diff --git a/README.md b/README.md
index 3308677..ba4251d 100644
--- a/README.md
+++ b/README.md
@@ -632,6 +632,51 @@ model:
 只加载 backbone，分类头仍随机初始化。`frozen_backbone: true` 可以单独衡量
 自监督特征本身有多好。
 
+## 从分类到定位：Grad-CAM 生成框标签
+
+分类模型只学过"这是什么缺陷"，没学过"缺陷在哪"。Grad-CAM 把分类分数对卷积特征的梯度
+当作通道重要性，加权求和得到空间热力图——这是分类模型隐含的定位线索。阈值化 +
+取连通域外接矩形，就得到弱监督的框标签：
+
+```bash
+uv run python scripts/gradcam_boxes.py --checkpoint runs/classify/best.pth
+```
+
+```
+runs/gradcam_boxes/
+├── labels/<类别>/<图名>.txt   # YOLO 格式 class_id cx cy w h，可直接喂给检测训练
+├── viz/<类别>/<图名>.png      # 原图 | 热力图 | 叠加+框，供人工核对
+├── data.yaml
+└── summary.json
+```
+
+**框的类别取自图片所在目录**（人工归类结果），不是模型预测——即使模型分错，
+要的也是该图真实类别的热力区域。
+
+### 分辨率是主要限制
+
+resnet18 在 128×128 输入下各层的 CAM 尺寸：layer4 只有 4×4、layer3 是 8×8、layer2 是 16×16。
+层越浅越精细，但类别判别性越弱。`--stage`（4~7 对应 layer1~4）可选，默认 6 即 **layer3**：
+
+| stage | CAM | 实测（每类抽 3 张，共 45 张） |
+| --- | --- | --- |
+| 7 (layer4) | 4×4 | 100% 出 1 个框，但只有 16 个格子，框很粗 |
+| **6 (layer3)** | **8×8** | **76% 出 1 框、20% 出 2 框，热力区域与缺陷贴合** |
+| 5 (layer2) | 16×16 | 40% 出不了框，响应太散 |
+
+### 几个调参点
+
+- `--threshold`（默认 0.4）：热力图相对最大值的阈值
+- `--peak-ratio`（默认 0.75）：次要连通域的峰值要达到全局最大值的这个比例才保留。
+  不加这条会在没有缺陷的角落刷出假框——实测把两框比例从 53% 降到 20%
+- `--padding`（默认 0.1）：框四周外扩。CAM 倾向只覆盖最具判别性的部位（缺陷中心），
+  往往比真实缺陷范围小
+- `--viz-per-class`（默认 20）：每类存多少张可视化；`--viz-all` 全存（很慢很占盘）
+
+> **不是所有类别都可定位。** `normal`/`black`/`edge_clean` 这类描述的是整图属性而非
+> 局部缺陷，它们的框没有物理意义。真正有用的是 `blackdot`/`whitedot`/`mosquito`/
+> `grease`/`zigzag` 这些有局部目标的类别。
+
 ## 方法
 
 - **骨架**：`tiny_conv`（自研深度可分离卷积，默认）或 `torchvision`（resnet / mobilenet / shufflenet）。
diff --git a/engine/gradcam.py b/engine/gradcam.py
new file mode 100644
index 0000000..bbcc153
--- /dev/null
+++ b/engine/gradcam.py
@@ -0,0 +1,159 @@
+"""Grad-CAM 与由热力图导出检测框。
+
+分类模型只学过"这是什么缺陷"，没学过"缺陷在哪"。Grad-CAM 把分类分数对某层特征图的
+梯度当作该层各通道的重要性，加权求和后得到一张空间热力图——分类模型隐含的定位线索。
+再对热力图阈值化、取连通域外接矩形，就得到弱监督的框标签。
+
+分辨率是主要限制：resnet18 在 128×128 输入下 layer4 只有 4×4、layer3 有 8×8。
+层越浅空间越精细，但类别判别性越弱，需要按数据实测取舍。
+"""
+
+from __future__ import annotations
+
+import numpy as np
+import torch
+from torch import nn
+from torch.nn import functional as F
+
+
+def grad_cam(
+    backbone: nn.Module,
+    head: nn.Module,
+    images: torch.Tensor,
+    targets: torch.Tensor,
+    stage: int | None = None,
+) -> torch.Tensor:
+    """对一个 batch 求 Grad-CAM，返回 (B, h, w) 的热力图（每张已归一化到 [0, 1]）。
+
+    Args:
+        stage: 取 `backbone.features` 的第几层输出做 CAM；None 表示用骨架最终输出
+            （即最后一个卷积块）。层越浅分辨率越高、类别判别性越弱。
+
+    对每个样本只回传它自己那一类的分数，所以整批可以一次 backward——
+    样本之间的梯度互不干扰。
+    """
+    layers = getattr(backbone, "features", None)
+    if stage is not None and not isinstance(layers, nn.Sequential):
+        raise ValueError("该骨架不是 nn.Sequential，无法按 stage 取中间层")
+
+    with torch.enable_grad():
+        if stage is None:
+            activations = backbone(images)
+            features = activations
+        else:
+            features = images
+            for index, layer in enumerate(layers):
+                features = layer(features)
+                if index == stage:
+                    break
+            else:
+                raise ValueError(f"stage {stage} 超出骨架层数 {len(layers)}")
+            activations = features
+            for layer in list(layers)[stage + 1 :]:
+                features = layer(features)
+
+        activations.retain_grad()
+        logits = head(features)
+        # 只取每个样本对应目标类别的分数；求和后一次 backward 即可
+        score = logits.gather(1, targets.view(-1, 1)).sum()
+        backbone.zero_grad(set_to_none=True)
+        head.zero_grad(set_to_none=True)
+        score.backward()
+
+    gradients = activations.grad
+    if gradients is None:
+        raise RuntimeError("没有拿到梯度，检查目标层是否参与了前向计算")
+
+    weights = gradients.mean(dim=(2, 3), keepdim=True)   # 各通道的重要性
+    cam = F.relu((weights * activations).sum(dim=1))     # (B, h, w)
+
+    flat = cam.flatten(1)
+    lo = flat.min(dim=1, keepdim=True).values
+    hi = flat.max(dim=1, keepdim=True).values
+    cam = ((flat - lo) / (hi - lo).clamp(min=1e-8)).view_as(cam)
+    return cam.detach()
+
+
+def connected_components(mask: np.ndarray) -> list[np.ndarray]:
+    """4-邻接连通域标记，返回每个连通域的布尔掩码。
+
+    自己写而不引 scipy/opencv：项目只依赖 numpy/torch/PIL，
+    为这一处再加一个重依赖不划算。
+    """
+    height, width = mask.shape
+    seen = np.zeros_like(mask, dtype=bool)
+    components = []
+    for start_y in range(height):
+        for start_x in range(width):
+            if not mask[start_y, start_x] or seen[start_y, start_x]:
+                continue
+            stack = [(start_y, start_x)]
+            seen[start_y, start_x] = True
+            pixels = []
+            while stack:
+                y, x = stack.pop()
+                pixels.append((y, x))
+                for ny, nx in ((y - 1, x), (y + 1, x), (y, x - 1), (y, x + 1)):
+                    if 0 <= ny < height and 0 <= nx < width and mask[ny, nx] and not seen[ny, nx]:
+                        seen[ny, nx] = True
+                        stack.append((ny, nx))
+            component = np.zeros_like(mask, dtype=bool)
+            ys, xs = zip(*pixels, strict=True)
+            component[list(ys), list(xs)] = True
+            components.append(component)
+    return components
+
+
+def cam_to_boxes(
+    cam: np.ndarray,
+    threshold: float = 0.4,
+    min_area_ratio: float = 0.01,
+    max_boxes: int = 3,
+    padding: float = 0.0,
+    peak_ratio: float = 0.75,
+) -> list[tuple[float, float, float, float]]:
+    """把一张热力图变成若干归一化 xyxy 框。
+
+    Args:
+        threshold: 相对最大值的阈值；CAM 已归一化到 [0,1]，所以直接比较。
+        min_area_ratio: 连通域面积小于整图这个比例就丢掉，滤掉零星的高响应点。
+        max_boxes: 最多保留几个框（按连通域内热力总和排序）。
+        padding: 框的四周外扩比例——CAM 倾向只覆盖最具判别性的部位（比如缺陷中心），
+            往往比真实缺陷范围小，适当外扩更贴合。
+        peak_ratio: 连通域的峰值至少要达到全局最大值的这个比例才保留。
+            CAM 归一化后最强的区域峰值恒为 1，次强区域若只有 0.45 多半是噪声——
+            实测不加这条会在没有缺陷的角落刷出假框。
+
+    返回的坐标已归一化到 [0, 1]，与图像实际尺寸无关，可直接写 YOLO 标签。
+    """
+    height, width = cam.shape
+    mask = cam >= threshold
+    if not mask.any():
+        return []
+
+    boxes = []
+    for component in connected_components(mask):
+        if component.sum() < max(1, min_area_ratio * height * width):
+            continue
+        if cam[component].max() < peak_ratio:   # 弱响应区多半是噪声，不是第二个缺陷
+            continue
+        ys, xs = np.where(component)
+        # +1 让框覆盖住整个格子，而不是格子中心之间的距离
+        x1, x2 = xs.min() / width, (xs.max() + 1) / width
+        y1, y2 = ys.min() / height, (ys.max() + 1) / height
+        if padding:
+            dx, dy = (x2 - x1) * padding, (y2 - y1) * padding
+            x1, x2 = max(0.0, x1 - dx), min(1.0, x2 + dx)
+            y1, y2 = max(0.0, y1 - dy), min(1.0, y2 + dy)
+        boxes.append((float(cam[component].sum()), (x1, y1, x2, y2)))
+
+    boxes.sort(key=lambda item: -item[0])
+    return [box for _, box in boxes[:max_boxes]]
+
+
+def to_yolo_line(box: tuple[float, float, float, float], class_id: int) -> str:
+    """归一化 xyxy -> YOLO 的 `class_id cx cy w h`。"""
+    x1, y1, x2, y2 = box
+    return (
+        f"{class_id} {(x1 + x2) / 2:.6f} {(y1 + y2) / 2:.6f} {x2 - x1:.6f} {y2 - y1:.6f}"
+    )
diff --git a/scripts/gradcam_boxes.py b/scripts/gradcam_boxes.py
new file mode 100644
index 0000000..db233db
--- /dev/null
+++ b/scripts/gradcam_boxes.py
@@ -0,0 +1,247 @@
+#!/usr/bin/env python
+"""用训练好的分类模型 + Grad-CAM 为每张图生成框标签。
+
+    uv run python scripts/gradcam_boxes.py --checkpoint runs/classify/best.pth
+
+分类模型只学过"这是什么缺陷"。Grad-CAM 把分类分数对卷积特征的梯度当作通道重要性，
+加权求和得到空间热力图——这是分类模型隐含的定位线索。阈值化 + 取连通域外接矩形，
+就得到弱监督的框标签。
+
+产出：
+
+```
+runs/gradcam_boxes/
+├── labels/<类别>/<图名>.txt   # YOLO 格式：class_id cx cy w h（归一化）
+├── viz/<类别>/<图名>.png      # 原图 | 热力图 | 叠加+框，供人工核对匹配程度
+├── data.yaml                  # 类别表，可直接喂给检测训练
+└── summary.json
+```
+
+**框的类别取自图片所在目录**（人工归类的结果），不是模型预测——即使模型分错，
+我们要的也是该图真实类别的热力区域。
+
+分辨率是主要限制：resnet18 在 128×128 输入下 layer4 只有 4×4、layer3 有 8×8。
+`--stage` 可选层（4~7 对应 layer1~4），层越浅越精细但类别判别性越弱。
+"""
+
+from __future__ import annotations
+
+import argparse
+import json
+import sys
+from collections import Counter
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
+
+import numpy as np  # noqa: E402
+import torch  # noqa: E402
+from PIL import Image  # noqa: E402
+from torch.utils.data import DataLoader  # noqa: E402
+
+from datasets import FolderClassificationDataset, classification_collate  # noqa: E402
+from datasets.yolo import write_data_yaml  # noqa: E402
+from engine import load_checkpoint  # noqa: E402
+from engine.gradcam import cam_to_boxes, grad_cam, to_yolo_line  # noqa: E402
+from models import build_model  # noqa: E402
+from utils import load_config, setup_logging  # noqa: E402
+from utils.device import DeviceNotAvailableError, describe_device, resolve_device  # noqa: E402
+
+logger = None
+
+
+def save_visualization(path: Path, image_path: Path, cam: np.ndarray, boxes, title: str) -> None:
+    """三联图：原图 | 热力图 | 叠加+框。人工一眼就能看出热力区域与框合不合。"""
+    import matplotlib
+
+    matplotlib.use("Agg")
+    import matplotlib.pyplot as plt
+    from matplotlib.patches import Rectangle
+
+    with Image.open(image_path) as raw:
+        image = np.asarray(raw.convert("RGB"))
+    height, width = image.shape[:2]
+    heat = np.asarray(
+        Image.fromarray((cam * 255).astype(np.uint8)).resize((width, height), Image.BILINEAR)
+    ) / 255.0
+
+    fig, axes = plt.subplots(1, 3, figsize=(11, 4))
+    axes[0].imshow(image)
+    axes[0].set_title("original")
+    axes[1].imshow(heat, cmap="jet", vmin=0, vmax=1)
+    axes[1].set_title(f"grad-cam {cam.shape[0]}x{cam.shape[1]}")
+    axes[2].imshow(image)
+    axes[2].imshow(heat, cmap="jet", alpha=0.45, vmin=0, vmax=1)
+    for x1, y1, x2, y2 in boxes:
+        axes[2].add_patch(
+            Rectangle(
+                (x1 * width, y1 * height), (x2 - x1) * width, (y2 - y1) * height,
+                fill=False, edgecolor="lime", linewidth=2,
+            )
+        )
+    axes[2].set_title(f"overlay + {len(boxes)} box")
+    for ax in axes:
+        ax.axis("off")
+    fig.suptitle(title, fontsize=10)
+    fig.tight_layout()
+    path.parent.mkdir(parents=True, exist_ok=True)
+    fig.savefig(path, dpi=110)
+    plt.close(fig)
+
+
+def main() -> int:
+    global logger
+    parser = argparse.ArgumentParser(
+        description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter
+    )
+    parser.add_argument("--checkpoint", default="runs/classify/best.pth", help="分类模型权重")
+    parser.add_argument("--config", default="configs/classify/film.yaml", help="训练时的配置")
+    parser.add_argument("--out", default="runs/gradcam_boxes", help="输出目录")
+    parser.add_argument("--set", nargs="*", default=[], metavar="KEY=VALUE")
+    parser.add_argument("--device", default=None)
+    parser.add_argument(
+        "--stage", type=int, default=6,
+        help="取骨架 features 的第几层做 CAM（4~7 = layer1~4）；默认 6 即 layer3",
+    )
+    parser.add_argument("--threshold", type=float, default=0.4, help="热力图阈值（相对最大值）")
+    parser.add_argument("--padding", type=float, default=0.1, help="框四周外扩比例")
+    parser.add_argument("--max-boxes", type=int, default=2, help="每张图最多几个框")
+    parser.add_argument(
+        "--min-area-ratio", type=float, default=0.02, help="连通域面积下限（占整图比例）"
+    )
+    parser.add_argument(
+        "--peak-ratio", type=float, default=0.75,
+        help="次要连通域的峰值至少要达到全局最大值的这个比例才保留，用于滤掉弱响应假框",
+    )
+    parser.add_argument(
+        "--viz-per-class", type=int, default=20, help="每个类别存多少张可视化，0 = 不存"
+    )
+    parser.add_argument("--viz-all", action="store_true", help="每张图都存可视化（很慢很占盘）")
+    parser.add_argument("--limit", type=int, default=0, help="只处理前 N 张，用于快速试参数")
+    args = parser.parse_args()
+
+    out_dir = Path(args.out)
+    out_dir.mkdir(parents=True, exist_ok=True)
+    logger = setup_logging(out_dir / "gradcam.log")
+
+    cfg = load_config(args.config, args.set)
+    try:
+        device = resolve_device(args.device or cfg.get("device", "auto"))
+    except DeviceNotAvailableError as exc:
+        logger.error("%s", exc)
+        return 1
+
+    data_cfg = cfg["data"]
+    dataset = FolderClassificationDataset(
+        root=data_cfg["root"],
+        split="all",                      # 为每个样本都生成，不分训练/验证
+        classes=data_cfg.get("classes"),
+        transforms=data_cfg["val_transforms"],
+        exclude=data_cfg.get("exclude") or [],
+    )
+    if args.limit:
+        dataset.samples = dataset.samples[: args.limit]
+    classes = dataset.classes
+
+    model_cfg = dict(cfg["model"])
+    model_cfg.setdefault("num_classes", len(classes))
+    model_cfg["init_from"] = None         # 权重从 checkpoint 来，不要再读自监督骨架
+    model = build_model(model_cfg)
+    load_checkpoint(args.checkpoint, model, map_location=device)
+    model.to(device).eval()
+    for param in model.parameters():      # Grad-CAM 需要梯度，但不更新权重
+        param.requires_grad_(True)
+
+    logger.info("设备: %s | 权重: %s", describe_device(device), args.checkpoint)
+    logger.info("数据集 %s：%d 张，%d 类", data_cfg["root"], len(dataset), len(classes))
+    logger.info(
+        "CAM 目标层 features[%d] | 阈值 %.2f | 外扩 %.2f | 每图最多 %d 框",
+        args.stage, args.threshold, args.padding, args.max_boxes,
+    )
+
+    loader = DataLoader(
+        dataset,
+        batch_size=int(data_cfg.get("loader", {}).get("batch_size", 64)),
+        shuffle=False,
+        num_workers=int(data_cfg.get("loader", {}).get("num_workers", 4)),
+        collate_fn=classification_collate,
+    )
+
+    labels_dir = out_dir / "labels"
+    viz_dir = out_dir / "viz"
+    viz_done: Counter[str] = Counter()
+    box_counts: Counter[int] = Counter()
+    per_class_boxes: Counter[str] = Counter()
+    correct = 0
+    cam_shape = None
+
+    for step, batch in enumerate(loader):
+        images = batch["images"].to(device)
+        targets = batch["labels"].to(device)
+        cams = grad_cam(model.backbone, model.head, images, targets, stage=args.stage)
+        with torch.no_grad():
+            correct += int((model(images).argmax(1) == targets).sum())
+        cam_shape = tuple(cams.shape[-2:])
+
+        for cam, target, meta in zip(cams.cpu().numpy(), targets.cpu().tolist(), batch["meta"],
+                                     strict=True):
+            name = classes[target]
+            boxes = cam_to_boxes(
+                cam,
+                threshold=args.threshold,
+                min_area_ratio=args.min_area_ratio,
+                max_boxes=args.max_boxes,
+                padding=args.padding,
+                peak_ratio=args.peak_ratio,
+            )
+            box_counts[len(boxes)] += 1
+            per_class_boxes[name] += len(boxes)
+
+            source = Path(meta["path"])
+            label_path = labels_dir / name / f"{source.stem}.txt"
+            label_path.parent.mkdir(parents=True, exist_ok=True)
+            label_path.write_text(
+                "\n".join(to_yolo_line(box, target) for box in boxes) + ("\n" if boxes else ""),
+                encoding="utf-8",
+            )
+
+            if args.viz_all or viz_done[name] < args.viz_per_class:
+                viz_done[name] += 1
+                save_visualization(
+                    viz_dir / name / f"{source.stem}.png", source, cam, boxes,
+                    f"{name}  |  {len(boxes)} box  |  thr={args.threshold}",
+                )
+
+        if step % 20 == 0:
+            logger.info("处理 [%d/%d]", step, len(loader))
+
+    total = sum(box_counts.values())
+    write_data_yaml(out_dir / "data.yaml", classes, root=out_dir)
+    summary = {
+        "checkpoint": args.checkpoint,
+        "dataset": str(data_cfg["root"]),
+        "num_images": total,
+        "cam_shape": list(cam_shape) if cam_shape else None,
+        "stage": args.stage,
+        "threshold": args.threshold,
+        "padding": args.padding,
+        "max_boxes": args.max_boxes,
+        "min_area_ratio": args.min_area_ratio,
+        "peak_ratio": args.peak_ratio,
+        "classification_accuracy_on_all": correct / total if total else 0.0,
+        "boxes_per_image": {str(k): v for k, v in sorted(box_counts.items())},
+        "boxes_per_class": dict(sorted(per_class_boxes.items())),
+    }
+    (out_dir / "summary.json").write_text(
+        json.dumps(summary, ensure_ascii=False, indent=2), encoding="utf-8"
+    )
+
+    logger.info("CAM 分辨率 %s | 全量分类准确率 %.4f", cam_shape, summary["classification_accuracy_on_all"])
+    for count, number in sorted(box_counts.items()):
+        logger.info("  生成 %d 个框的图片: %d 张 (%.1f%%)", count, number, 100 * number / total)
+    logger.info("标签写入 %s，可视化 %d 张写入 %s", labels_dir, sum(viz_done.values()), viz_dir)
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/tests/test_gradcam.py b/tests/test_gradcam.py
new file mode 100644
index 0000000..2f7ca21
--- /dev/null
+++ b/tests/test_gradcam.py
@@ -0,0 +1,180 @@
+"""Grad-CAM 与由热力图导出框。"""
+
+import pytest
+
+torch = pytest.importorskip("torch")
+
+import numpy as np  # noqa: E402
+from torch import nn  # noqa: E402
+
+from engine.gradcam import (  # noqa: E402
+    cam_to_boxes,
+    connected_components,
+    grad_cam,
+    to_yolo_line,
+)
+from models import build_model  # noqa: E402
+
+
+def test_connected_components_separates_blobs():
+    mask = np.array([[1, 1, 0, 0], [1, 1, 0, 0], [0, 0, 0, 1], [0, 0, 1, 1]], dtype=bool)
+    components = connected_components(mask)
+    assert len(components) == 2
+    assert sorted(int(c.sum()) for c in components) == [3, 4]
+    assert not (components[0] & components[1]).any()   # 互不重叠
+    assert (components[0] | components[1] == mask).all()
+
+
+def test_connected_components_is_four_connected():
+    """对角相邻不算连通——否则细长划痕会和邻近噪点粘成一个框。"""
+    mask = np.array([[1, 0], [0, 1]], dtype=bool)
+    assert len(connected_components(mask)) == 2
+
+
+def test_cam_to_boxes_finds_the_hot_region():
+    cam = np.zeros((8, 8), dtype=np.float32)
+    cam[2:5, 3:7] = 1.0
+    (box,) = cam_to_boxes(cam, threshold=0.5, min_area_ratio=0.0, peak_ratio=0.75)
+
+    x1, y1, x2, y2 = box
+    assert (x1, x2) == pytest.approx((3 / 8, 7 / 8))   # 框覆盖整格，不是格心距离
+    assert (y1, y2) == pytest.approx((2 / 8, 5 / 8))
+
+
+def test_padding_expands_within_bounds():
+    cam = np.zeros((8, 8), dtype=np.float32)
+    cam[3:5, 3:5] = 1.0
+    plain = cam_to_boxes(cam, threshold=0.5, min_area_ratio=0.0)[0]
+    padded = cam_to_boxes(cam, threshold=0.5, min_area_ratio=0.0, padding=0.5)[0]
+
+    assert padded[0] < plain[0] and padded[2] > plain[2]
+    assert all(0.0 <= v <= 1.0 for v in padded)        # 外扩不会越界
+
+    corner = np.zeros((8, 8), dtype=np.float32)
+    corner[0:2, 0:2] = 1.0
+    assert cam_to_boxes(corner, threshold=0.5, min_area_ratio=0.0, padding=1.0)[0][:2] == (0.0, 0.0)
+
+
+def test_peak_ratio_drops_weak_secondary_blobs():
+    """次强区域峰值只有 0.45 时多半是噪声——实测不过滤会在无缺陷处刷出假框。"""
+    cam = np.zeros((8, 8), dtype=np.float32)
+    cam[1:3, 1:3] = 1.0      # 主区域
+    cam[5:7, 5:7] = 0.45     # 弱响应区
+
+    assert len(cam_to_boxes(cam, threshold=0.4, min_area_ratio=0.0, peak_ratio=0.0)) == 2
+    assert len(cam_to_boxes(cam, threshold=0.4, min_area_ratio=0.0, peak_ratio=0.75)) == 1
+
+
+def test_min_area_and_max_boxes_limits():
+    cam = np.zeros((10, 10), dtype=np.float32)
+    cam[0:1, 0:1] = 1.0      # 1 格，面积很小
+    cam[4:8, 4:8] = 1.0
+    assert len(cam_to_boxes(cam, threshold=0.5, min_area_ratio=0.05, peak_ratio=0.5)) == 1
+
+    many = np.zeros((12, 12), dtype=np.float32)
+    for offset in (0, 4, 8):
+        many[offset:offset + 2, offset:offset + 2] = 1.0
+    assert len(cam_to_boxes(many, threshold=0.5, min_area_ratio=0.0, max_boxes=2)) == 2
+
+
+def test_no_box_when_nothing_activates():
+    assert cam_to_boxes(np.zeros((8, 8), dtype=np.float32), threshold=0.5) == []
+
+
+def test_to_yolo_line_roundtrip():
+    line = to_yolo_line((0.2, 0.4, 0.6, 0.8), class_id=7)
+    parts = line.split()
+    assert parts[0] == "7"
+    cx, cy, w, h = (float(v) for v in parts[1:])
+    assert (cx, cy, w, h) == pytest.approx((0.4, 0.6, 0.4, 0.4))
+
+
+class _TinyNet(nn.Module):
+    """features 是 nn.Sequential，好按 stage 取中间层。"""
+
+    def __init__(self) -> None:
+        super().__init__()
+        self.features = nn.Sequential(
+            nn.Conv2d(3, 4, 3, padding=1), nn.ReLU(),
+            nn.Conv2d(4, 8, 3, stride=2, padding=1), nn.ReLU(),
+        )
+        self.out_channels = 8
+
+    def forward(self, x):
+        return self.features(x)
+
+
+def test_grad_cam_shapes_and_normalization():
+    torch.manual_seed(0)
+    backbone = _TinyNet()
+    head = nn.Sequential(nn.AdaptiveAvgPool2d(1), nn.Flatten(), nn.Linear(8, 3))
+    images = torch.randn(2, 3, 16, 16)
+    targets = torch.tensor([0, 2])
+
+    cam = grad_cam(backbone, head, images, targets)
+    assert cam.shape == (2, 8, 8)                       # 与最终特征图同分辨率
+    assert not cam.requires_grad
+    assert float(cam.min()) >= 0.0 and float(cam.max()) <= 1.0
+    # 每张单独归一化：非退化的那张峰值应当正好是 1
+    peaks = cam.flatten(1).max(dim=1).values
+    assert any(float(p) == pytest.approx(1.0) for p in peaks)
+
+
+def test_grad_cam_all_zero_when_relu_kills_everything():
+    """梯度全为负时 ReLU 会清零整张图，此时归一化结果仍是合法的全 0，
+    对应下游"这张图提取不出框"——全量数据里约 3% 属于这种情况。"""
+    backbone = _TinyNet()
+    head = nn.Sequential(nn.AdaptiveAvgPool2d(1), nn.Flatten(), nn.Linear(8, 3))
+    with torch.no_grad():   # 让目标类别的权重全为负，梯度随之为负
+        head[2].weight.fill_(-1.0)
+        head[2].bias.zero_()
+    with torch.no_grad():
+        backbone.features[0].weight.fill_(0.1)
+        backbone.features[2].weight.fill_(0.1)
+
+    cam = grad_cam(backbone, head, torch.rand(1, 3, 16, 16), torch.tensor([0]))
+    assert float(cam.max()) == pytest.approx(0.0)
+    assert cam_to_boxes(cam[0].numpy(), threshold=0.4) == []
+
+
+def test_grad_cam_stage_controls_resolution():
+    backbone = _TinyNet()
+    head = nn.Sequential(nn.AdaptiveAvgPool2d(1), nn.Flatten(), nn.Linear(8, 3))
+    images = torch.randn(1, 3, 16, 16)
+    targets = torch.tensor([1])
+
+    # 浅层分辨率更高：stage 1 在第一次下采样之前
+    assert grad_cam(backbone, head, images, targets, stage=1).shape == (1, 16, 16)
+    assert grad_cam(backbone, head, images, targets, stage=3).shape == (1, 8, 8)
+
+    with pytest.raises(ValueError, match="超出骨架层数"):
+        grad_cam(backbone, head, images, targets, stage=99)
+
+
+def test_grad_cam_responds_to_the_target_class():
+    """不同目标类别应当给出不同的热力图，否则 CAM 就没有类别判别性。"""
+    torch.manual_seed(0)
+    backbone = _TinyNet()
+    head = nn.Sequential(nn.AdaptiveAvgPool2d(1), nn.Flatten(), nn.Linear(8, 3))
+    images = torch.randn(1, 3, 16, 16)
+
+    a = grad_cam(backbone, head, images, torch.tensor([0]))
+    b = grad_cam(backbone, head, images, torch.tensor([1]))
+    assert not torch.allclose(a, b)
+
+
+def test_grad_cam_works_on_the_real_classifier():
+    model = build_model({
+        "name": "classifier", "num_classes": 5,
+        "backbone": {"name": "torchvision", "arch": "resnet18", "pretrained": False},
+    }).eval()
+    for param in model.parameters():
+        param.requires_grad_(True)
+
+    images = torch.randn(2, 3, 128, 128)
+    targets = torch.tensor([1, 4])
+    cam = grad_cam(model.backbone, model.head, images, targets, stage=6)
+
+    assert cam.shape == (2, 8, 8)        # layer3 在 128 输入下是 8x8
+    boxes = cam_to_boxes(cam[0].cpu().numpy(), threshold=0.4, min_area_ratio=0.0)
+    assert all(0.0 <= v <= 1.0 for box in boxes for v in box)