探针增加尺寸扫描：区分链路带宽不够与摄像头本身就是 15 fps - What: tools/camera_probe.py 新增 SWEEP_SIZES 与 size_sweep()，在帧率扫描之后对 MJPG@30 依次申请 1920x1080/1280x720/960x540/848x480/640x480 并打印实测速率，提示语写明判断方式：帧越小越快 ⇒ 链路带宽是瓶颈；每个尺寸都一样 ⇒ 是摄像头本身的速率上限。 - Why: 用户实测 MJPG 在 1080p 与 720p 都是 15 fps、YUYV 两档都是 1 fps，两种解释仍然都成立（链路只能扛约 3-4 MB/s，或这颗摄像头在这些格式上就是 15 fps 设备），而处置完全不同；帧率扫描只能区分"申请被砍半"与"固定上限"，尺寸扫描才能区分"带宽"与"设备"。 - Verification: compileall 通过；无摄像头时仍返回 1 并给出解释；studio 全量 91 passed（探针的无摄像头用例覆盖 main() 的降级路径）。 - Not in this change: 未改 SDK 采集参数；未做自动降级。

apps/studio/tools/camera_probe.py

@@ -49,6 +49,18 @@ BACKENDS: tuple[tuple[str, int], ...] = (
 #: follows the request or ignores it.
 SWEEP_RATES: tuple[float, ...] = (60.0, 30.0, 20.0, 15.0, 10.0)
 
+#: Sizes asked for at one fixed rate. This is what separates the two remaining
+#: explanations for a low rate: "the link cannot carry the pixels" (smaller
+#: frames go faster) from "this camera is a 15 fps device" (the rate does not
+#: move at all).
+SWEEP_SIZES: tuple[tuple[int, int], ...] = (
+    (1920, 1080),
+    (1280, 720),
+    (960, 540),
+    (848, 480),
+    (640, 480),
+)
+
 SAMPLES = 60
 #: A request is "missed" when the measured rate is below this fraction of it.
 TOLERANCE = 0.9
@@ -145,6 +157,24 @@ def sweep(index: int, backend_name: str, backend: int, size: tuple[int, int],
     return rows
 
 
+def size_sweep(index: int, backend_name: str, backend: int, fps: float = 30.0,
+               *, samples: int = SAMPLES) -> list[dict]:
+    """Ask for one rate at shrinking sizes: bandwidth, or a fixed ceiling?
+
+    A link that cannot carry the pixels gets faster as the frames get smaller. A
+    camera whose own rate is 15 fps delivers 15 whatever it is asked for.
+    """
+    rows = []
+    for width, height in SWEEP_SIZES:
+        result = probe(
+            index, backend_name, backend, ("MJPG", width, height, fps),
+            samples=samples,
+        )
+        if result is not None:
+            rows.append(result)
+    return rows
+
+
 def main(argv: list[str] | None = None) -> int:
     parser = argparse.ArgumentParser(description=__doc__.splitlines()[0])
     parser.add_argument("--index", type=int, default=None, help="camera index to probe")
@@ -198,6 +228,21 @@ def main(argv: list[str] | None = None) -> int:
                     f"(a ceiling that does not move with the request is the "
                     f"hardware's, not the request's)"
                 )
+            print(
+                f"\n  sizes at MJPG@{30:g} fps [{first_backend[0]}] -- bandwidth, "
+                f"or a fixed ceiling?"
+            )
+            for result in size_sweep(
+                index, first_backend[0], first_backend[1], samples=args.samples
+            ):
+                width, height = result["asked"][1], result["asked"][2]
+                print(
+                    f"    {width}x{height:<5} -> measured {result['measured_fps']:5.1f} fps"
+                )
+            print(
+                "    smaller frames going faster means the link is the limit; the "
+                "same number at every size means the camera is"
+            )
         if header:
             found += 1