From ab705bc95d19afe358e31a02daa825bc4abc32ab Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?St=C3=A9phane=20Brunner?= <stephane.brunner@camptocamp.com>
Date: Sat, 6 Nov 2021 09:16:54 +0100
Subject: [PATCH] Add num angle to increase the angle precision

---
 deskew/__init__.py   | 10 ++++++----
 deskew/cli.py        |  5 ++++-
 tests/test_deskew.py | 41 +++++++++++++++++++++++++++++++++--------
 3 files changed, 43 insertions(+), 13 deletions(-)

diff --git a/deskew/__init__.py b/deskew/__init__.py
index 6014136..f2839c2 100644
--- a/deskew/__init__.py
+++ b/deskew/__init__.py
@@ -47,7 +47,7 @@ def _calculate_deviation(angle: float) -> np.float64:
 
 
 def determine_skew_dev(
-    image: ImageType, sigma: float = 3.0, num_peaks: int = 20
+    image: ImageType, sigma: float = 3.0, num_peaks: int = 20, num_angles: int = 180
 ) -> Tuple[
     Optional[np.float64],
     List[List[np.float64]],
@@ -57,7 +57,7 @@ def determine_skew_dev(
     """Calculate skew angle."""
     img = image
     edges = canny(img, sigma=sigma)
-    out, angles, distances = hough_line(edges)
+    out, angles, distances = hough_line(edges, np.linspace(-np.pi / 2, np.pi / 2, num_angles, endpoint=False))
 
     _, angles_peaks, _ = hough_line_peaks(out, angles, distances, num_peaks=num_peaks)
 
@@ -119,11 +119,13 @@ def determine_skew_dev(
     return rot_angle, angles, average_deviation, (out, angles, distances)
 
 
-def determine_skew(image: ImageType, sigma: float = 3.0, num_peaks: int = 20) -> Optional[np.float64]:
+def determine_skew(
+    image: ImageType, sigma: float = 3.0, num_peaks: int = 20, num_angles: int = 180
+) -> Optional[np.float64]:
     """
     Calculate skew angle.
 
     Return None if no skew will be found
     """
-    angle, _, _, _ = determine_skew_dev(image, sigma=sigma, num_peaks=num_peaks)
+    angle, _, _, _ = determine_skew_dev(image, sigma=sigma, num_peaks=num_peaks, num_angles=num_angles)
     return angle
diff --git a/deskew/cli.py b/deskew/cli.py
index f230352..be59aa3 100644
--- a/deskew/cli.py
+++ b/deskew/cli.py
@@ -15,7 +15,10 @@ def main() -> None:
 
     parser.add_argument("-o", "--output", default=None, help="Output file name")
     parser.add_argument("--sigma", default=3.0, help="The use sigma")
-    parser.add_argument("--num-peaks", default=20, help="The used num peaks")
+    parser.add_argument("--num-peaks", default=20, help="The used number of peaks")
+    parser.add_argument(
+        "--num-angles", default=180, help="The used number of angle (determine the pressision)"
+    )
     parser.add_argument("--background", help="The used background color")
     parser.add_argument(default=None, dest="input", help="Input file name")
     options = parser.parse_args()
diff --git a/tests/test_deskew.py b/tests/test_deskew.py
index a97a8e8..536afad 100644
--- a/tests/test_deskew.py
+++ b/tests/test_deskew.py
@@ -10,14 +10,14 @@
 @pytest.mark.parametrize(
     "image,expected_angle",
     [
-        ("1", pytest.approx(-1.375, abs=0.37)),
-        ("2", pytest.approx(-2.185, abs=0.18)),
-        ("3", pytest.approx(-6.250, abs=0.22)),
-        ("4", pytest.approx(7.095, abs=0.1)),
-        ("5", pytest.approx(3.410, abs=0.7)),
-        ("6", pytest.approx(-2.810, abs=0.21)),
-        ("7", pytest.approx(3.395, abs=0.7)),
-        ("8", pytest.approx(15.084, abs=0.05)),
+        ("1", pytest.approx(-1.0, abs=0.01)),
+        ("2", pytest.approx(-2.0, abs=0.01)),
+        ("3", pytest.approx(-6.0, abs=0.01)),
+        ("4", pytest.approx(7.0, abs=0.01)),
+        ("5", pytest.approx(4.0, abs=0.01)),
+        ("6", pytest.approx(-3.0, abs=0.01)),
+        ("7", pytest.approx(3.0, abs=0.01)),
+        ("8", pytest.approx(15.0, abs=0.01)),
     ],
 )
 def test_deskew(image, expected_angle):
@@ -30,3 +30,28 @@ def test_deskew(image, expected_angle):
     angle = determine_skew(grayscale)
     print(angle - expected_angle.expected)
     assert angle == expected_angle
+
+
+@pytest.mark.parametrize(
+    "image,expected_angle",
+    [
+        ("1", pytest.approx(-1.4, abs=0.01)),
+        ("2", pytest.approx(-2.1, abs=0.01)),
+        ("3", pytest.approx(-6.2, abs=0.01)),
+        ("4", pytest.approx(7.1, abs=0.01)),
+        ("5", pytest.approx(3.4, abs=0.01)),
+        ("6", pytest.approx(-2.8, abs=0.01)),
+        ("7", pytest.approx(3.7, abs=0.01)),
+        ("8", pytest.approx(14.9, abs=0.01)),
+    ],
+)
+def test_deskew_higher_pressision(image, expected_angle):
+    root_folder = f"results/{image}"
+    if not os.path.exists(root_folder):
+        os.makedirs(root_folder)
+
+    image = io.imread(os.path.join(os.path.dirname(__file__), f"deskew-{image}.png"))
+    grayscale = rgb2gray(image)
+    angle = determine_skew(grayscale, num_angles=1800)
+    print(angle - expected_angle.expected)
+    assert angle == expected_angle