fix(badapple): flip potrace y-up polygons to the framebuffer

bad-apple/data/manifest.json

@@ -1,7 +1,7 @@
 {
   "cost_us": {
-    "max": 37991,
-    "mean": 7472.0695503699535,
+    "max": 37992,
+    "mean": 7472.069550369951,
     "p50": 7948,
     "p95": 12011
   },

bad-apple/pipeline/vectorize.py

@@ -113,6 +113,16 @@ def trace(mask: np.ndarray, params: dict) -> list[Polygon]:
         with open(json_path, "r", encoding="utf-8") as handle:
             polygons = parse_potrace_geojson(handle.read())
 
+    # potrace's GeoJSON uses a y-up coordinate system; the framebuffer is
+    # y-down.  Without this the whole movie is vertically mirrored.
+    flipped: list[Polygon] = []
+    for poly in polygons:
+        rings: Polygon = []
+        for ring in poly:
+            rings.append([(x, float(height) - y) for x, y in ring])
+        flipped.append(rings)
+    polygons = flipped
+
     simplified: list[Polygon] = []
     for poly in polygons:
         rings: Polygon = []

bad-apple/tests/test_vectorize.py

@@ -1,6 +1,6 @@
 import numpy as np
 
-from pipeline.vectorize import rdp, rasterize, trace
+from pipeline.vectorize import rdp, rasterize, trace, vectorize_frame
 
 
 def test_rdp_drops_collinear_point():
@@ -23,6 +23,14 @@ def test_rasterize_square_with_hole():
     assert not out[3, 3]
 
 
+def test_trace_preserves_vertical_position():
+    mask = np.zeros((32, 32), dtype=bool)
+    mask[2:6, 4:12] = True
+    out = vectorize_frame(mask, {"rdp_epsilon": 1.0, "min_segment": 2})
+    assert out[3, 5]
+    assert not out[28, 5]
+
+
 def test_trace_simple_shape_and_roundtrip():
     mask = np.zeros((16, 16), dtype=bool)
     mask[4:12, 4:12] = True