如何使用鼠标事件和标志绘制自由和水平线？

Question

我尝试构建一个代码，在其中我可以使用OpenCV在图像上方绘制线条。在我的代码中，我可以自由绘制线条，但是我想在按下Shift键的同时绘制水平线条。实际上，我不了解flags和鼠标event的确切逻辑。请让我知道，如何修复我的代码。

import cv2 
import numpy as np 

cropping = False

x_start, y_start, x_end, y_end = 0, 0, 0, 0

image = cv2.imread("C:/Users/USER-PC/Documents/python/image/sunflower.jpg")
oriImage = image.copy()

def draw_line(event, x,y, flags, param):
    global x_start, y_start, x_end, y_end, cropping 

    if event == cv2.EVENT_LBUTTONDOWN:
        x_start, y_start, x_end, y_end = x,y,x,y
        cropping = True

    elif event == cv2.EVENT_MOUSEMOVE and flags != cv2.EVENT_FLAG_SHIFTKEY:
        x_end, y_end = x,y
    elif event == cv2.EVENT_MOUSEMOVE and flags == cv2.EVENT_FLAG_SHIFTKEY:
        x_end, y_end = x,y_start

    elif event == cv2.EVENT_LBUTTONUP:
        x_end, y_end = x,y
        cropping = False
        cv2.line(image, (x_start, y_start),(x_end, y_end), (255,0,0),2)

cv2.namedWindow("image")
cv2.setMouseCallback("image", draw_line)

while (1):
    i = image.copy()

    if not cropping:

        cv2.imshow("image", image)
    elif cropping:


        cv2.line(i, (x_start, y_start),(x_end, y_end), (255,0,0),2)
        # cv2.rectangle(i, (x_start, y_start), (x_end, y_end), (255,0,0), 2)
        cv2.imshow("image", i)

    if cv2.waitKey(1) & 0xFF ==27:
        break

cv2.destroyAllWindows

Answer 1

flags由几个位组成，请参见。 mouse event flags上的文档。对于这种情况，您要检测到，已激活EVENT_FLAG_LBUTTON（值1）和EVENT_FLAG_SHIFTKEY（值16），因此在这种情况下为flags = 17。现在，检查flags == 17可能是一个选项，但是专门检查EVENT_FLAG_SHIFTKEY位是否已激活可能会有所帮助：

shift_pressed = (flags & cv2.EVENT_FLAG_SHIFTKEY) > 0

如果激活了True，则返回EVENT_FLAG_SHIFTKEY，否则返回False。

然后可以通过以下方式修改您的draw_line方法：

def draw_line(event, x,y, flags, param):
    global x_start, y_start, x_end, y_end, cropping

    shift_pressed = (flags & cv2.EVENT_FLAG_SHIFTKEY) > 0

    if event == cv2.EVENT_LBUTTONDOWN:
        x_start, y_start, x_end, y_end = x, y, x, y
        cropping = True

    elif event == cv2.EVENT_MOUSEMOVE and shift_pressed:
        x_end, y_end = x, y_start

    elif event == cv2.EVENT_MOUSEMOVE:
        x_end, y_end = x, y

    elif event == cv2.EVENT_LBUTTONUP:
        x_end, y_end = x, y
        cropping = False
        cv2.line(image, (x_start, y_start), (x_end, y_end), (255, 0, 0), 2)

根据我对您问题的理解，这确实是您想要实现的目标。

希望有帮助！