Add trackcrop: crop a video to a square around a tracked objectHEADmaster

Single-file Python script. The user boxes an object on the first frame,
a CSRT tracker follows it, the path is smoothed over ~0.5 s and the
square crop is encoded with h264_vaapi, audio copied.

--ak820 FPS limits the output to the AJAZZ AK820 Pro screen's 255
frames at FPS, prompting for which window to keep (start, end or a
start second) when the clip is longer.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

5 files changed, 580 insertions, 0 deletions

diff --git a/AGENTS.md b/AGENTS.md
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--- /dev/null
+++ b/AGENTS.md
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+# trackcrop
+
+Single-file Python script (`trackcrop.py`) that crops a video to a 1:1 square,
+keeping a user-selected object centered. See README.md for usage.
+
+## How it works
+
+1. `cv2.selectROI` on the first frame, the user draws a box.
+2. Pass 1: CSRT tracker (`cv2.TrackerCSRT_create`, needs opencv-contrib) records
+   the object center per frame; when tracking is lost the last center is reused.
+3. Centers are smoothed with a ~0.5s moving average (edge-padded, no lag).
+4. Pass 2: re-read the video, crop an `s x s` square (`s = min(w, h)`) clamped
+   to the frame, pipe raw BGR frames to ffmpeg.
+
+Two passes keep memory flat, frames are never all held in RAM.
+
+`--ak820 FPS` limits the output to the AK820 Pro screen's 255 frames at FPS:
+if the clip is longer, an `input()` prompt picks the window (start, end or a
+start second) before `selectROI`. Both passes skip to the window's first frame
+with `grab()` (exact on any codec, unlike `CAP_PROP_POS_FRAMES` seeking) and
+stop after `count` frames; audio is cut with `-ss`/`-t` on the second ffmpeg
+input.
+
+## Environment facts
+
+- The local ffmpeg is built without `libx264` and without the native `aac`
+  encoder. Encoding uses `h264_vaapi` on `/dev/dri/renderD128`. NVENC, AMF and
+  Vulkan H.264 encoders are listed but do not work on this machine.
+- Audio is stream-copied, so no audio encoder is needed. `--no-audio` drops it.
+- For test clips use `libvpx-vp9` video and `libopus` audio.
+- `drawbox` with a `t` expression did not animate in testing, use `overlay`
+  with `y='...*t'` to make a moving test object.
+
+## Testing
+
+No test suite. Verify end to end headless by generating a synthetic clip with a
+moving red square, stubbing the GUI, and measuring the square's offset from the
+output frame center:
+
+```python
+import sys, runpy, cv2
+cv2.namedWindow = lambda *a: None
+cv2.selectROI = lambda *a: (310, 200, 100, 100)  # box around the test object
+cv2.destroyAllWindows = lambda: None
+sys.argv = ["trackcrop.py", "in.mp4", "out.mp4"]
+runpy.run_path("trackcrop.py")
+```
+
+For `--ak820`, also stub the prompt (`builtins.input = lambda p: "e"`), use a
+`testsrc2` background (it has a frame counter), and check the output's first
+frame against the source with `cv2.matchTemplate`: the best match must be the
+window's first frame.
+
+Test clip:
+
+```sh
+ffmpeg -f lavfi -i "color=gray:s=720x1280:d=4:r=30" \
+       -f lavfi -i "color=red:s=100x100:d=4:r=30" -f lavfi -i "sine=d=4" \
+       -filter_complex "[0][1]overlay=x=310:y='200+200*t'" \
+       -c:v libvpx-vp9 -c:a libopus -shortest in.mp4
+```
+
+A solid square is hard for CSRT (no texture), expect some drift late in the
+clip; real footage tracks better.
+
+## Conventions
+
+- Keep it a single file, stdlib plus numpy/opencv only.
+- Every source file carries the GPLv2-only header
+  (`Copyright (C) <year> Danilo M. <danix@danix.xyz>`).
+- Work directly on master.
diff --git a/CLAUDE.md b/CLAUDE.md
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+++ b/CLAUDE.md
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+# trackcrop
+
+AGENTS.md is the single source of truth for this project, shared across every
+agent tool. Edit AGENTS.md, not this file.
+
+@AGENTS.md
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..9efa6fb
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,338 @@
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diff --git a/README.md b/README.md
new file mode 100644
index 0000000..f2d86e2
--- /dev/null
+++ b/README.md
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+# trackcrop
+
+Crop a video to a 1:1 square while keeping a chosen object centered. You pick
+the object on the first frame, OpenCV's CSRT tracker follows it, the path is
+smoothed to remove jitter, and the cropped frames are encoded with ffmpeg.
+
+## Requirements
+
+- Python 3 with numpy and `opencv-contrib-python` (a GUI build, not `-headless`)
+- ffmpeg with the `h264_vaapi` encoder and a VA-API device at `/dev/dri/renderD128`
+
+## Usage
+
+```sh
+./trackcrop.py [--no-audio] [--ak820 FPS] in.mp4 out.mp4
+```
+
+1. A window opens on the first frame. Drag a box around the object and press
+   Enter or Space (`c` cancels the selection).
+2. The script tracks the object and encodes the result. It is done when the
+   prompt returns.
+
+Audio is copied unchanged unless `--no-audio` is given.
+
+### AJAZZ AK820 Pro screen
+
+The AK820 Pro keyboard screen holds at most 255 frames, so at a given upload
+frame rate it can only show `255 / FPS` seconds (8.5 s at 30 fps). With
+`--ak820 FPS`, a clip longer than that asks which part to keep before the
+object selection:
+
+- `s` or Enter: the start
+- `e`: the end
+- a number: start at that many seconds
+
+The object box is then drawn on the first frame of the kept part, and the
+audio is trimmed to match. Upload the result with
+`ak820-upload out.mp4 FPS`, using the same `FPS`.
+
+## Notes
+
+- The object must be visible in the first frame. Trim the start otherwise:
+  `ffmpeg -ss 3 -i in.mp4 -c copy trimmed.mp4`.
+- The square never leaves the frame, so the object sits off center near the
+  edges.
+- Works on landscape videos too, the square is cut horizontally.
+
+## License
+
+GPLv2 only. See [LICENSE](LICENSE).
+
+## Development Approach
+
+This project is developed using AI-assisted tools. Code is generated with the help of AI based on human-provided specifications, design decisions, and iterative feedback.
+
+All contributions are reviewed, tested, and curated by the maintainer before being included in the codebase. AI is used as a productivity and exploration tool, while human oversight remains central to all decisions.
+
+The goal is to combine the flexibility of AI-assisted development with standard open-source practices such as transparency, review, and accountability.
diff --git a/trackcrop.py b/trackcrop.py
new file mode 100755
index 0000000..9a29270
--- /dev/null
+++ b/trackcrop.py
@@ -0,0 +1,107 @@
+#!/usr/bin/env python3
+# Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# Crop a vertical video to 1:1, keeping a tracked object centered.
+# usage: trackcrop.py [--no-audio] [--ak820 FPS] in.mp4 out.mp4 ; draw box around object on first frame, press Enter
+# needs: opencv-contrib-python (not -headless), ffmpeg with h264_vaapi
+import sys, subprocess, cv2, numpy as np
+
+AK820_FRAMES = 255  # AJAZZ AK820 Pro screen holds at most 255 frames (see ak820-upload)
+USAGE = "usage: trackcrop.py [--no-audio] [--ak820 FPS] in.mp4 out.mp4"
+
+args = sys.argv[1:]
+mute = "--no-audio" in args
+if mute:
+    args.remove("--no-audio")
+kfps = None
+if "--ak820" in args:
+    i = args.index("--ak820")
+    try:
+        kfps = float(args[i + 1])
+    except (IndexError, ValueError):
+        sys.exit(USAGE)
+    del args[i:i + 2]
+if len(args) != 2:
+    sys.exit(USAGE)
+src, dst = args
+
+def open_at(first):
+    """Open src positioned at frame `first` (grab, not seek: exact on any codec)."""
+    cap = cv2.VideoCapture(src)
+    for _ in range(first):
+        cap.grab()
+    return cap
+
+cap = cv2.VideoCapture(src)
+fps = cap.get(cv2.CAP_PROP_FPS)
+total = int(cap.get(cv2.CAP_PROP_FRAME_COUNT))  # container estimate
+cap.release()
+
+# --ak820: keep a window that fits the keyboard's frame limit at FPS
+first, count = 0, 10**9
+if kfps and total / fps > AK820_FRAMES / kfps:
+    dur, keep = total / fps, AK820_FRAMES / kfps
+    ans = input(f"{dur:.1f}s clip, the keyboard holds {keep:.1f}s at {kfps:g} fps. "
+                f"Keep [s]tart, [e]nd, or start at N seconds (0-{dur - keep:.1f}): ").strip().lower()
+    try:
+        t0 = 0.0 if ans in ("", "s") else dur - keep if ans == "e" else float(ans)
+    except ValueError:
+        sys.exit(f"not s, e or a number: {ans}")
+    t0 = min(max(t0, 0.0), dur - keep)
+    first, count = round(t0 * fps), int(keep * fps)
+    print(f"keeping {t0:.2f}s to {t0 + keep:.2f}s")
+
+cap = open_at(first)
+ok, frame = cap.read()
+if not ok:
+    sys.exit(f"cannot read {src}")
+h, w = frame.shape[:2]
+s = min(w, h)
+tr = cv2.TrackerCSRT_create()
+cv2.namedWindow("select object", cv2.WINDOW_NORMAL)  # resizable, tall frames fit the screen
+tr.init(frame, cv2.selectROI("select object", frame))
+cv2.destroyAllWindows()
+
+# pass 1: track centers (reuse last position when tracker loses it)
+cx, cy = [], []
+last = (w / 2, h / 2)
+while ok and len(cx) < count:
+    found, (x, y, bw, bh) = tr.update(frame)
+    if found:
+        last = (x + bw / 2, y + bh / 2)
+    cx.append(last[0]); cy.append(last[1])
+    ok, frame = cap.read()
+
+# smooth path to kill jitter (~0.5s moving average)
+k = max(int(fps // 2) | 1, 1)
+smooth = lambda a: np.convolve(np.pad(a, k // 2, mode="edge"), np.ones(k) / k, "valid")
+cx, cy = smooth(np.array(cx)), smooth(np.array(cy))
+
+# pass 2: crop and pipe to ffmpeg, keep original audio
+# ponytail: local ffmpeg has no libx264, encodes on GPU via VA-API; device path is hardcoded
+ff = subprocess.Popen(["ffmpeg", "-y", "-vaapi_device", "/dev/dri/renderD128",
+    "-f", "rawvideo", "-pix_fmt", "bgr24",
+    "-s", f"{s}x{s}", "-r", str(fps), "-i", "-",
+    *([] if mute else ["-ss", f"{first / fps}", "-t", f"{len(cx) / fps}", "-i", src,
+                       "-map", "0:v", "-map", "1:a?", "-c:a", "copy"]),
+    "-vf", "format=nv12,hwupload", "-c:v", "h264_vaapi", "-qp", "20", dst],
+    stdin=subprocess.PIPE)
+cap = open_at(first)
+for x, y in zip(cx, cy):
+    ok, frame = cap.read()
+    if not ok:
+        break
+    x0 = int(np.clip(x - s / 2, 0, w - s))
+    y0 = int(np.clip(y - s / 2, 0, h - s))
+    ff.stdin.write(frame[y0:y0 + s, x0:x0 + s].tobytes())
+ff.stdin.close()
+ff.wait()