Handle Ghostscript rasterization with DPI below 10

Ghostscript may fail when asked to rasterize at very low DPI values
(below 10 on either axis). This adds a workaround that uses a minimum
of 10 DPI for the Ghostscript call, then resizes the output image to
match the dimensions that would have resulted from the original low
DPI request.

Fixes #1612

src/ocrmypdf/_exec/ghostscript.py

@@ -127,6 +127,19 @@ def rasterize_pdf(
     if not page_dpi:
         page_dpi = raster_dpi
 
+    # Ghostscript may fail with very low DPI values (below 10). If the requested
+    # DPI is too low, use a minimum of 10 DPI and resize the output afterward.
+    MIN_RASTER_DPI = 10
+    needs_low_dpi_resize = (
+        raster_dpi.x < MIN_RASTER_DPI or raster_dpi.y < MIN_RASTER_DPI
+    )
+    if needs_low_dpi_resize:
+        effective_dpi = Resolution(
+            max(raster_dpi.x, MIN_RASTER_DPI), max(raster_dpi.y, MIN_RASTER_DPI)
+        )
+    else:
+        effective_dpi = raster_dpi
+
     args_gs = (
         [
             GS,
@@ -137,7 +150,7 @@ def rasterize_pdf(
             f'-sDEVICE={raster_device}',
             f'-dFirstPage={pageno}',
             f'-dLastPage={pageno}',
-            f'-r{raster_dpi.x:f}x{raster_dpi.y:f}',
+            f'-r{effective_dpi.x:f}x{effective_dpi.y:f}',
         ]
         + (['-dUseCropBox'] if use_cropbox else [])
         + (['-dFILTERVECTOR'] if filter_vector else [])
@@ -173,6 +186,16 @@ def rasterize_pdf(
 
     try:
         with Image.open(output_file) as im:
+            if needs_low_dpi_resize:
+                # Resize to the dimensions that would have resulted from the
+                # original low DPI request
+                scale_x = raster_dpi.x / effective_dpi.x
+                scale_y = raster_dpi.y / effective_dpi.y
+                new_size = (
+                    max(1, int(round(im.width * scale_x))),
+                    max(1, int(round(im.height * scale_y))),
+                )
+                im = im.resize(new_size, Image.Resampling.LANCZOS)
             if rotation is not None:
                 log.debug("Rotating output by %i", rotation)
                 # rotation is a clockwise angle and Image.ROTATE_* is

tests/test_ghostscript.py

@@ -81,6 +81,62 @@ def test_rasterize_rotated(francais, outdir, caplog):
         assert im.info['dpi'] == forced_dpi.flip_axis()
 
 
+def test_rasterize_low_dpi(francais, outdir):
+    """Test that very low DPI values (below 10) produce correctly sized output.
+
+    Ghostscript may fail with DPI values below 10. The workaround renders at
+    a minimum of 10 DPI and resizes the output to match the expected dimensions.
+    """
+    path, pdf = francais
+    page_size_pts = (pdf.pages[0].mediabox[2], pdf.pages[0].mediabox[3])
+    assert pdf.pages[0].mediabox[0] == pdf.pages[0].mediabox[1] == 0
+    page_size = (float(page_size_pts[0]) / 72, float(page_size_pts[1]) / 72)
+
+    # Request a very small output (DPI below 10 on both axes)
+    target_size = (5, 3)
+    forced_dpi = Resolution(72.0, 72.0)
+
+    rasterize_pdf(
+        path,
+        outdir / 'out_low_dpi.png',
+        raster_device=GhostscriptRasterDevice.PNGMONO,
+        raster_dpi=Resolution(
+            target_size[0] / page_size[0], target_size[1] / page_size[1]
+        ),
+        page_dpi=forced_dpi,
+    )
+
+    with Image.open(outdir / 'out_low_dpi.png') as im:
+        assert im.size == target_size
+        assert im.info['dpi'] == forced_dpi
+
+
+def test_rasterize_low_dpi_one_axis(francais, outdir):
+    """Test low DPI on only one axis produces correctly sized output."""
+    path, pdf = francais
+    page_size_pts = (pdf.pages[0].mediabox[2], pdf.pages[0].mediabox[3])
+    assert pdf.pages[0].mediabox[0] == pdf.pages[0].mediabox[1] == 0
+    page_size = (float(page_size_pts[0]) / 72, float(page_size_pts[1]) / 72)
+
+    # Request low DPI on X axis only (below 10), normal on Y axis
+    target_size = (5, 50)
+    forced_dpi = Resolution(72.0, 72.0)
+
+    rasterize_pdf(
+        path,
+        outdir / 'out_low_dpi_x.png',
+        raster_device=GhostscriptRasterDevice.PNGMONO,
+        raster_dpi=Resolution(
+            target_size[0] / page_size[0], target_size[1] / page_size[1]
+        ),
+        page_dpi=forced_dpi,
+    )
+
+    with Image.open(outdir / 'out_low_dpi_x.png') as im:
+        assert im.size == target_size
+        assert im.info['dpi'] == forced_dpi
+
+
 def test_gs_render_failure(resources, outpdf, caplog):
     exitcode = run_ocrmypdf_api(
         resources / 'blank.pdf',