luciayen/afrisign-exp1-casl-baseline

AfriSignEncoder Exp1 — CASL Baseline (LandmarkTransformer)

Part of the AfriSignEncoder research project: a multilingual African sign language recognition benchmark. This checkpoint is the Experiment 1 single-language baseline for Central African Sign Language (CASL).

Model Description

A ViT-style transformer that treats T=64 MediaPipe Holistic landmark frames as a token sequence.

Component	Value
Architecture	LandmarkTransformer (custom)
Input	(B, 64, 225) float32 — 75 keypoints × 3 coords per frame
Embedding dim	256
Attention heads	8
Encoder layers	4
Feed-forward dim	1,024
Positional encoding	Learned
Classification head	Linear 256 → 60
Parameters	~3.25 M

Dataset

CASL-W60 — 60 word-level Central African Sign Language glosses.

Split	Samples
Train	3,667
Test	2,222

Source: Kaggle mwakalucky/casl-w60 → parquet at luciayen/CASL-W60-Landmarks. Landmark format: MediaPipe Holistic (pose 33 + left hand 21 + right hand 21) × xyz = 225D per frame.

Training

Setting	Value
Optimiser	AdamW (lr=3e-4, wd=1e-4)
LR schedule	OneCycleLR cosine
Max epochs	60
Batch size	64
Loss	CrossEntropy + label_smoothing=0.1
Early stopping	patience=12 on val acc
Normalisation	Per-feature z-score (stats stored in checkpoint)

Results

Metric	Value
Best validation accuracy	71.92%
Best checkpoint epoch	35
Final epoch (early stop)	47
Random-chance baseline	1.67%

Checkpoint Contents

import torch
ck = torch.load("pytorch_model.bin", map_location="cpu")
# Keys: epoch, val_acc, model (state_dict), l2i (label→index dict),
#        mean (tensor 225,), std (tensor 225,)

The l2i dict maps 60 CASL gloss strings to integer class indices. mean and std are the per-feature normalisation statistics computed on the training set.

Limitations

• Landmark-only (no RGB appearance) — sets a lower bound for CASL recognition.
• Train/test split is from the original dataset; signer independence has not been verified.
• 60 glosses is a small fraction of full CASL vocabulary.

Citation / Project

AfriSignEncoder research project, CMU, 2026. GitHub: africansl_encoder.