feat: LTX-2 legal-frame helpers (core/ltx2.py)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/ltx2.py

@@ -0,0 +1,26 @@
+"""LTX-2 frame-count math. Legal F satisfy F % 8 == 1 (8x temporal + 1)."""
+
+
+def is_legal_frames(f: int) -> bool:
+    return f >= 9 and f % 8 == 1
+
+
+def legal_frames(min_f: int = 9, max_f: int = 1000) -> list[int]:
+    start = max(9, min_f + ((1 - min_f) % 8))   # first 8k+1 >= min_f
+    return list(range(start, max_f + 1, 8))
+
+
+def nearest_legal_frames(f: int) -> int:
+    if f <= 9:
+        return 9
+    low = ((f - 1) // 8) * 8 + 1
+    high = low + 8
+    return low if (f - low) <= (high - f) else high
+
+
+def duration_for_frames(frames: int, fps: float) -> float:
+    return frames / fps
+
+
+def frames_for_duration(duration: float, fps: float) -> int:
+    return nearest_legal_frames(round(duration * fps))

tests/test_utils.py

@@ -439,3 +439,26 @@ def test_apply_keyframes_before_first_uses_base():
     result = apply_keyframes_to_jobs(jobs, kfs, base_center=0.5, base_ratio="4:5",
                                      base_rand_p=True, base_rand_s=False)
     assert result == [(1.0, "/out/a", "4:5", 0.5, True, False)]
+
+
+# --- LTX-2 legal-frame math (core/ltx2.py) ---
+
+from core.ltx2 import is_legal_frames, nearest_legal_frames, frames_for_duration, duration_for_frames, legal_frames
+
+def test_ltx2_is_legal():
+    assert is_legal_frames(201) and is_legal_frames(9) and is_legal_frames(25)
+    assert not is_legal_frames(200) and not is_legal_frames(8)
+
+def test_ltx2_nearest():
+    assert nearest_legal_frames(200) == 201   # 200 -> nearest 8k+1
+    assert nearest_legal_frames(196) == 193
+    assert nearest_legal_frames(5) == 9        # floor at 9
+
+def test_ltx2_duration_roundtrip():
+    assert duration_for_frames(201, 25) == 201 / 25
+    assert frames_for_duration(8.0, 25) == 201   # 200 -> 201
+
+def test_ltx2_legal_series():
+    s = legal_frames(min_f=9, max_f=33)
+    assert s == [9, 17, 25, 33]
+