As you were

tests/test_imageops.py

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+# SPDX-FileCopyrightText: 2023 James R. Barlow
+# SPDX-License-Identifier: MPL-2.0
+
+from __future__ import annotations
+
+from ocrmypdf.imageops import bytes_per_pixel, calculate_downsample, downsample_image
+from PIL import Image
+
+
+def test_bytes_per_pixel():
+    assert bytes_per_pixel('RGB') == 4
+    assert bytes_per_pixel('RGBA') == 4
+    assert bytes_per_pixel('LA') == 2
+    assert bytes_per_pixel('L') == 1
+
+
+def test_calculate_downsample():
+    im = Image.new('RGB', (100, 100))
+    assert calculate_downsample(im, max_size=(50, 50)) == 0.5
+    assert calculate_downsample(im, max_pixels=2500) == 0.5
+    assert calculate_downsample(im, max_bytes=10000) == 0.25
+
+
+def test_downsample_image():
+    im = Image.new('RGB', (100, 100))
+    im.info['dpi'] = (300, 300)
+    assert downsample_image(im, 0.5).size == (50, 50)
+    assert im.info['dpi'] == (150, 150)