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May 28, 2022

CV2, Pillow and shapely for Polygons

python

Contents

CV2

  • Read image and correct channels 
    img = cv2.imread(img_path)
img = cv2.cvtColor(img, cv2.COLOR_BGR2RGB)
img = cv2.cvtColor(img, cv2.COLOR_RGB2BGR)
  • Draw rectangle 
    cv2.rectangle(img, (x1, y1), (x2, y2), config.bbox_color, config.bbox_stroke)
  • Write image into disk: 
    cv2.imwrite(img_path, img)
  • Fill a polygon by coordinates: 
    # [[x1,y1], [x2,y2], ... ]
points = np.array( [[[10,10],[100,10],[100,100],[10,100]]], dtype=np.int32 )
img = np.zeros([240,320],dtype=np.uint8)
cv2.fillPoly(img, pts=points, color=(0, 255, 0))

Pillow

  • Read image

    from PIL import Image
img = Image.open(img_path)

  • Resize image

    img = img.resize((new_width, new_height), Image.BILINEAR)

  • Rotate image

    img = img.rotate(angle, Image.BILINEAR) # in degree

  • Get image height and width:

    h, w = img.height, img.width

  • Crop image:

    box = (start_w, start_h, start_w + length, start_h + length)
region = img.crop(box)

  • Draw bounding box:

    from PIL import ImageDraw

draw = ImageDraw.Draw(img) # img: PIL.Image.Image
for box in boxes:
  xmin, ymin, xmax, ymax = box
  draw.rectangle(((xmin, ymin), (xmax, ymax)), outline=(255,255,255,150), width=1)

shapely.geometry.Polygon

To check intersection of two polygonal regions, we can:

from shapely.geometry import Polygon
poly1 = np.array([[x1, y1, x2, y2, x3, y3, x4, y4]])
poly2 = np.array([[x1_, y1_, x2_, y2_, x3_, y3_, x4_, y4_]])

p1 = Polygon(poly1).convex_hull
p2 = Polygon(poly2).convex_hull

inter_area = p1.intersection(p2).area
iou = inter_area / (p1.area + p2.area - inter_area)