opencv 画矩形roi截图

OpenCV是一个开源的计算机视觉库。它提供了许多处理数字图像和视频的函数，而其中之一就是画矩形ROI截图。

首先，要创建一个矩形ROI，需要知道需要截取的区域的左上角和右下角的坐标。可以使用OpenCV自带的鼠标事件函数，在图像上拖动鼠标选择需要截取的区域并获取该区域的坐标。

其次，使用cv2.rectangle函数将矩形绘制在图像上。函数参数包括：原始图像，矩形左上角坐标，矩形右下角坐标，颜色和线宽度。

最后，使用numpy的切片操作截取原始图像中的ROI区域。切片的参数是左上角和右下角坐标，保持一致性。

示例程序如下：

import cv2
import numpy as np

# Mouse callback function
def draw_rect(event, x, y, flags, params):
    global x_init, y_init, drawing, top_left_pt, bottom_right_pt

    if event == cv2.EVENT_LBUTTONDOWN:
        drawing = True
        x_init, y_init = x, y

    elif event == cv2.EVENT_MOUSEMOVE:
        if drawing:
            top_left_pt = (min(x_init, x), min(y_init, y))
            bottom_right_pt = (max(x_init, x), max(y_init, y))
            img[y_init:y, x_init:x] = 255 - img[y_init:y, x_init:x]

    elif event == cv2.EVENT_LBUTTONUP:
        drawing = False
        top_left_pt  = (min(x_init, x), min(y_init, y))
        bottom_right_pt  = (max(x_init, x), max(y_init, y))
        img[y_init:y, x_init:x] = 255 - img[y_init:y, x_init:x]

# MAIN SCRIPT
drawing = False
top_left_pt, bottom_right_pt = (-1, -1), (-1, -1)

# Open image
img = cv2.imread('test.png')
copy = img.copy()

# Create a window
cv2.namedWindow("Window")

# Set the mouse callback function to "draw_rect"
cv2.setMouseCallback("Window", draw_rect)

while True:
    cv2.imshow("Window", img)
    k = cv2.waitKey(1) & 0xFF

    # If "c" is pressed, reset the window
    if k == ord('c'):
        img = copy.copy()

    # If "r" is pressed, crop the ROI
    elif k == ord('r'):
        if top_left_pt != (-1, -1) and bottom_right_pt != (-1, -1):
            roi = copy[top_left_pt[1]:bottom_right_pt[1], top_left_pt[0]:bottom_right_pt[0]]
            cv2.imshow("ROI", roi)

    # If "q" is pressed, exit
    elif k == ord('q'):
        break

cv2.destroyAllWindows()

该程序允许用户在图像上拖动鼠标选择需要截取的区域。在选择完区域后，按下“r”键就可以截取该区域并显示在一个新窗口中。按下“c”键可以重置窗口。如果想要结束程序，按下“q”键即可。