Motion detection must reliably detect jumpers while ignoring environmental changes (wind, lighting, snow). Tuning involves balancing sensitivity against false triggers.
debug_motion=1 to see detection diagnosticsThe most effective tuning step is narrowing the motion region to exclude areas where non-jumper motion occurs (trees, spectators, wind-blown objects).
# Only monitor the landing hill area in feed 1
motion_start_x[1]=200
motion_start_y[1]=100
motion_end_x[1]=1700
motion_end_y[1]=900Use the viewer with showing_motion=1 to visualize what the detector sees.
These control what counts as a "changed" pixel between frames:
ydiff_min[N] — minimum brightness change to count as different (default 10). Increase if sensor noise causes false pixels. Decrease if the jumper is hard to detect against similar background.udiff_min[N], vdiff_min[N] — same for color channels. Color changes help detect jumpers wearing colored suits against white snow.These filter out whole-frame changes (lighting shifts, camera shake):
max_diff_y% — if more than this percentage of pixels changed, reject the frame (probably a lighting change, not a jumper). Default 0.5%.min_diff_y% — if fewer than this percentage changed, reject (just noise). Default 0.01%.max_diff_y%. If it misses jumpers in low contrast conditions, lower ydiff_min.
min_blob_sizes[N] — blobs smaller than this are noise (default 30 pixels). A jumper at distance typically produces blobs of 50–500 pixels.max_blob_size — blobs larger than this are rejected (default 50000). Prevents triggering on large lighting changes that pass the percentage filter.Nearby blobs are merged into one. Adjust merge tolerance:
blob_fuzz_x / blob_fuzz_y — merge blobs within this many pixels (default 30). Increase if the jumper appears as multiple scattered blobs.min_blob_distance[N] — minimum blob movement per frame (default 2 pixels). Filters out stationary noise blobs.max_blob_distance[N] — maximum blob movement per frame (default 200 pixels). Filters out random blobs that appear in different locations.The trigger requires multiple consecutive frames of motion to fire:
min_motion_count[N] — frames of motion needed to start recording (default 3). Increase to reduce false triggers; decrease for faster response.min_nomotion_count[N] — frames without motion to stop recording (default 10). Increase to avoid premature clip ending if jumper briefly leaves the motion region.min_blob_neighbors — minimum neighboring changed pixels for a pixel to be kept (default 2). This removes isolated noise pixels. Increase for noisy cameras; decrease if thin features (ski edges) are lost.| Problem | Adjustment |
|---|---|
| False triggers on wind/trees | Narrow motion_start/end to exclude trees. Increase min_blob_sizes. |
| False triggers on lighting changes | Lower max_diff_y% (e.g., 0.3). Increase min_blob_sizes. |
| Misses jumpers in fog/flat light | Lower ydiff_min (e.g., 5). Lower min_blob_sizes (e.g., 15). |
| Clips end too early | Increase min_nomotion_count (e.g., 20). |
| Clips start too late | Decrease min_motion_count (e.g., 2). Increase movie_pad. |
| Too many small noise blobs | Increase min_blob_neighbors (e.g., 3). Increase min_blob_sizes. |
| Jumper splits into multiple blobs | Increase blob_fuzz_x and blob_fuzz_y (e.g., 50). |
| Triggers on spectators | Narrow motion region to exclude spectator areas. |
Enable diagnostics to see what the detector is doing:
debug_motion=1
debug_distance=1This prints per-frame information to the console: