Anyone Having Success with Wand to Create Thumbnails from PDF files?

[Mark Phillips]

I am using ImageMagick 6.7.7-10 2017-07-31 Q16 on Ubuntu 14.04, through Wand 0.4.4, Python 3.4.3 and Django 1.11. I am trying to create a jpg thumbnail of a pdf file.

On the command line, I can do this with no errors:

convert -thumbnail x300 -background white -alpha remove Lucy.pdf[0] output_thumbnail.jpg

But when I try to use wand on the same image I get this error:

Traceback (most recent call last):

  File "/home/mark/python-projects/memorabilia-project/memorabilia/models.py", line 24, in make_thumb

    pages = Image(blob = b)

  File "/home/mark/.virtualenvs/memorabilia/lib/python3.4/site-packages/wand/image.py", line 2742, in __init__

    self.read(blob=blob, resolution=resolution)

  File "/home/mark/.virtualenvs/memorabilia/lib/python3.4/site-packages/wand/image.py", line 2822, in read

    self.raise_exception()

  File "/home/mark/.virtualenvs/memorabilia/lib/python3.4/site-packages/wand/resource.py", line 222, in raise_exception

    raise e

wand.exceptions.MissingDelegateError: no decode delegate for this image format `' @ error/blob.c/BlobToImage/367

I looked at the delegates.xml file for ImageMagic in /etc, and there are entries for pdf files.

My models.py (slightly abbreviated):

def make_thumb(instance, filename, newname, b):

    file_name, file_extension = os.path.splitext(filename)

    file_extension = file_extension.lower()

    print(os.path.join(settings.MEDIA_ROOT, filename))

    try:

        if file_extension in [".pdf"]:

            pages = Image(blob = b)

            first_page = pages.sequence[0]

            image = Image(first_page)

        else:

            image = Image(blob = b)

        image.transform(resize="x200")

        image.format = "jpg"

        thumb_name, thumb_extension = os.path.splitext(newname)

        thumb_extension = ".jpg"

        thumb_name = thumb_name + thumb_extension

        image.save(filename=os.path.join(settings.MEDIA_ROOT, 'thumbs', thumb_name))

        instance.thumb_storage = thumb_name

        return True

    except:

        instance.thumb_storage = ''

        print(traceback.format_exc())

        return False

        

def unique_file_path(instance, filename):

    """This function calculates the sha256 hash for the file as well

    as a UUID4 string for the file name. The hash is stored in the model

    and the function returns the new path for the file using the UUID4

    file name.

    """

    # Save original file name in model

    instance.original_file_name = filename

    

    # calculate the hash for the file

    h = hashlib.sha256()

    buff_size = 128*1024

    #with open(instance.storage_file_name, 'rb', buffering=0) as f:

    for b in iter(lambda : instance.storage_file_name.read(buff_size), b""):

        h.update(b)

    instance.computed_sha256 = h.hexdigest()

    print ("b="+str(len(b)))

        

    # Get new file name/upload path

    base, ext = splitext(filename)

    newname = "%s%s" % (uuid.uuid4(), ext)

    if not make_thumb(instance, filename, newname, b):

        print('Could not create thumbnail - is the file type valid?')

        

    #return os.path.join('documents', newname)

    return os.path.join('',newname)

# Document               

class Document(models.Model):

    document_id = models.AutoField(primary_key=True)

      

    # Document states

    PRIVATE = 1

    PENDING = 2

    PUBLISHED = 3

    DOCUMENT_STATE = (

        (PRIVATE, 'private'),

        (PENDING, 'pending'),

        (PUBLISHED, 'published'),

    )

    document_state = models.IntegerField(choices=DOCUMENT_STATE, default=PRIVATE)

    

    # Document types

    documentType_id = models.ForeignKey(DocumentType, on_delete=models.CASCADE,)

    

    # Document fields

    title = models.CharField('title', max_length=200)

    storage_file_name = models.FileField('File name', upload_to=unique_file_path)

    thumb_storage = models.CharField(editable=False, max_length=200, blank=True, null=True)

    original_file_name = models.CharField(editable=False, max_length=200)

    computed_sha256 = models.CharField(editable=False, max_length=64)

    created = models.DateTimeField(editable=False)

    updated = models.DateTimeField(editable=False)

    

    class Meta:

        ordering = ['title']

    def __str__(self):

        return "%s" % (self.title)

        

    def save(self, *args, **kwargs):

        if not self.document_id:

            # Change created timestamp only if it's new entry

            self.created = timezone.now() 

        self.updated = timezone.now()

        super(Document, self).save(*args, **kwargs)

@receiver(models.signals.post_delete, sender=Document)

def auto_delete_file_on_delete(sender, instance, **kwargs):

    """

    Deletes file from filesystem

    when corresponding `Document` object is deleted.

    """

    if instance.storage_file_name:

        if os.path.isfile(instance.storage_file_name.path):

            os.remove(instance.storage_file_name.path)

@receiver(models.signals.pre_save, sender=Document)

def auto_delete_file_on_change(sender, instance, **kwargs):

    """

    Deletes old file from filesystem

    when corresponding `Document` object is updated

    with new file.

    """

    if not instance.document_id:

        return False

    

    try:

        old_storage_file_name = Document.objects.get(pk=instance.document_id).storage_file_name

        old_computed_sha256 = Document.objects.get(pk=instance.document_id).computed_sha256

    except Document.DoesNotExist:

        return False 

    

    new_computed_sha256 = instance.computed_sha256

    if old_computed_sha256 == new_computed_sha256:

        if os.path.isfile(old_storage_file_name.path):

            os.remove(old_storage_file_name.path)

Thanks for any suggestions on how to get this conversion to work through wand. I am not married to wand. If there is an easier way to make a thumbnail of the first page of a pdf file, please let me know!

Thanks!

Mark