;
         float max_p1 = 0.0f;
+        double sum_p0 = 0.0;
+        double sum_p1 = 0.0;
         if (face_idx < 0) {
             for (int i = 0; i < n; ++i) {
                 const float s0 = conf.data[static_cast<size_t>(i) * 2 + 0];
@@ -694,8 +742,13 @@ private:
                 const float p0 = 1.0f - p1;
                 if (p0 > max_p0) max_p0 = p0;
                 if (p1 > max_p1) max_p1 = p1;
+                sum_p0 += p0;
+                sum_p1 += p1;
             }
-            face_idx = (max_p1 >= max_p0) ? 1 : 0;
+            const double mean_p0 = sum_p0 / static_cast<double>(n);
+            const double mean_p1 = sum_p1 / static_cast<double>(n);
+            // Face channel is typically sparse (low mean). Background channel has high mean.
+            face_idx = (mean_p1 <= mean_p0) ? 1 : 0;
         }
 
         if (!logged_conf_probe_.exchange(true)) {
@@ -712,8 +765,8 @@ private:
             return;
         }
 
-        const float sx = static_cast<float>(orig_w) / static_cast<float>(in_w);
-        const float sy = static_cast<float>(orig_h) / static_cast<float>(in_h);
+        const bool use_lb = letterbox_ && lb.scale > 0.0f;
+        const float inv_s = use_lb ? (1.0f / lb.scale) : 1.0f;
 
         std::vector<Rect> boxes;
         std::vector<float> scores;
@@ -756,10 +809,19 @@ private:
             float x2 = (cx + ww * 0.5f) * static_cast<float>(in_w);
             float y2 = (cy + hh * 0.5f) * static_cast<float>(in_h);
 
-            x1 *= sx;
-            x2 *= sx;
-            y1 *= sy;
-            y2 *= sy;
+            if (use_lb) {
+                x1 = (x1 - static_cast<float>(lb.pad_x)) * inv_s;
+                x2 = (x2 - static_cast<float>(lb.pad_x)) * inv_s;
+                y1 = (y1 - static_cast<float>(lb.pad_y)) * inv_s;
+                y2 = (y2 - static_cast<float>(lb.pad_y)) * inv_s;
+            } else {
+                const float sx = static_cast<float>(orig_w) / static_cast<float>(in_w);
+                const float sy = static_cast<float>(orig_h) / static_cast<float>(in_h);
+                x1 *= sx;
+                x2 *= sx;
+                y1 *= sy;
+                y2 *= sy;
+            }
 
             Rect bb;
             bb.x = static_cast<float>(ClampInt(static_cast<int>(x1), 0, orig_w - 1));
@@ -778,8 +840,17 @@ private:
                 for (int k = 0; k < 5; ++k) {
                     const float lx = lmk.data[static_cast<size_t>(i) * 10 + k * 2 + 0];
                     const float ly = lmk.data[static_cast<size_t>(i) * 10 + k * 2 + 1];
-                    const float px = (p.cx + lx * var0 * p.w) * static_cast<float>(in_w) * sx;
-                    const float py = (p.cy + ly * var0 * p.h) * static_cast<float>(in_h) * sy;
+                    float px = (p.cx + lx * var0 * p.w) * static_cast<float>(in_w);
+                    float py = (p.cy + ly * var0 * p.h) * static_cast<float>(in_h);
+                    if (use_lb) {
+                        px = (px - static_cast<float>(lb.pad_x)) * inv_s;
+                        py = (py - static_cast<float>(lb.pad_y)) * inv_s;
+                    } else {
+                        const float sx = static_cast<float>(orig_w) / static_cast<float>(in_w);
+                        const float sy = static_cast<float>(orig_h) / static_cast<float>(in_h);
+                        px *= sx;
+                        py *= sy;
+                    }
                     pts[k].x = static_cast<float>(ClampInt(static_cast<int>(px), 0, orig_w - 1));
                     pts[k].y = static_cast<float>(ClampInt(static_cast<int>(py), 0, orig_h - 1));
                 }
@@ -825,6 +896,7 @@ private:
     int max_faces_ = 10;
     bool output_landmarks_ = true;
     int face_class_index_ = -1;  // -1=auto, 0/1=explicit
+    bool letterbox_ = false;
 
     std::string input_format_ = "rgb";