thumbnailer.py

# -*- coding: utf-8 -*-
# Copyright 2014-2016 OpenMarket Ltd
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import logging
from io import BytesIO

from PIL import Image

logger = logging.getLogger(__name__)

EXIF_ORIENTATION_TAG = 0x0112
EXIF_TRANSPOSE_MAPPINGS = {
    2: Image.FLIP_LEFT_RIGHT,
    3: Image.ROTATE_180,
    4: Image.FLIP_TOP_BOTTOM,
    5: Image.TRANSPOSE,
    6: Image.ROTATE_270,
    7: Image.TRANSVERSE,
    8: Image.ROTATE_90,
}


class ThumbnailError(Exception):
    """An error occurred generating a thumbnail."""


class Thumbnailer:

    FORMATS = {"image/jpeg": "JPEG", "image/png": "PNG"}

    def __init__(self, input_path):
        try:
            self.image = Image.open(input_path)
        except OSError as e:
            # If an error occurs opening the image, a thumbnail won't be able to
            # be generated.
            raise ThumbnailError from e

        self.width, self.height = self.image.size
        self.transpose_method = None
        try:
            # We don't use ImageOps.exif_transpose since it crashes with big EXIF
            image_exif = self.image._getexif()
            if image_exif is not None:
                image_orientation = image_exif.get(EXIF_ORIENTATION_TAG)
                self.transpose_method = EXIF_TRANSPOSE_MAPPINGS.get(image_orientation)
        except Exception as e:
            # A lot of parsing errors can happen when parsing EXIF
            logger.info("Error parsing image EXIF information: %s", e)

    def transpose(self):
        """Transpose the image using its EXIF Orientation tag

        Returns:
            Tuple[int, int]: (width, height) containing the new image size in pixels.
        """
        if self.transpose_method is not None:
            self.image = self.image.transpose(self.transpose_method)
            self.width, self.height = self.image.size
            self.transpose_method = None
            # We don't need EXIF any more
            self.image.info["exif"] = None
        return self.image.size

    def aspect(self, max_width, max_height):
        """Calculate the largest size that preserves aspect ratio which
        fits within the given rectangle::

            (w_in / h_in) = (w_out / h_out)
            w_out = min(w_max, h_max * (w_in / h_in))
            h_out = min(h_max, w_max * (h_in / w_in))

        Args:
            max_width: The largest possible width.
            max_height: The larget possible height.
        """

        if max_width * self.height < max_height * self.width:
            return max_width, (max_width * self.height) // self.width
        else:
            return (max_height * self.width) // self.height, max_height

    def _resize(self, width, height):
        # 1-bit or 8-bit color palette images need converting to RGB
        # otherwise they will be scaled using nearest neighbour which
        # looks awful
        if self.image.mode in ["1", "P"]:
            self.image = self.image.convert("RGB")
        return self.image.resize((width, height), Image.ANTIALIAS)

    def scale(self, width, height, output_type):
        """Rescales the image to the given dimensions.

        Returns:
            BytesIO: the bytes of the encoded image ready to be written to disk
        """
        scaled = self._resize(width, height)
        return self._encode_image(scaled, output_type)

    def crop(self, width, height, output_type):
        """Rescales and crops the image to the given dimensions preserving
        aspect::
            (w_in / h_in) = (w_scaled / h_scaled)
            w_scaled = max(w_out, h_out * (w_in / h_in))
            h_scaled = max(h_out, w_out * (h_in / w_in))

        Args:
            max_width: The largest possible width.
            max_height: The larget possible height.

        Returns:
            BytesIO: the bytes of the encoded image ready to be written to disk
        """
        if width * self.height > height * self.width:
            scaled_height = (width * self.height) // self.width
            scaled_image = self._resize(width, scaled_height)
            crop_top = (scaled_height - height) // 2
            crop_bottom = height + crop_top
            cropped = scaled_image.crop((0, crop_top, width, crop_bottom))
        else:
            scaled_width = (height * self.width) // self.height
            scaled_image = self._resize(scaled_width, height)
            crop_left = (scaled_width - width) // 2
            crop_right = width + crop_left
            cropped = scaled_image.crop((crop_left, 0, crop_right, height))
        return self._encode_image(cropped, output_type)

    def _encode_image(self, output_image, output_type):
        output_bytes_io = BytesIO()
        fmt = self.FORMATS[output_type]
        if fmt == "JPEG":
            output_image = output_image.convert("RGB")
        output_image.save(output_bytes_io, fmt, quality=80)
        return output_bytes_io