Toggling pregrading and Identifying blank solutions

This merge request does two things:

• Automatically identify blank solutions (either multiple choice or not) and set a feedback option named 'blank' for each one.
• When creating an exam, there are three options to specify which questions should be graded automatically.
• Pre-grade a question again if a submission is reuploaded and is automatically graded (by 'Zesje')

The grading policy is specified in the following way:

• 0/set_nothing: Manually approve everything
• 1/set_blank: Automatically approve blank solutions
• 2/set_blank_single: Automatically approve blank solutions or solutions with one answer

Add a new field to a problem in the database with this number.

For a more detailed description, see #61.

Closes #33 and #61.

zesje/blanks.py

+import numpy as np
+import os
+
+from .image_extraction import extract_images
+from .images import get_box
+from PIL import Image
+from flask import current_app
+
+
+def get_blank(problem, dpi, widget_area_in, sub):
+    page = problem.widget.page
+
+    app_config = current_app.config
+    data_directory = app_config.get('DATA_DIRECTORY', 'data')
+    output_directory = os.path.join(data_directory, f'{problem.exam_id}_data')
+
+    generated_path = os.path.join(output_directory, 'blanks', f'{dpi}')
+
+    if not os.path.exists(generated_path):
+        set_blank(sub.copy_number, dpi, output_directory)
+
+    image_path = os.path.join(generated_path, f'page{page:02d}.jpg')
+    blank_page = Image.open(image_path)
+    return get_box(np.array(blank_page), widget_area_in, padding=0)
+
+
+def set_blank(copy_number, dpi, output_directory):
+    pdf_path = os.path.join(output_directory, 'generated_pdfs', f'{copy_number:05d}.pdf')
+    pages = extract_images(pdf_path, dpi)
+
+    for image, page in pages:
+        save_image(np.array(image), page, dpi, output_directory)
+
+
+def save_image(image, page, dpi, output_directory):
+    """Save an image at an appropriate location.
+
+    Parameters
+    ----------
+    image : numpy array
+        Image data.
+    barcode : ExtractedBarcode
+        The barcode identifying the page.
+    base_path : string
+        The folder corresponding to a correct exam.
+
+    Returns
+    -------
+    image_path : string
+        Location of the image.
+    """
+
+    submission_path = os.path.join(output_directory, 'blanks', f'{dpi}')
+    os.makedirs(submission_path, exist_ok=True)
+    image_path = os.path.join(submission_path, f'page{page-1:02d}.jpg')
+    Image.fromarray(image).save(image_path)
+    return image_path