Reading PASCAL VOC annotations in python

Question

I have annotations in xml files such as this one, which follows the PASCAL VOC convention:

<annotation>
<folder>training</folder>
<filename>chanel1.jpg</filename>
<source>
<database>synthetic initialization</database>
<annotation>PASCAL VOC2007</annotation>
<image>synthetic</image>
<flickrid>none</flickrid>
</source>
<owner>
<flickrid>none</flickrid>
<name>none</name>
</owner>
<size>
<width>640</width>
<height>427</height>
<depth>3</depth>
</size>
<segmented>0</segmented>
<object>
<name>chanel</name>
<pose>Unspecified</pose>
<truncated>0</truncated>
<difficult>0</difficult>
<bndbox>
<xmin>344</xmin>
<ymin>10</ymin>
<xmax>422</xmax>
<ymax>83</ymax>
</bndbox>
</object>
<object>
<name>chanel</name>
<pose>Unspecified</pose>
<truncated>0</truncated>
<difficult>0</difficult>
<bndbox>
<xmin>355</xmin>
<ymin>165</ymin>
<xmax>443</xmax>
<ymax>206</ymax>
</bndbox>
</object>
</annotation>

What is the cleanest way of retrieving for example the fields filename and bndbox in Python?

I was trying to ElementTree, which seems to be the official Python solution, but I can’t make it work.

My code so far:

from xml.etree import ElementTree as ET
tree = ET.parse("data/all/annotations/" + file)
fn = tree.find('filename').text
boxes = tree.findall('bndbox')

this produces

fn == 'chanel1.jpg'
boxes == []

So it succesfully extracts the filename field, but not the bndbox‘es.

Asked By: Jsevillamol

||

Source

Answer 1

That’s a quite easy solution for your problem:

This will return your box coordinates in a nested list [xmin, ymin, xmax, ymax] and the filename
Once I struggled with bndbox tags which where mixed up (ymin, xmin,…) or any other strange combinations, so this code read the tags not only the position.

Finally I updated the code. Thanks to craq and Pritesh Gohil, you were absolutely right.

Hope it helps…

import xml.etree.ElementTree as ET


def read_content(xml_file: str):

    tree = ET.parse(xml_file)
    root = tree.getroot()

    list_with_all_boxes = []

    for boxes in root.iter('object'):

        filename = root.find('filename').text

        ymin, xmin, ymax, xmax = None, None, None, None

        ymin = int(boxes.find("bndbox/ymin").text)
        xmin = int(boxes.find("bndbox/xmin").text)
        ymax = int(boxes.find("bndbox/ymax").text)
        xmax = int(boxes.find("bndbox/xmax").text)

        list_with_single_boxes = [xmin, ymin, xmax, ymax]
        list_with_all_boxes.append(list_with_single_boxes)

    return filename, list_with_all_boxes

name, boxes = read_content("file.xml")

Answered By: pix_1

Answer 2

Another option is to use the standard xmldict library to load the VOC XML in a python dict.

import xmltodict

with open('/path/to/voc.xml') as file:
        file_data = file.read()
        dict_data = xmltodict.parse(file_data)
        print(dict_data)

Answered By: Octave

Answer 3

My attempt at it, slightly more readable than the accepted answer, offering the option to convert to 0-based pixel coordinates, and pairing the name of the object rather than the name of the file with each box’s coordinates.

Output example:

{'excavator': {'xmin': 0, 'ymin': 0, 'xmax': 1265, 'ymax': 587},
 'dump_truck': {'xmin': 259, 'ymin': 159, 'xmax': 713, 'ymax': 405}}

import xml.etree.ElementTree as ET

def read_Pascal_VOC(xml_file,do_0_based):
    # Pascal VOC is 1-based, but more recent formats like MS COCO are 0-based
    # see, e.g., https://github.com/Ricardozzf/maskrcnn-benchmark/commit/da8f99927eb945d3e66985d5e070fb55db472de6
    if do_0_based:
        to_subtract = 1
    else:
        to_subtract = 0

    tree = ET.parse(xml_file)
    root = tree.getroot()

    boxes = dict()

    for box in root.iter('object'):

        name = box.find('name').text
        
        bb = box.find('bndbox')
        
        # dict to remove any ambiguity ordering-wise
        coords = dict(xmin = bb.find('xmin').text, 
                      ymin = bb.find('ymin').text, 
                      xmax = bb.find('xmax').text, 
                      ymax = bb.find('ymax').text)
            
        coords = {k:int(v)-to_subtract for k,v in coords.items()}
        
        if name in boxes:
            boxes[name] = boxes[name] + [coords]
        else:
            boxes[name] = [coords]

    return boxes

Answered By: Antoine

Reading PASCAL VOC annotations in python

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