OpenCV in Python

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tagsdev, opencv, computer-vision, python

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OpenCV (cv2) covers video/image I/O, drawing primitives, and interactive windows for computer-vision work in Python.

Read Video Frame by Frame

VideoCapture.read() returns a (success, frame) pair — ret is False once the video ends or the source disconnects, which is the loop’s exit condition.

cap = cv2.VideoCapture(video_path)
while cap.isOpened():
    ret, frame = cap.read()
    if not ret:
        break

Draw Text and Shapes

cv2.putText and cv2.line both draw directly onto a frame’s pixel array and return it, so you can chain them or reassign in place.

new_frame = cv2.putText(frame, text, (x, y),
                        cv2.FONT_HERSHEY_SIMPLEX, line_width,
                        (0, 255, 0), 5, lineType=cv2.LINE_AA)
cv2.line(to_show, (x0, y0), (x1, y1), (0, 0, 255), 5)

Mouse Callback

A mouse callback receives the event type and cursor position on every mouse action inside the window it’s bound to — check event against cv2.EVENT_* constants to react to specific actions (click, drag, release).

def mouse_callback(event, x, y, flags, param):
    if event == cv2.EVENT_LBUTTONDOWN:
        pass

Bind it to a named window after creating it:

self.window = cv2.namedWindow('frame')
cv2.setMouseCallback('frame', self.mouse_callback)

Warp Perspective

See perspective-transform for computing and applying the homography matrix.

Plot Curves

cv2.polylines is OpenCV’s equivalent of matplotlib.pyplot.plot — it needs points as an integer (N, 2) array rather than separate x/y sequences, and draws each element of the outer array as its own curve.

# matplotlib
matplotlib.pyplot.plot(x_values, y_values, color='yellow')
 
# OpenCV equivalent
curve = numpy.column_stack((x_values.astype(numpy.int32), y_values.astype(numpy.int32)))
cv2.polylines(image, [curve], False, (0, 255, 255))
 
# multiple curves: add each as an element of the array
curve1 = numpy.column_stack((x1.astype(numpy.int32), y1.astype(numpy.int32)))
curve2 = numpy.column_stack((x2.astype(numpy.int32), y2.astype(numpy.int32)))
cv2.polylines(image, [curve1, curve2], False, (0, 255, 255))

More information: https://docs.opencv.org/4.x/d6/d6e/group__imgproc__draw.html

Cheatsheet

Video I/O

cap = cv2.VideoCapture(video_path)
ret, frame = cap.read() # ret is False at end of stream

Drawing

cv2.putText(frame, text, (x, y), cv2.FONT_HERSHEY_SIMPLEX, line_width, (0, 255, 0), 5, lineType=cv2.LINE_AA)
cv2.line(frame, (x0, y0), (x1, y1), (0, 0, 255), 5)
cv2.polylines(image, [curve], False, (0, 255, 255)) # curve: np.int32 (N, 2) array

Mouse callback

cv2.namedWindow('frame')
cv2.setMouseCallback('frame', mouse_callback) # mouse_callback(event, x, y, flags, param)

Graph View