contours

Example Python program contours.py Python version 3.x or newer. To check the Python version use:

python --version

Modules

• import os
• import cv2
• import copy
• import pprint
• import numpy as np
• from shapely.geometry import Polygon
• from descartes import PolygonPatch
• import matplotlib.pyplot as plt

Methods

• def find_contours(img_url_local, class_id_study=0, only_classes=0,verbose=0, show_image=0):
• def get_contour_pts(contours, hierarchy, verbose=0):
• def draw_scatter(polygons_pts, h, w):
• def draw_polygon(polygons_pts, h, w, title='', save=False):

Code

Python example

import os
import cv2
import copy
import pprint
import numpy as np
from shapely.geometry import Polygon
from descartes import PolygonPatch
import matplotlib.pyplot as plt

%matplotlib inline

def find_contours(img_url_local, class_id_study=0, only_classes=0,verbose=0, show_image=0):
    if os.path.exists(img_url_local):
        img = cv2.imread(img_url_local, cv2.IMREAD_UNCHANGED)
        h, w = len(img), len(img[0])
        img_class_ids, counts = np.unique(img, return_counts=True)
        if verbose or only_classes:
            pprint.pprint(dict(zip(img_class_ids, counts)))
            if only_classes:
                return [], [], [], h, w

        for class_id in img_class_ids:
            if class_id not in [class_id_study]:
                continue
            if verbose:
                print (' - class_id : ', class_id)
            img_tmp = copy.deepcopy(img)
            img_tmp[img_tmp != class_id] = 0
            img_tmp[img_tmp == class_id] = 255
            if verbose:
                print (np.unique(img_tmp, return_counts=True))
            if show_image:
                plt.imshow(img_tmp, cmap='gray')

            im2, contours, hierarchy = cv2.findContours(img_tmp, cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)

            return im2, contours, hierarchy, h, w
    else:
        print (' Check image URL : ', img_url_local)
        return []

def get_contour_pts(contours, hierarchy, verbose=0):
    polygons_pts = []
    for contour_id, contour_topology in enumerate(hierarchy[0]):
        if verbose:
            print (' - hierarchy : ', contour_topology)
        next_contour    =  contour_topology[0]
        prev_contour    =  contour_topology[1]
        child_contour   =  contour_topology[2]
        parent_countour =  contour_topology[3]

        if child_contour == -1 and parent_countour == -1:
            polygon_pts = np.array([pts[0] for pts in contours[contour_id]])
            polygons_pts.append(polygon_pts)
        else:
            print (' - Special case noticed! Please handle.')

    return polygons_pts

def draw_scatter(polygons_pts, h, w):
    f, axarr= plt.subplots(1, figsize=(10,10))
    axarr.set_xlim(0, w)
    axarr.set_ylim(h, 0)

    for polygon_pts in polygons_pts:
        plt.scatter(polygon_pts[:,0], polygon_pts[:,1]) 

def draw_polygon(polygons_pts, h, w, title='', save=False):
    fig, ax = plt.subplots(1, figsize=(10,10))
    ax.set_xlim(0, w)
    ax.set_ylim(h, 0)

    for polygon_pts in polygons_pts:
        print (polygon_pts.shape)
        if len(polygon_pts) > 2:
            poly    = Polygon(polygon_pts)
            patch   = PolygonPatch(poly, facecolor=[0,0,0.5], edgecolor=[1,1,1], alpha=0.8)
            ax.add_patch(patch)

    plt.title(title)
    plt.tight_layout()    



if __name__ == "__main__":
    ## PARAMS
    img_url_local                  = '../data/cityscapes/stuttgart_000189_000019_gtFine_labelIds.png'

    ## STEPS
    _, contours, hierarchy, h, w   = find_contours(img_url_local, class_id_study=4, only_classes=0
                                                   , verbose=0, show_image=0)
    if len(contours):
        polygons_pts                = get_contour_pts(contours, hierarchy, verbose=0)
        # draw_scatter(polygons_pts, h, w) 
        draw_polygon(polygons_pts, h, w)

Useful Links

• Articles: https://python-commandments.org/
• Python shell: https://bsdnerds.org/learn-python/
• Tutorial: https://pythonprogramminglanguage.com/