Add --ghostscript-jpeg-quality and --ghostscript-jpeg-maxdpi

Expose Ghostscript's -dJPEGQ and image downsampling switches as
advanced, plugin-scoped options for tuning PDF/A output, without
polluting the central OcrOptions registry. The optimizer's existing
--jpeg-quality remains the recommended JPEG quality control.

- GhostscriptOptions gains jpeg_quality and jpeg_maxdpi fields and CLI
  args (advanced help text). jpeg_quality=0 is honored as Ghostscript's
  maximum compression rather than being silently coerced to the default.
- _exec.ghostscript.generate_pdfa() forwards both values; when
  jpeg_maxdpi is set, downsample threshold is pinned at 1.0.
- _get_plugin_options falls back to extra_attrs for namespaced fields
  so plugins can own their options without registering them centrally.
- Documentation explains the rationale: Ghostscript is the legacy path
  (pypdfium + verapdf is preferred in v17+), the optimizer is the
  supported file-size lever, and lowering quality is almost always a
  better trade than downsampling.

docs/advanced.md

@@ -419,6 +419,70 @@ curves. In this case, you may want to use a different color conversion
 strategy. The `--color-conversion-strategy` option allows you to select a
 different strategy, such as `RGB`.
 
+## Advanced Ghostscript tuning
+
+:::{versionadded} 17.5.0
+:::
+
+OCRmyPDF intentionally hides most Ghostscript controls because Ghostscript
+is a legacy code path. The preferred PDF/A pipeline in v17+ uses pypdfium2
+as the rasterizer and verapdf to validate speculative PDF/A output, with
+Ghostscript reserved as a fallback for PDFs that cannot be made compliant
+without it. OCRmyPDF's separate optimizer (controlled by `--optimize`,
+`--jpeg-quality`, `--png-quality`, etc.) is the supported way to shrink
+output PDFs: it gives consistent results across input files, and isolates
+Ghostscript so it can focus on producing a PDF/A with as few image
+transformations as possible.
+
+The two options below are exposed for advanced users who want to tune
+Ghostscript's intermediate PDF/A output directly. Most users will get
+more predictable results from the optimizer.
+
+### `--ghostscript-jpeg-quality Q`
+
+Sets Ghostscript's `-dJPEGQ` switch for images that Ghostscript chooses
+to recompress to JPEG while building a PDF/A. `Q=0` requests maximum
+compression and `Q=100` requests best quality; if the flag is omitted,
+OCRmyPDF passes `95` (the historical default). This only affects images
+Ghostscript transcodes — existing JPEGs pass through unchanged on modern
+Ghostscript releases. For end-to-end JPEG quality tuning, prefer
+`--jpeg-quality`, which is implemented by the OCRmyPDF optimizer and is
+applied independently of whatever Ghostscript decides to do.
+
+Note: setting both `--ghostscript-jpeg-quality` and `--jpeg-quality` can
+result in double JPEG recompression, since the optimizer may re-encode
+images that Ghostscript already recompressed. This can degrade quality
+in subtle ways.
+
+### `--ghostscript-jpeg-maxdpi DPI`
+
+Enables Ghostscript's image downsampling and caps color, grayscale, and
+monochrome image resolution to `DPI`. The downsample threshold is set to
+`1.0`, so any image whose effective DPI exceeds the cap will be
+downsampled.
+
+Reducing JPEG quality is almost always a better trade than downsampling
+at the same compression budget: a 400 DPI JPEG at modest quality usually
+looks much better than a 200 DPI JPEG, because the JPEG codec can spend
+bits where they count. Downsampling is also dangerous for PDFs that
+combine a low-resolution color image with a high-resolution monochrome
+mask — capping the mask resolution can produce visible quality loss.
+For these reasons, prefer `--jpeg-quality` over `--ghostscript-jpeg-maxdpi`
+unless you specifically want to force a hard DPI cap.
+
+Example:
+
+```bash
+ocrmypdf --output-type pdfa \
+    --ghostscript-jpeg-quality 80 \
+    --ghostscript-jpeg-maxdpi 150 \
+    in.pdf out.pdf
+```
+
+These options only take effect when Ghostscript is invoked for PDF/A
+conversion (`--output-type pdfa`, `pdfa-1`, `pdfa-2`, or `pdfa-3`, or
+when `--output-type auto` falls back to Ghostscript).
+
 ## PDF/A output modes
 
 :::{versionchanged} 17.0.0

docs/cookbook.md

@@ -31,6 +31,16 @@ ocrmypdf --output-type pdf input.pdf output.pdf
 ocrmypdf --output-type pdfa --pdfa-image-compression jpeg input.pdf output.pdf
 ```
 
+### Reduce JPEG quality with the optimizer
+
+This is the recommended way to shrink JPEG content in the output. The
+optimizer applies regardless of `--output-type`, so it works on both
+plain PDFs and Ghostscript-produced PDF/A files.
+
+```bash
+ocrmypdf --optimize 2 --jpeg-quality 60 input.pdf output.pdf
+```
+
 ### Modify a file in place
 
 The file will only be overwritten if OCRmyPDF is successful.

docs/introduction.md

@@ -183,6 +183,11 @@ v17 addresses through alternative codepaths. When Ghostscript is used:
   `jpeg` or `lossless` to set all images to one type or the other.
   Ghostscript lacks an option to maintain the input image's format.
   (Modern Ghostscript can copy JPEG images without transcoding them.)
+  Advanced users can also tune Ghostscript's image recompression with
+  `--ghostscript-jpeg-quality` and `--ghostscript-jpeg-maxdpi`; see
+  [Advanced Ghostscript tuning](advanced.md#advanced-ghostscript-tuning).
+  Most users should prefer `--jpeg-quality` (applied by the OCRmyPDF
+  optimizer) over those Ghostscript-scoped controls.
 - Ghostscript's PDF/A conversion removes any XMP metadata that is not
   one of the standard XMP metadata namespaces for PDFs. In particular,
   PRISM Metadata is removed.

docs/optimizer.md

@@ -98,7 +98,16 @@ If `pngquant` is installed, OCRmyPDF will use it to perform quantize
 paletted images to reduce their size.
 
 The quality of JPEGs may be lowered, on the assumption that a lower
-quality image may be suitable for storage after OCR.
+quality image may be suitable for storage after OCR. Use `--jpeg-quality`
+to control the optimizer's JPEG quality target. The optimizer is the
+recommended way to reduce JPEG image sizes: it applies consistently
+regardless of whether Ghostscript was used to produce a PDF/A.
+
+If you specifically need to tune Ghostscript's own PDF/A image handling
+(for example, to force a hard DPI cap), see
+[Advanced Ghostscript tuning](advanced.md#advanced-ghostscript-tuning)
+for the separate `--ghostscript-jpeg-quality` and
+`--ghostscript-jpeg-maxdpi` options.
 
 It is not possible to optimize all image types. Uncommon image types may
 be skipped by the optimizer.

misc/completion/ocrmypdf.bash

@@ -46,6 +46,8 @@ __ocrmypdf_arguments()
 --rasterizer                    (PDF page rasterizer)
 --rotate-pages-threshold        (page rotation confidence)
 --pdfa-image-compression        (set PDF/A image compression options)
+--ghostscript-jpeg-quality      (Ghostscript JPEG quality during PDF/A [0..100])
+--ghostscript-jpeg-maxdpi       (cap Ghostscript image DPI during PDF/A)
 --fast-web-view                 (if file size if above this amount in MB linearize PDF)
 --continue-on-soft-render-error (continue after recoverable render errors)
 --plugin                        (name of plugin to import)
@@ -337,6 +339,7 @@ __ocrmypdf_check_previous()
 
         --title|--author|--subject|--keywords|--unpaper-args|--pages|--plugin|\
         --jpeg-quality|--png-quality|--image-dpi|--oversample|--skip-big|--max-image-mpixels|\
+        --ghostscript-jpeg-quality|--ghostscript-jpeg-maxdpi|\
         --tesseract-timeout|--tesseract-non-ocr-timeout|--tesseract-downsample-above|\
         --rotate-pages-threshold|--fast-web-view)
             # argument required but no completions available

misc/completion/ocrmypdf.fish

@@ -102,6 +102,8 @@ function __fish_ocrmypdf_pdfa_compression
     echo -e "lossless\t"(_ "convert color and grayscale images to lossless (PNG)")
 end
 complete -c ocrmypdf -x -l pdfa-image-compression -a '(__fish_ocrmypdf_pdfa_compression)' -d "set PDF/A image compression options"
+complete -c ocrmypdf -x -l ghostscript-jpeg-quality -d "Ghostscript JPEG quality during PDF/A [0..100]"
+complete -c ocrmypdf -x -l ghostscript-jpeg-maxdpi -d "cap Ghostscript image DPI during PDF/A"
 
 complete -c ocrmypdf -x -s j -l jobs -d "how many worker processes to use"
 complete -c ocrmypdf -x -l title -d "set metadata"

src/ocrmypdf/_exec/ghostscript.py

@@ -278,6 +278,8 @@ def generate_pdfa(
     *,
     compression: str,
     color_conversion_strategy: str,
+    jpeg_quality: int | None = None,
+    jpeg_maxdpi: int | None = None,
     pdf_version: str = '1.5',
     pdfa_part: str = '2',
     progressbar_class=None,
@@ -318,15 +320,38 @@ def generate_pdfa(
         # Windows has lots of fatal "permission denied" errors
         stop_on_error = False
 
-    # 1. nb no need to specify ProcessColorModel when ColorConversionStrategy
+    # `-dJPEGQ=N` tells Ghostscript to use a JPEG quality of N, IF it decides
+    # to transcode an image to JPEG. When there are existing JPEG images,
+    # Ghostscript uses passthrough mode, so the quality level is not changed.
+    # OCRmyPDF's optimizer separately uses the `--jpeg-quality` command line
+    # option to potentially re-encode JPEG images, regardless of whether
+    # Ghostscript decided to transcode them to JPEG or not.
+    # `jpeg_quality=0` is meaningful to Ghostscript (maximum compression), so
+    # only fall back to the default when the value is None.
+    effective_jpeg_quality = jpeg_quality if jpeg_quality is not None else 95
+
+    # Downsampling images is a blunt-force way to reduce file size and almost
+    # always degrades quality more than lowering JPEG quality at the original
+    # resolution. We expose this for users with very specific needs (e.g.
+    # producing very small files for screen-only viewing); the optimizer is
+    # usually a better choice.
+    downsample_args: list[str] = []
+    if jpeg_maxdpi is not None:
+        downsample_args = [
+            "-dDownsampleColorImages=true",
+            "-dColorImageDownsampleThreshold=1.0",
+            "-dDownsampleGrayImages=true",
+            "-dGrayImageDownsampleThreshold=1.0",
+            "-dDownsampleMonoImages=true",
+            "-dMonoImageDownsampleThreshold=1.0",
+            f"-dColorImageResolution={jpeg_maxdpi}",
+            f"-dGrayImageResolution={jpeg_maxdpi}",
+            f"-dMonoImageResolution={jpeg_maxdpi}",
+        ]
+
+    # nb no need to specify ProcessColorModel when ColorConversionStrategy
     # is set; see:
     # https://bugs.ghostscript.com/show_bug.cgi?id=699392
-    # 2. `-dJPEGQ=95` tells Ghostscript to use a JPEG quality of 95, IF it
-    # decides to transcode an image to JPEG. When there are existing JPEG
-    # images, Ghostscript uses passthrough mode, so the quality level is not
-    # changed. OCRmyPDF's optimizer uses the `--jpeg-quality` command line
-    # option to potentially re-encoded JPEG images, regardless of whether
-    # Ghostscript decided to transcode them to JPEG or not.
     args_gs = (
         [
             GS,
@@ -340,8 +365,9 @@ def generate_pdfa(
         ]
         + (['-dPDFSTOPONERROR'] if stop_on_error else [])
         + compression_args
+        + downsample_args
         + [
-            "-dJPEGQ=95",  # See note above on JPEG quality
+            f"-dJPEGQ={effective_jpeg_quality}",  # See note above on JPEG quality
             "-dSubsetFonts=false",  # Prevents GS from messing up some encodings
             f"-dPDFA={pdfa_part}",
             "-dPDFACompatibilityPolicy=1",

src/ocrmypdf/_options.py

@@ -582,6 +582,13 @@ class OcrOptions(BaseModel):
                 value = getattr(self, flat_name)
                 if value is not None:
                     kwargs[field_name] = _convert_value(value)
+            # Plugin-scoped fields that aren't in the central OcrOptions
+            # registry: argparse stores them in extra_attrs under the
+            # namespace_field name.
+            elif flat_name in self.extra_attrs:
+                value = self.extra_attrs[flat_name]
+                if value is not None:
+                    kwargs[field_name] = _convert_value(value)
             # Also check direct field name (for fields like jbig2_lossy)
             elif field_name in OcrOptions.model_fields:
                 value = getattr(self, field_name)

src/ocrmypdf/builtin_plugins/ghostscript.py

@@ -54,6 +54,27 @@ class GhostscriptOptions(BaseModel):
     pdfa_image_compression: Annotated[
         PdfaImageCompression, Field(description="PDF/A image compression method")
     ] = PdfaImageCompression.AUTO
+    jpeg_quality: Annotated[
+        int | None,
+        Field(
+            ge=0,
+            le=100,
+            description=(
+                "JPEG quality (0-100) for Ghostscript image recompression during "
+                "PDF/A generation; None uses Ghostscript's default."
+            ),
+        ),
+    ] = None
+    jpeg_maxdpi: Annotated[
+        int | None,
+        Field(
+            ge=1,
+            description=(
+                "Maximum DPI for Ghostscript image downsampling during PDF/A "
+                "generation."
+            ),
+        ),
+    ] = None
 
     @classmethod
     def add_arguments_to_parser(cls, parser, namespace: str = 'ghostscript'):
@@ -86,6 +107,35 @@ class GhostscriptOptions(BaseModel):
             "skipped.  Not supported for --output-type=pdf ; that setting "
             "preserves the original compression of all images.",
         )
+        gs.add_argument(
+            '--ghostscript-jpeg-quality',
+            type=int,
+            metavar='Q',
+            default=None,
+            dest=f'{namespace}_jpeg_quality',
+            help=(
+                "Advanced: Set Ghostscript's -dJPEGQ for images that Ghostscript "
+                "transcodes to JPEG during PDF/A generation. 0 is maximum "
+                "compression; 100 is best quality. If omitted, Ghostscript's "
+                "default is used. This only affects images Ghostscript chooses "
+                "to recompress; for general JPEG quality tuning prefer "
+                "--jpeg-quality, which is applied by the OCRmyPDF optimizer."
+            ),
+        )
+        gs.add_argument(
+            '--ghostscript-jpeg-maxdpi',
+            type=int,
+            metavar='DPI',
+            default=None,
+            dest=f'{namespace}_jpeg_maxdpi',
+            help=(
+                "Advanced: Force Ghostscript to downsample color, grayscale, "
+                "and monochrome images in PDF/A output to the given maximum DPI. "
+                "Reducing JPEG quality usually gives better results than "
+                "downsampling at the same file size, and can degrade quality "
+                "of high-resolution monochrome masks."
+            ),
+        )
 
 
 @hookimpl
@@ -352,6 +402,8 @@ def generate_pdfa(
         output_file=output_file,
         compression=context.options.ghostscript.pdfa_image_compression,
         color_conversion_strategy=context.options.ghostscript.color_conversion_strategy,
+        jpeg_quality=context.options.ghostscript.jpeg_quality,
+        jpeg_maxdpi=context.options.ghostscript.jpeg_maxdpi,
         pdf_version=pdf_version,
         pdfa_part=pdfa_part,
         progressbar_class=progressbar_class,

tests/test_ghostscript.py

@@ -137,6 +137,129 @@ def test_rasterize_low_dpi_one_axis(francais, outdir):
         assert im.info['dpi'] == forced_dpi
 
 
+def test_generate_pdfa_default_jpeg_quality(outdir):
+    """When jpeg_quality is None, Ghostscript receives -dJPEGQ=95 (default)."""
+    with (
+        patch('ocrmypdf._exec.ghostscript.version', return_value=Version('10.05.1')),
+        patch('ocrmypdf._exec.ghostscript.run_polling_stderr') as run_mock,
+    ):
+        run_mock.return_value = subprocess.CompletedProcess(
+            ['gs'], returncode=0, stdout='', stderr=''
+        )
+        ghostscript.generate_pdfa(
+            pdf_pages=[outdir / 'input.pdf'],
+            output_file=outdir / 'out.pdf',
+            compression='auto',
+            color_conversion_strategy='LeaveColorUnchanged',
+        )
+
+    args = run_mock.call_args.args[0]
+    assert '-dJPEGQ=95' in args
+    # No downsample switches when jpeg_maxdpi is not set
+    assert not any(a.startswith('-dDownsampleColorImages') for a in args)
+    assert not any(a.startswith('-dColorImageResolution') for a in args)
+
+
+def test_generate_pdfa_uses_user_jpeg_quality(outdir):
+    with (
+        patch('ocrmypdf._exec.ghostscript.version', return_value=Version('10.05.1')),
+        patch('ocrmypdf._exec.ghostscript.run_polling_stderr') as run_mock,
+    ):
+        run_mock.return_value = subprocess.CompletedProcess(
+            ['gs'], returncode=0, stdout='', stderr=''
+        )
+        ghostscript.generate_pdfa(
+            pdf_pages=[outdir / 'input.pdf'],
+            output_file=outdir / 'out.pdf',
+            compression='jpeg',
+            color_conversion_strategy='RGB',
+            jpeg_quality=72,
+        )
+
+    args = run_mock.call_args.args[0]
+    assert '-dJPEGQ=72' in args
+    assert '-dJPEGQ=95' not in args
+
+
+def test_generate_pdfa_jpeg_quality_zero_is_max_compression(outdir):
+    """Explicit jpeg_quality=0 must reach Ghostscript as -dJPEGQ=0.
+
+    Ghostscript accepts 0 as a valid quality value (maximum compression);
+    it must not be silently replaced by the default 95.
+    """
+    with (
+        patch('ocrmypdf._exec.ghostscript.version', return_value=Version('10.05.1')),
+        patch('ocrmypdf._exec.ghostscript.run_polling_stderr') as run_mock,
+    ):
+        run_mock.return_value = subprocess.CompletedProcess(
+            ['gs'], returncode=0, stdout='', stderr=''
+        )
+        ghostscript.generate_pdfa(
+            pdf_pages=[outdir / 'input.pdf'],
+            output_file=outdir / 'out.pdf',
+            compression='jpeg',
+            color_conversion_strategy='RGB',
+            jpeg_quality=0,
+        )
+
+    args = run_mock.call_args.args[0]
+    assert '-dJPEGQ=0' in args
+    assert '-dJPEGQ=95' not in args
+
+
+def test_generate_pdfa_honors_jpeg_maxdpi(outdir):
+    with (
+        patch('ocrmypdf._exec.ghostscript.version', return_value=Version('10.05.1')),
+        patch('ocrmypdf._exec.ghostscript.run_polling_stderr') as run_mock,
+    ):
+        run_mock.return_value = subprocess.CompletedProcess(
+            ['gs'], returncode=0, stdout='', stderr=''
+        )
+        ghostscript.generate_pdfa(
+            pdf_pages=[outdir / 'input.pdf'],
+            output_file=outdir / 'out.pdf',
+            compression='auto',
+            color_conversion_strategy='LeaveColorUnchanged',
+            jpeg_maxdpi=300,
+        )
+
+    args = run_mock.call_args.args[0]
+    assert '-dJPEGQ=95' in args
+    assert '-dDownsampleColorImages=true' in args
+    assert '-dColorImageDownsampleThreshold=1.0' in args
+    assert '-dDownsampleGrayImages=true' in args
+    assert '-dGrayImageDownsampleThreshold=1.0' in args
+    assert '-dDownsampleMonoImages=true' in args
+    assert '-dMonoImageDownsampleThreshold=1.0' in args
+    assert '-dColorImageResolution=300' in args
+    assert '-dGrayImageResolution=300' in args
+    assert '-dMonoImageResolution=300' in args
+
+
+def test_ghostscript_jpeg_options_via_cli(resources, outpdf):
+    """End-to-end: CLI flags reach the ghostscript plugin namespace."""
+    with patch(
+        'ocrmypdf._exec.ghostscript.generate_pdfa',
+        wraps=ghostscript.generate_pdfa,
+    ) as gen_mock:
+        run_ocrmypdf_api(
+            resources / 'francais.pdf',
+            outpdf,
+            '--output-type',
+            'pdfa',
+            '--ghostscript-jpeg-quality',
+            '60',
+            '--ghostscript-jpeg-maxdpi',
+            '150',
+            '--plugin',
+            'tests/plugins/tesseract_noop.py',
+        )
+    assert gen_mock.called
+    call_kwargs = gen_mock.call_args.kwargs
+    assert call_kwargs['jpeg_quality'] == 60
+    assert call_kwargs['jpeg_maxdpi'] == 150
+
+
 def test_gs_render_failure(resources, outpdf, caplog):
     exitcode = run_ocrmypdf_api(
         resources / 'blank.pdf',
@@ -176,9 +299,9 @@ def test_ghostscript_pdfa_failure(resources, outpdf, caplog):
         '--plugin',
         'tests/plugins/gs_pdfa_failure.py',
     )
-    assert (
+    assert exitcode == ExitCode.pdfa_conversion_failed, (
-        exitcode == ExitCode.pdfa_conversion_failed
+        "Unexpected return when PDF/A fails"
-    ), "Unexpected return when PDF/A fails"
+    )
 
 
 def test_ghostscript_feature_elision(resources, outpdf):
@@ -439,7 +562,9 @@ class TestGs106JpegCorruptionRepair:
                     repaired_bytes_list.append(obj.read_raw_bytes())
 
         assert len(repaired_bytes_list) == len(original_bytes_list)
-        for orig, repaired_bytes in zip(original_bytes_list, repaired_bytes_list, strict=False):
+        for orig, repaired_bytes in zip(
+            original_bytes_list, repaired_bytes_list, strict=False
+        ):
             assert orig == repaired_bytes, "Repaired bytes should match original"
 
         # Check that error/warning was logged