fix: the single-color filter no longer takes line art for a blank image (#4483)

Five of Todding's Discord doodles (pencil lines on white, 3000px) were skipped
on import as "single color", so their posts showed text and no image. The
predicate sampled a 64px BILINEAR thumbnail, which blends thin strokes into
the paper, and 0.95 "one color" is below how white a doodle is.

- Sample 256x256 by NEAREST, so each sample is a real pixel.
- Default threshold 0.995: blank means essentially blank. Migration 0116 moves
  a stored 0.95 (the old default) with it; the settings slider now spans
  0.9-1 in 0.005 steps so the value is reachable.
- The Cleanup audit shares the predicate, so it stops flagging sketches too.

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

alembic/versions/0116_single_color_threshold.py

@@ -0,0 +1,34 @@
+"""The single-color filter's default becomes near-total: 0.95 → 0.995.
+
+#4483. At 0.95 the filter rejected line art: a pencil doodle on white is
+mostly white, and five of Todding's Discord doodles were skipped on import as
+"single color", leaving their posts with text and no image. The predicate now
+samples real pixels instead of a blurred thumbnail, and the default only
+calls an image blank when it essentially is one.
+
+A stored 0.95 is the old default, so it moves with it. Any other value was
+chosen by hand and is left alone.
+
+Revision ID: 0116
+Revises: 0115
+Create Date: 2026-09-28
+
+"""
+from alembic import op
+
+revision = "0116"
+down_revision = "0115"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.alter_column("import_settings", "single_color_threshold", server_default="0.995")
+    op.execute(
+        "UPDATE import_settings SET single_color_threshold = 0.995 "
+        "WHERE single_color_threshold = 0.95"
+    )
+
+
+def downgrade() -> None:
+    op.alter_column("import_settings", "single_color_threshold", server_default="0.95")

backend/app/models/import_settings.py

@@ -39,7 +39,7 @@ class ImportSettings(Base):
     transparency_threshold: Mapped[float] = mapped_column(Float, nullable=False, default=0.9, server_default="0.9")
 
     skip_single_color: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False, server_default="false")
-    single_color_threshold: Mapped[float] = mapped_column(Float, nullable=False, default=0.95, server_default="0.95")
+    single_color_threshold: Mapped[float] = mapped_column(Float, nullable=False, default=0.995, server_default="0.995")
     single_color_tolerance: Mapped[int] = mapped_column(Integer, nullable=False, default=30, server_default="30")
 
     # Hamming distance over a 256-bit pHash (utils.phash, hash_size=16). The

backend/app/services/audits/single_color.py

@@ -4,11 +4,19 @@ predicate for BOTH surfaces: FC-Cleanup's retroactive audit and — since
 2026-07-02 — the import-side filter (Importer._single_color_hit /
 SkipReason.single_color), so what the audit flags and what the import
 skips can never disagree.
+
+It is meant to catch the blank: a placeholder, an error tile, a solid fill.
+Line art is the case it must not catch (#4483): a pencil doodle on white can be
+well over 95% white even at full size, and a smoothing downsample blends its strokes
+into the paper until it measures as blank. So the sample is taken by NEAREST
+(each sampled pixel is a real pixel, strokes keep their contrast), at 256px
+so thin strokes are still hit, and the default threshold is near-total
+(0.995) — a few percent of ink is a drawing, not an empty image.
 """
 
 from PIL import Image
 
-_THUMB_SIZE = (64, 64)
+_THUMB_SIZE = (256, 256)
 
 
 def evaluate(
@@ -20,7 +28,8 @@ def evaluate(
     """True iff the fraction of pixels within `tolerance` (Euclidean RGB
     distance) of the dominant color exceeds `threshold`.
 
-    Downsamples to 64x64 for speed (~4ms regardless of source size).
+    Samples 256x256 by NEAREST (see the module docstring for why not a
+    smoothing resample).
     Alpha channels are stripped; only RGB is considered. Animated images
     use frame 0 (PIL's default after Image.open without seek).
     """
@@ -30,7 +39,7 @@ def evaluate(
     elif im.mode not in ("RGB", "L"):
         im = im.convert("RGB")
     if im.size != _THUMB_SIZE:
-        im = im.resize(_THUMB_SIZE, Image.Resampling.BILINEAR)
+        im = im.resize(_THUMB_SIZE, Image.Resampling.NEAREST)
     pixels = list(im.getdata())
     if not pixels:
         return False

frontend/src/components/cleanup/SingleColorAuditCard.vue

@@ -101,7 +101,7 @@ import MaintenanceTile from '../common/MaintenanceTile.vue'
 import { useCleanupStore } from '../../stores/cleanup.js'
 
 const store = useCleanupStore()
-const threshold = ref(0.95)
+const threshold = ref(0.995)
 const tolerance = ref(30)
 const audit = ref(null)
 const busy = ref(false)

frontend/src/components/settings/ImportFiltersForm.vue

@@ -77,7 +77,7 @@
         <v-col cols="12" sm="6">
           <v-slider
             v-model="local.single_color_threshold" label="Single-color threshold"
-            min="0.5" max="1" step="0.05" thumb-label hide-details color="accent"
+            min="0.9" max="1" step="0.005" thumb-label hide-details color="accent"
             :disabled="!local.skip_single_color" @end="save"
           />
         </v-col>
@@ -142,7 +142,7 @@ const PHASH_TICKS = { 0: 'Exact', 12: 'Strict', 24: 'Default', 48: 'Loose' }
 const local = reactive({
   min_width: 0, min_height: 0,
   skip_transparent: false, transparency_threshold: 0.9,
-  skip_single_color: false, single_color_threshold: 0.95,
+  skip_single_color: false, single_color_threshold: 0.995,
   phash_threshold: 24,
   wip_title_tagging_enabled: true,
 })

frontend/src/stores/cleanup.js

@@ -12,7 +12,7 @@ export const useCleanupStore = defineStore('cleanup', () => {
     min_width: 0,
     min_height: 0,
     transparency_threshold: 0.9,
-    single_color_threshold: 0.95,
+    single_color_threshold: 0.995,
     single_color_tolerance: 30,
   })
 
@@ -24,7 +24,7 @@ export const useCleanupStore = defineStore('cleanup', () => {
       min_width: s.min_width ?? 0,
       min_height: s.min_height ?? 0,
       transparency_threshold: s.transparency_threshold ?? 0.9,
-      single_color_threshold: s.single_color_threshold ?? 0.95,
+      single_color_threshold: s.single_color_threshold ?? 0.995,
       single_color_tolerance: s.single_color_tolerance ?? 30,
     }
   }

tests/test_audits_single_color.py

@@ -40,3 +40,48 @@ def test_single_color_evaluate_handles_rgba_input():
     # Alpha channel should be ignored — only RGB matters for the rule.
     im = Image.new("RGBA", (50, 50), (100, 100, 100, 128))
     assert single_color.evaluate(im, threshold=0.9, tolerance=10) is True
+
+
+# -- line art is a drawing, not a blank (#4483) ---------------------------------
+
+_T = 0.995  # the default since alembic 0116
+_TOL = 30
+
+
+def _doodle(size=(1500, 2000), lines=12):
+    """Pencil line art on white: a few thin dark strokes plus faint
+    construction lines — a couple of percent ink, the rest paper."""
+    from PIL import ImageDraw
+
+    im = Image.new("RGB", size, (255, 255, 255))
+    draw = ImageDraw.Draw(im)
+    w, h = size
+    for i in range(lines):
+        x = (i + 1) * w // (lines + 1)
+        draw.line([(x, 0), (w - x, h)], fill=(70, 60, 60), width=3)
+        draw.line([(0, (i + 1) * h // (lines + 1)), (w, h - x)], fill=(235, 235, 235), width=2)
+    return im
+
+
+def test_line_art_on_white_is_not_single_color():
+    """Five of Todding's Discord doodles were skipped as blank: a smoothing
+    64px thumbnail blended the strokes into the paper."""
+    assert single_color.evaluate(_doodle(), threshold=_T, tolerance=_TOL) is False
+
+
+def test_a_blank_page_with_a_small_mark_is_still_single_color():
+    from PIL import ImageDraw
+
+    im = Image.new("RGB", (1000, 1000), (250, 250, 250))
+    ImageDraw.Draw(im).rectangle([480, 480, 520, 520], fill=(40, 40, 40))
+    assert single_color.evaluate(im, threshold=_T, tolerance=_TOL) is True
+
+
+def test_a_noisy_solid_fill_is_still_single_color():
+    """JPEG noise on a placeholder stays inside the tolerance."""
+    im = Image.new("RGB", (600, 600), (30, 90, 160))
+    for i in range(0, 600 * 600, 7):
+        x, y = i % 600, i // 600
+        d = (i % 21) - 10
+        im.putpixel((x, y), (30 + d, 90 + d, 160 + d))
+    assert single_color.evaluate(im, threshold=_T, tolerance=_TOL) is True