fix(render): retain source tile indices for PDF subsets

render/src/pixelrag_render/backends/pdf.py

@@ -32,6 +32,7 @@ def render_pdf(
         output_dir: Directory to write the tile subdirectory into.
         dpi: Resolution for rendering (default 200 gives ~1650×2200px for A4).
         pages: 1-based list of page numbers to render. ``None`` renders all pages.
+            Tile filenames and indices retain their zero-based source page index.
         quality: JPEG quality 1-100 (default 85).
         stem: Override for the tile directory name. Defaults to the PDF filename
             stem. The pipeline passes the article_id here so directory names
@@ -82,10 +83,11 @@ def render_pdf(
 
     saved_tiles: list[str] = []
     chunks_info: list[dict] = []
-    for idx, img in enumerate(images):
+    first_index = convert_kwargs.get("first_page", 1) - 1
+    for idx, img in enumerate(images, start=first_index):
         # If caller provided a sparse page list, skip pages not in the list
         if pages is not None:
-            page_num = min(pages) + idx
+            page_num = idx + 1
             if page_num not in pages:
                 continue
 

tests/test_pdf_page_indices.py

@@ -0,0 +1,43 @@
+"""Selected PDF pages retain the same zero-based IDs as a full render."""
+
+import json
+import sys
+from types import ModuleType
+
+import pytest
+from PIL import Image
+from pixelrag_render import render_pdf
+
+
+@pytest.mark.parametrize("pages", [None, [1], [3], [4], [2, 3], [1, 3], [2, 4], [4, 2]])
+def test_pdf_page_indices_match_full_render(tmp_path, monkeypatch, pages):
+    source = tmp_path / "document.pdf"
+    source.touch()
+    colors = ["red", "green", "blue", "yellow"]
+    converter = ModuleType("pdf2image")
+
+    def convert_from_path(**kwargs):
+        first = kwargs.get("first_page", 1)
+        last = kwargs.get("last_page", 4)
+        return [
+            Image.new("RGB", (16, 16), colors[i - 1]) for i in range(first, last + 1)
+        ]
+
+    converter.convert_from_path = convert_from_path
+    monkeypatch.setitem(sys.modules, "pdf2image", converter)
+    full_dir = render_pdf(source, tmp_path / "full")[0]
+    selected_dir = render_pdf(source, tmp_path / "selected", pages=pages)[0]
+    expected_ids = [0, 1, 2, 3] if pages is None else sorted(p - 1 for p in pages)
+    expected_files = [f"tile_{i:04d}.jpg" for i in expected_ids]
+    manifest = json.loads((selected_dir / "tiles.json").read_text())
+    chunks = json.loads((selected_dir / "chunks.json").read_text())["chunks"]
+
+    assert manifest["tiles"] == expected_files
+    assert [c["tile_index"] for c in chunks] == expected_ids
+    assert [c["file"] for c in chunks] == expected_files
+    assert [c["tile"] for c in chunks] == expected_files
+    assert sorted(p.name for p in selected_dir.glob("tile_*.jpg")) == expected_files
+    for filename in expected_files:
+        assert (selected_dir / filename).read_bytes() == (
+            full_dir / filename
+        ).read_bytes()