Consider text when determining page raster DPI

Previously if we found vectors of any sort on a page, we would bump
the DPI up to 400. We did nothing
about pages with text. As a result,
pages with a low image resolution
and printable text would have the text downgraded to image
resolution when --force-ocr was used.

We don't try to determine if the text is visible or invisible OCR text, since
that is a slower test. --redo-ocr would improve such cases anyway.

tests/test_pipeline.py

@@ -0,0 +1,63 @@
+# © 2021 James R. Barlow: github.com/jbarlow83
+#
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+from unittest.mock import Mock
+
+import pytest
+from PIL import Image
+from reportlab.lib.units import inch
+from reportlab.lib.utils import ImageReader
+from reportlab.pdfgen.canvas import Canvas
+
+from ocrmypdf import _pipeline, pdfinfo
+from ocrmypdf.helpers import Resolution
+
+
+@pytest.fixture(scope='session')
+def rgb_image():
+    im = Image.new('RGB', (8, 8))
+    im.putpixel((4, 4), (255, 0, 0))
+    im.putpixel((5, 5), (0, 255, 0))
+    im.putpixel((6, 6), (0, 0, 255))
+    return ImageReader(im)
+
+
+DUMMY_OVERSAMPLE_RESOLUTION = Resolution(42.0, 42.0)
+VECTOR_RESOLUTION = Resolution(_pipeline.VECTOR_PAGE_DPI, _pipeline.VECTOR_PAGE_DPI)
+
+
+@pytest.mark.parametrize(
+    'image, text, vector, result',
+    [
+        (False, False, False, VECTOR_RESOLUTION),
+        (False, True, False, VECTOR_RESOLUTION),
+        (True, False, False, DUMMY_OVERSAMPLE_RESOLUTION),
+        (True, True, False, VECTOR_RESOLUTION),
+        (False, False, True, VECTOR_RESOLUTION),
+        (False, True, True, VECTOR_RESOLUTION),
+        (True, False, True, VECTOR_RESOLUTION),
+        (True, True, True, VECTOR_RESOLUTION),
+    ],
+)
+def test_dpi_needed(image, text, vector, result, rgb_image, outdir):
+
+    c = Canvas(str(outdir / 'dpi.pdf'), pagesize=(5 * inch, 5 * inch))
+    if image:
+        c.drawImage(rgb_image, 1 * inch, 1 * inch, width=1 * inch, height=1 * inch)
+    if text:
+        c.drawString(1 * inch, 4 * inch, "Actual text")
+    if vector:
+        c.ellipse(3 * inch, 3 * inch, 4 * inch, 4 * inch)
+    c.showPage()
+    c.save()
+
+    mock = Mock()
+    mock.oversample = DUMMY_OVERSAMPLE_RESOLUTION[0]
+
+    pi = pdfinfo.PdfInfo(outdir / 'dpi.pdf')
+
+    assert _pipeline.get_canvas_square_dpi(pi[0], mock) == result
+    assert _pipeline.get_page_square_dpi(pi[0], mock) == result