qemu: structural hash relative to the background, taps that land on the pixel

imgtools.structural() marked a cell occupied when its grey exceeded an
absolute 10/255, and the WardenOS page background is grey 16: every cell
of every region read occupied and no structural check could ever fail
(#19). Occupancy is now grey deviating from the crop's own median by more
than DEVIATION_THRESHOLD, or edge energy above EDGE_THRESHOLD. Measured on
real captures: a switch knob left/right differs in 240 of 256 cells (was
0), a dark card reads its icon and text and nothing else. Every committed
reference is recaptured with flare-edge tools/flow-run-all.sh --capture.

qmp.py to_axis() truncated the pixel-to-axis conversion and LVGL's
evdev calibration truncates on the way back, so many pixels landed one
short (130 -> 5924 -> 129) and a tap could miss the control hit had just
confirmed at that pixel (#21). It now rounds up to the smallest axis value
that truncates to the requested pixel; test_qmp_drive.py asserts the round
trip for every pixel at three panel sizes.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N3G6m9Aw5RyVY4ZowtKzEj

qemu/tests/imgtools.py

@@ -37,8 +37,12 @@ from PIL import Image, ImageFilter
 # occupancy thresholds for structural(): a downsampled cell counts as
 # "occupied" if it is meaningfully brighter than black, or sits on an edge.
 # Both are 0..255 greyscale/edge-magnitude averages over the cell.
-NONBLACK_THRESHOLD = 10
+# structural(): a cell is occupied when its grey is this far from the crop's
-EDGE_THRESHOLD = 10
+# median (its background) or its FIND_EDGES energy exceeds the edge threshold.
+# Validated on real captures: a switch knob left/right differs in 240/256
+# cells, a dark card's icon and text stand out from its (7,13,29) ground.
+DEVIATION_THRESHOLD = 28
+EDGE_THRESHOLD = 24
 
 
 # ---------------------------------------------------------------------------
@@ -181,25 +185,48 @@ def hamming(a, b):
 
 def structural(img):
     """16x16 binary occupancy mask as 32 bytes (256 bits, MSB first,
-    row-major). A cell is "occupied" if it is meaningfully non-black or
+    row-major). A cell is "occupied" if it carries ink relative to the crop's
-    sits on an edge, so the mask is robust to a recolour (still occupied)
+    OWN background -- its grey deviates from the crop's median by more than
-    but sensitive to a shape disappearing (goes from occupied to empty)."""
+    DEVIATION_THRESHOLD -- or sits on an edge, so the mask is robust to a
+    recolour (still occupied) but sensitive to a shape disappearing (goes
+    from occupied to empty).
+
+    Relative to the median, not to black: the WardenOS page background is
+    (7,13,29), grey 16, and an absolute non-black test read every cell of
+    every region as occupied, so no structural check could ever fail
+    (SDK #19, flare-edge #156). Measured on real captures with this rule: a
+    Bluetooth switch off/on differs in 240 of 256 cells, a dark card reads
+    its icon and text and nothing else."""
     grey = _to_pil(img).convert("L")
     edges = grey.filter(ImageFilter.FIND_EDGES)
     grey_small = grey.resize((16, 16), Image.BOX)
     edge_small = edges.resize((16, 16), Image.BOX)
+    median = _median_grey(grey)
     gpx, epx = grey_small.load(), edge_small.load()
     bits = bytearray(32)
     idx = 0
     for y in range(16):
         for x in range(16):
-            occupied = gpx[x, y] > NONBLACK_THRESHOLD or epx[x, y] > EDGE_THRESHOLD
+            occupied = abs(gpx[x, y] - median) > DEVIATION_THRESHOLD or epx[x, y] > EDGE_THRESHOLD
             if occupied:
                 bits[idx // 8] |= 1 << (7 - (idx % 8))
             idx += 1
     return bytes(bits)
 
 
+def _median_grey(grey):
+    """The crop's dominant luminance: the background of a card, the fill of a
+    switch, whatever most of the pixels are."""
+    hist = grey.histogram()
+    total = sum(hist)
+    acc = 0
+    for value, count in enumerate(hist):
+        acc += count
+        if acc * 2 >= total:
+            return value
+    return 0
+
+
 def _structural_diff(a, b):
     """Count of differing bits between two 32-byte occupancy masks."""
     return sum(bin(x ^ y).count("1") for x, y in zip(a, b))

qemu/tests/qmp.py

@@ -97,6 +97,7 @@ A step that names an unknown command is a FATAL error rather than a skip: a
 silently-ignored line in a scenario is a test that proves nothing.
 """
 import json
+import math
 import os
 import re
 import shutil
@@ -157,9 +158,18 @@ def btn_ev(down):
 
 
 def to_axis(px, size):
-    """Panel pixels -> the tablet's absolute axis, clamped to the panel."""
+    """Panel pixels -> the tablet's absolute axis, clamped to the panel.
+
+    LVGL's evdev driver maps the axis back with integer truncation
+    (lv_evdev.c _evdev_calibrate: px = axis * (width - 1) / AXIS_MAX), so the
+    axis value must be the SMALLEST one that truncates to px, i.e. rounded up.
+    Truncating here as well composed two floors and landed one pixel short for
+    many values (130 -> 5924 -> 129), which is how a tap could miss the control
+    that `hit` at the same pixel had just confirmed (flare-edge #148 triage).
+    tools/touch-inject writes the pixel itself on a panel, so hardware never
+    had this seam."""
     px = max(0, min(size - 1, int(px)))
-    return int(px * AXIS_MAX / (size - 1))
+    return min(AXIS_MAX, math.ceil(px * AXIS_MAX / (size - 1)))
 
 
 def do_tap(s, f, ax, ay, hold=0.2):

qemu/tests/test_qmp_drive.py

@@ -85,6 +85,17 @@ def run_script(text, refs=None):
 
 
 class PureHelpers(unittest.TestCase):
+    def test_every_pixel_round_trips_through_lvgl_calibration(self):
+        # lv_evdev.c _evdev_calibrate: px = axis * (width - 1) / AXIS_MAX,
+        # integer division. A tap requested at px must land at px, for every
+        # px, or a hit-confirmed target can be missed by one pixel.
+        for size in (720, 480, 1024):
+            for px in range(size):
+                axis = qmp.to_axis(px, size)
+                self.assertTrue(0 <= axis <= qmp.AXIS_MAX)
+                back = axis * (size - 1) // qmp.AXIS_MAX
+                self.assertEqual(back, px, f"size {size}: px {px} -> axis {axis} -> {back}")
+
     def test_resolve_path_and_ops(self):
         doc = {"a": {"b": [5, 6]}, "s": "connected"}
         self.assertEqual(qmp.resolve_path(doc, "a.b[1]"), (6, None))