add forgotten helper module

zesje/helpers/image_helper.py

+"""Utilities for dealing with images"""
+
+import numpy as np
+import cv2
+
+def get_widget_image(image_path, widget):
+    box = (widget.top, widget.bottom, widget.left, widget.right)
+    raw_image = get_box(cv2.imread(image_path), box, padding=0.3)
+    return cv2.imencode(".jpg", raw_image)[1].tostring()
+
+
+def guess_dpi(image_array):
+    h, *_ = image_array.shape
+    resolutions = np.array([1200, 600, 300, 200, 150, 120, 100, 75, 60, 50, 40])
+    return resolutions[np.argmin(abs(resolutions - 25.4 * h / 297))]
+
+
+def get_box(image_array, box, padding):
+    """Extract a subblock from an array corresponding to a scanned A4 page.
+
+    Parameters:
+    -----------
+    image_array : 2D or 3D array
+        The image source.
+    box : 4 floats (top, bottom, left, right)
+        Coordinates of the bounding box in inches. By due to differing
+        traditions, box coordinates are counted from the bottom left of the
+        image, while image array coordinates are from the top left.
+    padding : float
+        Padding around box borders in inches.
+    """
+    h, w, *_ = image_array.shape
+    dpi = guess_dpi(image_array)
+    box = np.array(box)
+    box += (padding, -padding, -padding, padding)
+    box = (np.array(box) * dpi).astype(int)
+    # Here we are not returning the lowest pixel of the image because otherwise
+    # the numpy slicing is not correct.
+    top, bottom = min(h, box[0]), max(1, box[1])
+    left, right = max(0, box[2]), min(w, box[3])
+    return image_array[-top:-bottom, left:right]