Chore: Supersample the PDF thumbnail render 2x

Render the PDF page at twice the target size and downsample, with edge case
handling for small and odd sized pages, simplified helpers and tests, and
trimmed comments.

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

src/documents/parsers.py

@@ -79,8 +79,10 @@ _THUMBNAIL_MAX_WIDTH = 500
 _THUMBNAIL_MAX_HEIGHT = 5000
 # Used only when the page geometry cannot be read
 _THUMBNAIL_FALLBACK_DPI = 150
-# Matches the density the thumbnail was previously rendered at before scaling
+# Applied before supersampling, so tiny pages are not enlarged
 _THUMBNAIL_MAX_DPI = 300
+# Rendering at a multiple and downsampling keeps text crisper
+_THUMBNAIL_SUPERSAMPLE = 2
 
 
 def rasterize_pdf_page_to_png(
@@ -123,9 +125,11 @@ def rasterize_pdf_page_to_png(
 def encode_thumbnail_webp(
     png_path: Path,
     out_path: Path,
+    *,
+    supersample: int = 1,
 ) -> None:
     """
-    Flattens alpha onto white, shrinks to fit and saves as WebP.
+    Flattens alpha onto white, undoes supersampling, shrinks to fit and saves as WebP.
     """
     from PIL import Image
 
@@ -137,16 +141,25 @@ def encode_thumbnail_webp(
             else:
                 flattened = im.convert("RGB")
 
+        if supersample > 1:
+            flattened = flattened.resize(
+                (
+                    max(1, round(flattened.width / supersample)),
+                    max(1, round(flattened.height / supersample)),
+                ),
+                Image.Resampling.LANCZOS,
+            )
+
         flattened.thumbnail((_THUMBNAIL_MAX_WIDTH, _THUMBNAIL_MAX_HEIGHT))
         flattened.save(out_path, format="WEBP")
     except (OSError, Image.DecompressionBombError) as e:
         raise ParseError(f"Unable to encode thumbnail from {png_path}") from e
 
 
-def _compute_thumbnail_dpi(in_path: Path, logging_group=None) -> int:
+def _compute_thumbnail_dpi(in_path: Path, logging_group=None) -> tuple[int, int]:
     """
-    Computes the DPI which renders the first page of the PDF at or just above
-    the thumbnail size in one pass, never above 300.
+    Returns (dpi, supersample). Unknown geometry is not supersampled, since
+    the render size cannot be bounded.
     """
     from paperless.parsers.utils import get_pdf_first_page_size_points
 
@@ -156,17 +169,19 @@ def _compute_thumbnail_dpi(in_path: Path, logging_group=None) -> int:
             "Could not read PDF page size, using fallback DPI",
             extra={"group": logging_group},
         )
-        return _THUMBNAIL_FALLBACK_DPI
+        return _THUMBNAIL_FALLBACK_DPI, 1
 
     width_pts, height_pts = size
     dpi_for_width = _THUMBNAIL_MAX_WIDTH * 72 / width_pts
     dpi_for_height = _THUMBNAIL_MAX_HEIGHT * 72 / height_pts
-    # Round up so the render is never a few pixels short of the target; the
-    # shrink-only clamp in encode_thumbnail_webp trims the excess.
-    return max(
+    # Round up so the downsampled render is never a few pixels short of the
+    # target; the shrink-only clamp in encode_thumbnail_webp trims the excess.
+    dpi = max(
         1,
         math.ceil(min(_THUMBNAIL_MAX_DPI, dpi_for_width, dpi_for_height)),
     )
+    # At the 1 DPI floor the page is already oversized, so do not supersample
+    return dpi, 1 if dpi == 1 else _THUMBNAIL_SUPERSAMPLE
 
 
 def _render_pdf_thumbnail(
@@ -175,14 +190,14 @@ def _render_pdf_thumbnail(
     out_path: Path,
     logging_group=None,
 ) -> None:
-    dpi = _compute_thumbnail_dpi(in_path, logging_group=logging_group)
+    dpi, supersample = _compute_thumbnail_dpi(in_path, logging_group=logging_group)
     rasterize_pdf_page_to_png(
         in_path,
         png_path,
-        dpi=dpi,
+        dpi=dpi * supersample,
         logging_group=logging_group,
     )
-    encode_thumbnail_webp(png_path, out_path)
+    encode_thumbnail_webp(png_path, out_path, supersample=supersample)
 
 
 def _repair_pdf_with_qpdf(in_path: Path, out_path: Path) -> None: