Measure the target's detail at the size candidates are rendered at

src/vectrify/vector/runner.py

@@ -167,11 +167,14 @@ def run_vector_search(
 
     scoring_img = resize_long_side(original_img, DEFAULT_CONFIG.target_long_side)
     pixel_ref = prepare(scoring_img)
-    # The target's own detail, measured once. Candidates are charged for the
-    # distance from it, so this has to come from the same render size they do.
-    _ref_buf = io.BytesIO()
-    scoring_img.save(_ref_buf, format="PNG")
-    reference_detail = detail(_ref_buf.getvalue())
+    # The target's own detail, measured once, at the size candidates are
+    # rasterized at -- original_png_bytes, not the smaller image the pixel
+    # comparison resizes to. Compressed size grows with pixel count, so reading
+    # the reference at scoring resolution and candidates at render resolution
+    # charges every candidate for the difference between the two: measured on
+    # the duck, the same image reads 7,607 at 256 and 41,602 at 700, and every
+    # candidate in a run came out 58-142% "busier" than a target it matched.
+    reference_detail = detail(original_png_bytes)
     log.info(
         "Round scoring: edge overlap, colour distance and a detail budget, no model. "
         f"Front evaluator: {ScorerType(scorer_type).value} ({vision_model})."

tests/score/test_complexity.py

@@ -74,3 +74,32 @@ def test_an_empty_candidate_wins_this_axis_and_loses_the_rest():
 
 def test_survives_a_blank_reference():
     assert detail_excess(0.0, _noisy()) == 0.0
+
+
+def test_detail_grows_with_pixel_count_at_equal_busyness():
+    """Why the reference has to be measured at the size candidates render at.
+    Compressed size counts bytes, so the same picture reads larger when there
+    is more of it -- reading the reference at scoring resolution and candidates
+    at render resolution charges every candidate for the gap between the two.
+    """
+    from vectrify.image_utils import resize_long_side
+
+    rng = random.Random(5)
+    big = Image.new("RGB", (256, 256))
+    big.putdata(
+        [
+            (rng.randrange(256), rng.randrange(256), rng.randrange(256))
+            for _ in range(256 * 256)
+        ]
+    )
+    small = resize_long_side(big, 96)
+
+    def as_png(img):
+        buf = io.BytesIO()
+        img.save(buf, format="PNG")
+        return buf.getvalue()
+
+    assert detail(as_png(big)) > detail(as_png(small)) * 2
+    # Which is exactly the false charge: the same image against itself at the
+    # wrong scale reads as substantially busier than the reference.
+    assert detail_excess(detail(as_png(small)), as_png(big)) > 1.0