feat: promote raster device for color/gray image masks; default pngmonod (#1688)

src/ocrmypdf/_pipeline.py

@@ -44,7 +44,7 @@ from ocrmypdf.pdfa import (
     generate_pdfa_ps,
     speculative_pdfa_conversion,
 )
-from ocrmypdf.pdfinfo import Colorspace, Encoding, FloatRect, PageInfo, PdfInfo
+from ocrmypdf.pdfinfo import Colorspace, Encoding, FloatRect, Ink, PageInfo, PdfInfo
 from ocrmypdf.pluginspec import GhostscriptRasterDevice, OrientationConfidence
 
 try:
@@ -508,6 +508,49 @@ def calculate_raster_dpi(page_context: PageContext):
     return canvas_dpi, page_dpi
 
 
+def _select_raster_device(pageinfo: PageInfo) -> GhostscriptRasterDevice:
+    """Choose the minimum raster device that preserves the page's color depth.
+
+    The device escalates from 1-bit mono through grayscale, indexed, and full
+    color as required by the page's images, image masks, and vector content.
+    Image masks are painted with the current fill color, so a mask painted in
+    gray or color escalates the device even though the mask itself is 1-bit.
+    """
+    colorspaces = [
+        GhostscriptRasterDevice.PNGMONOD,
+        GhostscriptRasterDevice.PNGGRAY,
+        GhostscriptRasterDevice.PNG256,
+        GhostscriptRasterDevice.PNG16M,
+    ]
+    device_idx = 0
+
+    def at_least(colorspace):
+        return max(device_idx, colorspaces.index(colorspace))
+
+    for image in pageinfo.images:
+        if image.type_ == 'stencil':
+            # The fill color used to paint the mask, not the 1-bit mask data,
+            # determines the color depth OCR needs.
+            if image.ink == Ink.color:
+                device_idx = at_least(GhostscriptRasterDevice.PNG16M)
+            elif image.ink == Ink.gray:
+                device_idx = at_least(GhostscriptRasterDevice.PNGGRAY)
+            continue
+        if image.bpc > 1:
+            if image.color == Colorspace.index:
+                device_idx = at_least(GhostscriptRasterDevice.PNG256)
+            elif image.color == Colorspace.gray:
+                device_idx = at_least(GhostscriptRasterDevice.PNGGRAY)
+            else:
+                device_idx = at_least(GhostscriptRasterDevice.PNG16M)
+
+    if pageinfo.has_vector:
+        log.debug(f"Page has vector content, using {GhostscriptRasterDevice.PNG16M}")
+        device_idx = at_least(GhostscriptRasterDevice.PNG16M)
+
+    return colorspaces[device_idx]
+
+
 def rasterize(
     input_file: Path,
     page_context: PageContext,
@@ -529,39 +572,13 @@ def rasterize(
     Returns:
         Path: The output PNG file path.
     """
-    colorspaces = [
-        GhostscriptRasterDevice.PNGMONO,
-        GhostscriptRasterDevice.PNGGRAY,
-        GhostscriptRasterDevice.PNG256,
-        GhostscriptRasterDevice.PNG16M,
-    ]
-    device_idx = 0
-
     if remove_vectors is None:
         remove_vectors = page_context.options.remove_vectors
 
     output_file = page_context.get_path(f'rasterize{output_tag}.png')
     pageinfo = page_context.pageinfo
 
-    def at_least(colorspace):
-        return max(device_idx, colorspaces.index(colorspace))
-
-    for image in pageinfo.images:
-        if image.type_ != 'image':
-            continue  # ignore masks
-        if image.bpc > 1:
-            if image.color == Colorspace.index:
-                device_idx = at_least(GhostscriptRasterDevice.PNG256)
-            elif image.color == Colorspace.gray:
-                device_idx = at_least(GhostscriptRasterDevice.PNGGRAY)
-            else:
-                device_idx = at_least(GhostscriptRasterDevice.PNG16M)
-
-    if pageinfo.has_vector:
-        log.debug(f"Page has vector content, using {GhostscriptRasterDevice.PNG16M}")
-        device_idx = at_least(GhostscriptRasterDevice.PNG16M)
-
-    device = colorspaces[device_idx]
+    device = _select_raster_device(pageinfo)
 
     log.debug(
         f"Rasterize with {device}, rotation {correction}, mediabox {pageinfo.mediabox}"

tests/test_pipeline.py

@@ -6,6 +6,7 @@ from __future__ import annotations
 import warnings
 from unittest.mock import Mock
 
+import pikepdf
 import pytest
 from PIL import Image
 from reportlab.lib.units import inch
@@ -13,8 +14,10 @@ from reportlab.lib.utils import ImageReader
 from reportlab.pdfgen.canvas import Canvas
 
 from ocrmypdf import _pipeline, pdfinfo
+from ocrmypdf._pipeline import _select_raster_device
 from ocrmypdf.helpers import Resolution
 from ocrmypdf.pdfinfo import Encoding
+from ocrmypdf.pluginspec import GhostscriptRasterDevice
 
 warnings.filterwarnings(
     "ignore", category=DeprecationWarning, module="reportlab.lib.rl_safe_eval"
@@ -176,3 +179,39 @@ def test_should_visible_page_image_use_jpg(encodings, expected):
     pageinfo = Mock()
     pageinfo.images = [Mock(enc=enc) for enc in encodings]
     assert _pipeline.should_visible_page_image_use_jpg(pageinfo) == expected
+
+
+def _make_image_mask_pdf(path, content_fill: bytes):
+    pdf = pikepdf.Pdf.new()
+    pdf.add_blank_page(page_size=(72, 72))
+    mask = pikepdf.Stream(pdf, bytes([0x7E] * 8))
+    mask.Type = pikepdf.Name.XObject
+    mask.Subtype = pikepdf.Name.Image
+    mask.Width = 8
+    mask.Height = 8
+    mask.ImageMask = True
+    mask.BitsPerComponent = 1
+    name = pdf.pages[0].add_resource(mask, pikepdf.Name.XObject)
+    pdf.pages[0].Contents = pikepdf.Stream(
+        pdf, b"q 72 0 0 72 0 0 cm %s %s Do Q" % (content_fill, bytes(name))
+    )
+    pdf.save(path)
+    return path
+
+
+def test_select_device_gray_mask(tmp_path):
+    p = _make_image_mask_pdf(tmp_path / 'g.pdf', b"0.263 0.263 0.263 rg")
+    pageinfo = pdfinfo.PdfInfo(p)[0]
+    assert _select_raster_device(pageinfo) == GhostscriptRasterDevice.PNGGRAY
+
+
+def test_select_device_color_mask(tmp_path):
+    p = _make_image_mask_pdf(tmp_path / 'c.pdf', b"0.8 0.2 0.2 rg")
+    pageinfo = pdfinfo.PdfInfo(p)[0]
+    assert _select_raster_device(pageinfo) == GhostscriptRasterDevice.PNG16M
+
+
+def test_select_device_black_mask_stays_mono(tmp_path):
+    p = _make_image_mask_pdf(tmp_path / 'b.pdf', b"0 g")
+    pageinfo = pdfinfo.PdfInfo(p)[0]
+    assert _select_raster_device(pageinfo) == GhostscriptRasterDevice.PNGMONOD