Python - Drawing True North line on equirectangular picture based on GPano:PoseHeadingDegrees

watermark-north.py

#
#   Dependencies:
#    - pip install Pillow (easy_install Pillow works as well)
#

import os
import xml.etree.ElementTree as ET

from PIL import Image, ImageDraw, ImageFont

def get_xmp_heading(fname):

    xmpstart = '<x:xmpmeta'
    xmpend   = '</x:xmpmeta>'

    binary_encoding = 'latin-1'

    open_mode = 'r'

    gpano_heading_tag = '{http://ns.google.com/photos/1.0/panorama/}PoseHeadingDegrees'

    with open(fname, open_mode, encoding=binary_encoding) as f:
        contents = str(f.read())

    # extract xmp data
    xmpdata = contents[contents.find(xmpstart):contents.find(xmpend) + len(xmpend)]
    xmp = ET.fromstring(xmpdata)

    # find GPano:PoseHeadingDegrees tag in subtree of XMP
    # data recusively and take the first result
    gpano_heading = next(xmp.iter(gpano_heading_tag)).text

    return float(gpano_heading)

def watermark_north(fname, degrees, fname2 = None):

    save_format = 'PNG'

    if fname2 is None:
        _, extension = os.path.splitext(fname)
        fname2 = filename.replace(extension, '-e' + extension)

    image1 = Image.open(fname).convert('RGBA')
    w = image1.size[0]
    h = image1.size[1]

    # calculate offset x, the amount of pixels converted
    # from the number of degrees
    offset = (degrees / 360) * w

    # calculate actual x coordinate of true north line
    x2 = (w / 2) - offset
    if x2 < 0:
        # if x2 is negative, wrap around width of image
        x2 = w + x2

    # draw true north line on image
    draw = ImageDraw.Draw(image1)
    draw.line([(x2, 0), (x2, h)], fill=128, width=8)

    # print degrees for debugging
    draw.text((10, 10), str(degrees), font=ImageFont.truetype('arial.ttf', 100), fill=255)

    del draw

    image1.save(fname2, save_format)

filename = "input.jpg"

degrees = get_xmp_heading(filename)
watermark_north(filename, degrees)