Let’s add a custom gesture for the **ASL letter “B”** (flat hand, fingers together, thumb folded across the palm) using MediaPipe **Gesture Recognizer (Model Maker)**.

# Plan (what you’ll build)

* A custom model with a new class label, e.g. `ASL_B`, plus the required `none` class.
* A small, labeled image dataset (Model Maker will extract hand landmarks for you).
* A trained `.task` file you can drop into your Python/JS app and allowlist.

---

# 1) Pick labels

Use:

* `ASL_B`  ← your new gesture
* `none`   ← anything that’s not one of your target gestures (mandatory)

Folder layout:

```
dataset/
  ASL_B/
    ...images...
  none/
    ...images...
```

---

# 2) Collect the right data (what to capture)

Target handshape for **B**:

* **Fingers**: index–pinky fully extended and **pressed together**
* **Thumb**: folded across palm (not sticking out to the side)
* **Palm**: facing camera (front) and also a few angles

Suggested minimums (per label):

| Bucket                                              | Shots                |
| --------------------------------------------------- | -------------------- |
| Distances: close (\~40–60 cm), medium (\~80–120 cm) | 80                   |
| View angles: front, \~30°, \~60° yaw                | 80                   |
| Rotations: slight roll/tilt                         | 40                   |
| Lighting: bright, dim, backlit                      | 40                   |
| Backgrounds: plain wall, cluttered office/outdoor   | 40                   |
| Hands: left & right (both)                          | included across all  |
| Skin tones / several people                         | as many as practical |

Do **at least \~300–500** `ASL_B` images to start.
For **`none`**, include: open palm (“High-Five”), slightly spread fingers, thumbs-up, fist, pointing, random objects/background frames, other ASL letters—especially **Open\_Palm** look-alikes so the model learns “not B”.

Quick ways to get images:

* Record short clips on laptop/phone and extract frames (e.g., 2 fps).
* Ask 3–5 colleagues to contribute a short 10–20s clip each.

Frame extraction example:

```bash
# Extract 2 frames/sec from a video into dataset/ASL_B/
ffmpeg -i b_sign.mov -vf fps=2 dataset/ASL_B/b_%05d.jpg
# Do the same for negatives into dataset/none/
```

---

# 3) Train with Model Maker (Python)

Create and activate a venv, then:

```bash
pip install --upgrade pip
pip install mediapipe-model-maker
```

Training script (save as `train_asl_b.py` and run it):

```python
from mediapipe_model_maker import gesture_recognizer as gr

DATA_DIR = "dataset"
EXPORT_DIR = "exported_model"

# Load & auto-preprocess (runs hand detection, keeps images with a detected hand)
data = gr.Dataset.from_folder(
    dirname=DATA_DIR,
    hparams=gr.HandDataPreprocessingParams(  # you can tweak these if needed
        min_detection_confidence=0.5
    )
)

# Split
train_data, rest = data.split(0.8)
validation_data, test_data = rest.split(0.5)

# Hyperparameters (start small; bump epochs if needed)
hparams = gr.HParams(
    export_dir=EXPORT_DIR,
    epochs=12,
    batch_size=16,
    learning_rate=0.001,
)

# Optional model head size & dropout
options = gr.GestureRecognizerOptions(
    hparams=hparams,
    model_options=gr.ModelOptions(layer_widths=[128, 64], dropout_rate=0.1)
)

model = gr.GestureRecognizer.create(
    train_data=train_data,
    validation_data=validation_data,
    options=options
)

# Evaluate
loss, acc = model.evaluate(test_data, batch_size=1)
print(f"Test loss={loss:.4f}, acc={acc:.4f}")

# Export .task
model.export_model()   # writes exported_model/gesture_recognizer.task
print("Exported:", EXPORT_DIR + "/gesture_recognizer.task")
```

Tips:

* If many `ASL_B` images get dropped at load time (no hand detected), back up the camera a little or ensure the whole hand is visible.
* If `none` is weak, add more “near-miss” negatives: open palm with fingers slightly apart, thumb slightly out, partial occlusions.

---

# 4) Plug it into your app

**Python (Tasks API example):**

```python
import mediapipe as mp
BaseOptions = mp.tasks.BaseOptions
GestureRecognizer = mp.tasks.vision.GestureRecognizer
GestureRecognizerOptions = mp.tasks.vision.GestureRecognizerOptions
VisionRunningMode = mp.tasks.vision.RunningMode
ClassifierOptions = mp.tasks.components.processors.ClassifierOptions

options = GestureRecognizerOptions(
    base_options=BaseOptions(model_asset_path="exported_model/gesture_recognizer.task"),
    running_mode=VisionRunningMode.LIVE_STREAM,
    custom_gesture_classifier_options=ClassifierOptions(
        score_threshold=0.6,                 # tighten until false positives drop
        category_allowlist=["ASL_B"]         # only report your class
    ),
)
recognizer = GestureRecognizer.create_from_options(options)
```

**Web (JS):**

```js
const recognizer = await GestureRecognizer.createFromOptions(fileset, {
  baseOptions: { modelAssetPath: "exported_model/gesture_recognizer.task" },
  runningMode: "LIVE_STREAM",
  customGesturesClassifierOptions: {
    scoreThreshold: 0.6,
    categoryAllowlist: ["ASL_B"]
  }
});
```

---

# 5) Troubleshooting & tuning

* **False positives with Open Palm:** Add more `none` examples where fingers are together but **thumb is visible** to the side. The model needs to see “almost B but not B.”
* **Left vs right hand:** Include both in training. If you only trained on right hands, left hands may underperform.
* **Distance issues:** If far-away hands fail, capture more medium/far shots. Landmarks get noisier when small.
* **Thresholds:** Raise `score_threshold` to reduce spurious detections; lower it if you miss true B’s.
* **Confusion matrix:** If accuracy is fine but live results wobble, collect more from the exact camera/lighting you’ll use.

---