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/image_extraction.py

+from io import BytesIO
+
+import numpy as np
+from PIL import Image
+from pikepdf import Pdf, PdfImage
+from tempfile import SpooledTemporaryFile
+from wand.image import Image as WandImage
+
+
+def extract_images(filename, dpi=300):
+    """Yield all images from a PDF file.
+
+    Tries to use PikePDF to extract the images from the given PDF.
+    If PikePDF is not able to extract the image from a page,
+    it continues to use Wand to flatten the rest of the pages.
+    """
+
+    with Pdf.open(filename) as pdf_reader:
+        use_wand = False
+
+        total = len(pdf_reader.pages)
+
+        for pagenr in range(total):
+            if not use_wand:
+                try:
+                    # Try to use PikePDF, but catch any error it raises
+                    img = extract_image_pikepdf(pagenr, pdf_reader)
+
+                except Exception:
+                    # Fallback to Wand if extracting with PikePDF failed
+                    use_wand = True
+
+            if use_wand:
+                img = extract_image_wand(pagenr, pdf_reader, dpi)
+
+            if img.mode == 'L':
+                img = img.convert('RGB')
+
+            yield img, pagenr+1
+
+
+def extract_image_pikepdf(pagenr, reader):
+    """Extracts an image as an array from the designated page
+
+    This method uses PikePDF to extract the image and only works
+    when there is a single image present on the page with the
+    same aspect ratio as the page.
+
+    We do not check for the actual size of the image on the page,
+    since this size depends on the draw instruction rather than
+    the embedded image object available to pikepdf.
+
+    Raises an error if not exactly image is present or the image
+    does not have the same aspect ratio as the page.
+
+    Parameters
+    ----------
+    pagenr : int
+        Page number to extract
+    reader : pikepdf.Pdf instance
+        The pdf reader to read the page from
+
+    Returns
+    -------
+    img_array : PIL Image
+        The extracted image data
+
+    Raises
+    ------
+    ValueError
+        if not exactly one image is found on the page or the image
+        does not have the same aspect ratio as the page
+    AttributeError
+        if no XObject or MediaBox is present on the page
+    """
+
+    page = reader.pages[pagenr]
+
+    xObject = page.Resources.XObject
+
+    if sum((xObject[obj].Subtype == '/Image')
+            for obj in xObject) != 1:
+        raise ValueError('Not exactly 1 image present on the page')
+
+    for obj in xObject:
+        if xObject[obj].Subtype == '/Image':
+            pdfimage = PdfImage(xObject[obj])
+
+            pdf_width = float(page.MediaBox[2] - page.MediaBox[0])
+            pdf_height = float(page.MediaBox[3] - page.MediaBox[1])
+
+            ratio_width = pdfimage.width / pdf_width
+            ratio_height = pdfimage.height / pdf_height
+
+            # Check if the aspect ratio of the image is the same as the
+            # aspect ratio of the page up to a 3% relative error
+            if abs(ratio_width - ratio_height) > 0.03 * ratio_width:
+                raise ValueError('Image has incorrect dimensions')
+
+            return pdfimage.as_pil_image()
+
+
+def extract_image_wand(pagenr, reader, dpi):
+    """Flattens a page from a PDF to an image array
+
+    This method uses Wand to flatten the page and creates an image.
+
+    Parameters
+    ----------
+    pagenr : int
+        Page number to extract, starting at 0
+    reader : pikepdf.Pdf instance
+        The pdf reader to read the page from
+
+    Returns
+    -------
+    img_array : PIL Image
+        The extracted image data
+    """
+    page = reader.pages[pagenr]
+
+    page_pdf = Pdf.new()
+    page_pdf.pages.append(page)
+
+    with SpooledTemporaryFile() as page_file:
+
+        page_pdf.save(page_file)
+
+        with WandImage(blob=page_file._file.getvalue(), format='pdf', resolution=dpi) as page_image:
+            page_image.format = 'jpg'
+            img_array = np.asarray(bytearray(page_image.make_blob(format="jpg")), dtype=np.uint8)
+            img = Image.open(BytesIO(img_array))
+            img.load()  # Load the data into the PIL image from the Wand image
+
+    return img