Image utilities

arkindex_worker.image

Helper methods to download and open IIIF images, and manage polygons.

Classes

Functions

open_image

open_image(
    path, mode="RGB", rotation_angle=0, mirrored=False
)

Open an image from a path or a URL.

Warns: Prefer arkindex_worker.models.Element.open_image whenever possible.

Parameters:

Name	Type	Description	Default
path	Union[str, Path]	Path or URL to open the image from. This parameter will be interpreted as a URL when it has a http or https scheme and no file exist with this path locally.	required
mode	Optional[str]	Pillow mode for the image. See the Pillow documentation.	'RGB'
rotation_angle	Optional[int]	Rotation angle to apply to the image, in degrees. If it is not a multiple of 90°, then the rotation can cause empty pixels of the mode’s default color to be added for padding.	0
mirrored	Optional[bool]	Whether or not to mirror the image horizontally.	False

Returns:

Type	Description
Image	A Pillow image.

Source code in arkindex_worker/image.py

def open_image(
    path: Union[str, Path],
    mode: Optional[str] = "RGB",
    rotation_angle: Optional[int] = 0,
    mirrored: Optional[bool] = False,
) -> Image:
    """
    Open an image from a path or a URL.

    Warns:
    Prefer [arkindex_worker.models.Element.open_image][] whenever possible.

    :param path: Path or URL to open the image from.
       This parameter will be interpreted as a URL when it has a `http` or `https` scheme
       and no file exist with this path locally.
    :param mode: Pillow mode for the image. See [the Pillow documentation](https://pillow.readthedocs.io/en/stable/handbook/concepts.html#modes).
    :param rotation_angle: Rotation angle to apply to the image, in degrees.
       If it is not a multiple of 90°, then the rotation can cause empty pixels of
       the mode's default color to be added for padding.
    :param mirrored: Whether or not to mirror the image horizontally.
    :returns: A Pillow image.
    """
    if (
        path.startswith("http://")
        or path.startswith("https://")
        or not os.path.exists(path)
    ):
        image = download_image(path)
    else:
        try:
            image = Image.open(path)
        except (IOError, ValueError):
            image = download_image(path)

    if image.mode != mode:
        image = image.convert(mode)

    if mirrored:
        image = image.transpose(Image.Transpose.FLIP_LEFT_RIGHT)

    if rotation_angle:
        image = image.rotate(-rotation_angle, expand=True)

    return image

download_image

download_image(url)

Download an image and open it with Pillow.

Parameters:

Name	Type	Description	Default
url	str	URL of the image.	required

Returns:

Type	Description
Image	A Pillow image.

Source code in arkindex_worker/image.py

def download_image(url: str) -> Image:
    """
    Download an image and open it with Pillow.

    :param url: URL of the image.
    :returns: A Pillow image.
    """
    assert url.startswith("http"), "Image URL must be HTTP(S)"

    # Download the image
    # Cannot use stream=True as urllib's responses do not support the seek(int) method,
    # which is explicitly required by Image.open on file-like objects
    try:
        resp = requests.get(url, timeout=DOWNLOAD_TIMEOUT)
    except requests.exceptions.SSLError:
        logger.warning(
            "An SSLError occurred during image download, retrying with a weaker and unsafe SSL configuration"
        )

        # Saving current ciphers
        previous_ciphers = requests.packages.urllib3.util.ssl_.DEFAULT_CIPHERS

        # Downgrading ciphers to download the image
        requests.packages.urllib3.util.ssl_.DEFAULT_CIPHERS = "ALL:@SECLEVEL=1"
        resp = requests.get(url, timeout=DOWNLOAD_TIMEOUT)

        # Restoring previous ciphers
        requests.packages.urllib3.util.ssl_.DEFAULT_CIPHERS = previous_ciphers

    resp.raise_for_status()

    # Preprocess the image and prepare it for classification
    image = Image.open(BytesIO(resp.content))
    logger.info(
        "Downloaded image {} - size={}x{} in {}".format(
            url, image.size[0], image.size[1], resp.elapsed
        )
    )

    return image

polygon_bounding_box

polygon_bounding_box(polygon)

Compute the rectangle bounding box of a polygon.

Parameters:

Name	Type	Description	Default
polygon	List[List[Union[int, float]]]	Polygon to get the bounding box of.	required

Returns:

Type	Description
BoundingBox	Bounding box of this polygon.

Source code in arkindex_worker/image.py

def polygon_bounding_box(polygon: List[List[Union[int, float]]]) -> BoundingBox:
    """
    Compute the rectangle bounding box of a polygon.

    :param polygon: Polygon to get the bounding box of.
    :returns: Bounding box of this polygon.
    """
    x_coords, y_coords = zip(*polygon)
    x, y = min(x_coords), min(y_coords)
    width, height = max(x_coords) - x, max(y_coords) - y
    return BoundingBox(x, y, width, height)

download_tiles

download_tiles(url)

Reconstruct a full IIIF image on servers that cannot serve the full-sized image, using tiles.

Parameters:

Name	Type	Description	Default
url	str	URL of the image.	required

Returns:

Type	Description
Image	A Pillow image.

Source code in arkindex_worker/image.py

def download_tiles(url: str) -> Image:
    """
    Reconstruct a full IIIF image on servers that cannot serve the full-sized image, using tiles.

    :param url: URL of the image.
    :returns: A Pillow image.
    """
    if not url.endswith("/"):
        url += "/"
    logger.debug("Downloading image information")
    info = _retried_request(url + "info.json").json()

    image_width, image_height = info.get("width"), info.get("height")
    assert image_width and image_height, "Missing image dimensions in info.json"
    assert info.get(
        "tiles"
    ), "Image cannot be retrieved at full size and tiles are not supported"

    # Take the biggest available tile size
    tile = sorted(info["tiles"], key=lambda tile: tile.get("width", 0), reverse=True)[0]
    tile_width = tile["width"]
    # Tile height is optional and defaults to the width
    tile_height = tile.get("height", tile_width)

    full_image = Image.new("RGB", (image_width, image_height))

    for tile_x in range(ceil(image_width / tile_width)):
        for tile_y in range(ceil(image_height / tile_height)):
            region_x = tile_x * tile_width
            region_y = tile_y * tile_height

            # Prevent trying to crop outside the bounds of an image
            region_width = min(tile_width, image_width - region_x)
            region_height = min(tile_height, image_height - region_y)

            logger.debug(f"Downloading tile {tile_x},{tile_y}")
            resp = _retried_request(
                f"{url}{region_x},{region_y},{region_width},{region_height}/full/0/default.jpg"
            )

            tile_img = Image.open(BytesIO(resp.content))

            # Some bad IIIF image server implementations may sometimes return tiles with a few pixels of difference
            # with the expected sizes, causing Pillow to raise ValueError('images do not match').
            actual_width, actual_height = tile_img.size
            if actual_width < region_width or actual_height < region_height:
                # Fail when tiles are too small
                raise ValueError(
                    f"Expected size {region_width}×{region_height} for tile {tile_x},{tile_y}, "
                    f"but got {actual_width}×{actual_height}"
                )

            if actual_width > region_width or actual_height > region_height:
                # Warn and crop when tiles are too large
                logger.warning(
                    f"Cropping tile {tile_x},{tile_y} from {actual_width}×{actual_height} "
                    f"to {region_width}×{region_height}"
                )
                tile_img = tile_img.crop((0, 0, region_width, region_height))

            full_image.paste(
                tile_img,
                box=(
                    region_x,
                    region_y,
                    region_x + region_width,
                    region_y + region_height,
                ),
            )

    return full_image

trim_polygon

trim_polygon(polygon, image_width, image_height)

Trim a polygon to an image’s boundaries, with non-negative coordinates.

Parameters:

Name	Type	Description	Default
polygon	List[List[int]]	A polygon to trim.	required
image_width	int	Width of the image.	required
image_height	int	Height of the image.	required

Returns:

Type	Description
List[List[int]]	A polygon trimmed to the image’s bounds. Some points may appear as missing, as the trimming can deduplicate points. The first and last point are always equal, to reproduce the behavior of the Arkindex backend.

Raises:

Type	Description
AssertionError	When argument types are invalid or when the trimmed polygon is entirely outside of the image’s bounds.

Source code in arkindex_worker/image.py

def trim_polygon(
    polygon: List[List[int]], image_width: int, image_height: int
) -> List[List[int]]:
    """
    Trim a polygon to an image's boundaries, with non-negative coordinates.

    :param polygon: A polygon to trim.
    :param image_width: Width of the image.
    :param image_height: Height of the image.
    :returns: A polygon trimmed to the image's bounds.
       Some points may appear as missing, as the trimming can deduplicate points.
       The first and last point are always equal, to reproduce the behavior
       of the Arkindex backend.
    :raises AssertionError: When argument types are invalid or when the trimmed polygon
       is entirely outside of the image's bounds.
    """

    assert isinstance(
        polygon, (list, tuple)
    ), "Input polygon must be a valid list or tuple of points."
    assert all(
        isinstance(point, (list, tuple)) for point in polygon
    ), "Polygon points must be tuples or lists."
    assert all(
        len(point) == 2 for point in polygon
    ), "Polygon points must be tuples or lists of 2 elements."
    assert all(
        isinstance(point[0], int) and isinstance(point[1], int) for point in polygon
    ), "Polygon point coordinates must be integers."
    assert any(
        point[0] <= image_width and point[1] <= image_height for point in polygon
    ), "This polygon is entirely outside the image's bounds."

    trimmed_polygon = [
        [
            min(image_width, max(0, x)),
            min(image_height, max(0, y)),
        ]
        for x, y in polygon
    ]

    updated_polygon = []
    for point in trimmed_polygon:
        if point not in updated_polygon:
            updated_polygon.append(point)

    # Add back the matching last point, if it was present in the original polygon
    if polygon[-1] == polygon[0]:
        updated_polygon.append(updated_polygon[0])

    return updated_polygon

revert_orientation

revert_orientation(element, polygon, reverse=False)

Update the coordinates of the polygon of a child element based on the orientation of its parent.

This method should be called before sending any polygon to Arkindex, to undo the possible orientation applied by arkindex_worker.models.Element.open_image.

In some cases, we want to apply the parent’s orientation on the child’s polygon instead. This is done by enabling reverse=True.

Parameters:

Name	Type	Description	Default
element	Union[Element, CachedElement]	Parent element.	required
polygon	List[List[Union[int, float]]]	Polygon corresponding to the child element.	required
reverse	Optional[bool]	Whether we should revert or apply the parent’s orientation.	False

Returns:

Type	Description
List[List[int]]	A polygon with updated coordinates.

Source code in arkindex_worker/image.py

def revert_orientation(
    element: Union["Element", "CachedElement"],
    polygon: List[List[Union[int, float]]],
    reverse: Optional[bool] = False,
) -> List[List[int]]:
    """
    Update the coordinates of the polygon of a child element based on the orientation of
    its parent.

    This method should be called before sending any polygon to Arkindex, to undo the possible
    orientation applied by [arkindex_worker.models.Element.open_image][].

    In some cases, we want to apply the parent's orientation on the child's polygon instead. This is done
    by enabling `reverse=True`.

    :param element: Parent element.
    :param polygon: Polygon corresponding to the child element.
    :param reverse: Whether we should revert or apply the parent's orientation.
    :return: A polygon with updated coordinates.
    """
    from arkindex_worker.cache import CachedElement
    from arkindex_worker.models import Element

    assert element and isinstance(
        element, (Element, CachedElement)
    ), "element shouldn't be null and should be an Element or CachedElement"
    assert polygon and isinstance(
        polygon, list
    ), "polygon shouldn't be null and should be a list"
    assert isinstance(reverse, bool), "Reverse should be a bool"
    # Rotating with Pillow can cause it to move the image around, as the image cannot have negative coordinates
    # and must be a rectangle.  This means the origin point of any coordinates from an image is invalid, and the
    # center of the bounding box of the rotated image is different from the center of the element's bounding box.
    # To properly undo the mirroring and rotation implicitly applied by open_image, we first need to find the center
    # of the rotated bounding box.
    if isinstance(element, Element):
        assert (
            element.zone and element.zone.polygon
        ), "element should have a zone and a polygon"
        parent_ring = LinearRing(element.zone.polygon)
    elif isinstance(element, CachedElement):
        assert element.polygon, "cached element should have a polygon"
        parent_ring = LinearRing(element.polygon)

    rotated_ring = rotate(parent_ring, element.rotation_angle, origin="center")

    # This rotated ring might have negative coordinates, so we get the vector that Pillow applies to offset the
    # image to non-negative coordinates using the rotated bounding box.
    offset_x, offset_y, _, _ = rotated_ring.bounds

    # This uses the same calculation as what Shapely does for rotate/scale(origin='center').
    # We will use this below to rotate around the center of the parent bounding box and not of each child polygon.
    # https://github.com/Toblerity/Shapely/blob/462de3aa7a8bbd80408762a2d5aaf84b04476e4d/shapely/affinity.py#L98-L101
    minx, miny, maxx, maxy = parent_ring.bounds
    origin = ((maxx + minx) / 2.0, (maxy + miny) / 2.0)

    ring = LinearRing(polygon)

    if reverse:
        # Apply the parent's orientation on the child's polygon
        # Apply mirroring
        if element.mirrored:
            ring = scale(ring, xfact=-1, origin=origin)
        # Apply rotation
        if element.rotation_angle:
            ring = rotate(ring, element.rotation_angle, origin=origin)
        # At last translate coordinates offset
        ring = translate(ring, xoff=-offset_x, yoff=-offset_y)
    else:
        # First undo the negative coordinates offset, since this is the last step of the original transformation
        ring = translate(ring, xoff=offset_x, yoff=offset_y)
        # Revert any rotation
        if element.rotation_angle:
            ring = rotate(ring, -element.rotation_angle, origin=origin)
        # Revert any mirroring
        if element.mirrored:
            ring = scale(ring, xfact=-1, origin=origin)

    return [[int(x), int(y)] for x, y in ring.coords]