Python+OpenCV实现鼠标画瞄准星的方法详解
本文介绍如何使用Python和OpenCV实现动态瞄准星功能，通过鼠标回调函数控制圆圈与十字线的绘制。支持滚轮调整半径、加减号调节变化量，旨在提升关键点标注的精准度，特别适用于回归任务及人脸关键点定位场景。
瞄准星由一个外圆及内部十字线构成，模拟射击游戏中狙击枪开镜的视觉效果。本方案并非在静态图像上直接绘制固定图形，而是实现瞄准星随鼠标位置动态移动的功能。在图像标注任务中，尤其是回归类任务，借助该辅助工具可显著提升关键点标注的精确度。
关于鼠标回调函数的具体实现细节，可参考：opencv-python的鼠标交互操作
函数说明
导入cv2模块后，可通过help(cv2.circle)和help(cv2.line)分别查看绘制圆形与直线的函数帮助文档：
cv2.circle()
该函数包含四个必需参数：
img：作为底图的图像，数据类型需为uint8的ndarray
center：圆心坐标，必须为包含两个数值的tuple类型，格式为(x, y)
radius：圆的半径，必须为整数类型
color：颜色值，为包含三个数值的tuple或list，格式为(b, g, r)
其余参数为可选：
thickness：线条宽度，必须为大于0的整数，默认值为1
lineType：线条类型，可选值包括cv2.LINE_4、cv2.LINE_8和cv2.LINE_AA。其中cv2.LINE_AA表示抗锯齿处理，能使线条更加平滑，绘制圆形时推荐使用此类型。
cv2.line()
line(img, pt1, pt2, color[, thickness[, lineType[, shift]]]) -> img . @brief Draws a line segment connecting two points. . . The function line draws the line segment between pt1 and pt2 points in the image. The line is . clipped by the image boundaries. For non-antialiased lines with integer coordinates, the 8-connected . or 4-connected Bresenham algorithm is used. Thick lines are drawn with rounding endings. Antialiased . lines are drawn using Gaussian filtering. . . @param img Image. . @param pt1 First point of the line segment. . @param pt2 Second point of the line segment. . @param color Line color. . @param thickness Line thickness. . @param lineType Type of the line. See #LineTypes. . @param shift Number of fractional bits in the point coordinates.
该函数同样包含四个必需参数：
img：作为底图的图像，数据类型需为uint8的ndarray
pt1：线段起点坐标，必须为包含两个数值的tuple类型，格式为(x, y)
pt2：线段终点坐标，类型要求同上
color：颜色值，为包含三个数值的tuple或list，格式为(b, g, r)
其余参数为可选：
thickness：线条宽度，必须为大于0的整数，默认值为1
lineType：线条类型，可选值包括cv2.LINE_4、cv2.LINE_8和cv2.LINE_AA。其中cv2.LINE_AA表示抗锯齿处理，能使线条更加平滑，绘制圆形时推荐使用此类型。
简单的例子
# -*- coding: utf-8 -*-
import cv2
import numpy as np
def imshow(winname, image):
cv2.namedWindow(winname, 1)
cv2.imshow(winname, image)
cv2.waitKey(0)
cv2.destroyAllWindows()
if __name__ == '__main__':
image = np.zeros((256, 256, 3), np.uint8)
center = (128, 128)
radius = 50
color = (0, 255, 0)
thickness = 2
pt_left = (center[0] - radius, center[1])
pt_right = (center[0] + radius, center[1])
pt_top = (center[0], center[1] - radius)
pt_bottom = (center[0], center[1] + radius)
cv2.circle(image, center, radius, color, thickness, lineType=cv2.LINE_AA)
cv2.line(image, pt_left, pt_right, color, thickness)
cv2.line(image, pt_top, pt_bottom, color, thickness)
imshow('draw_crosshair', image)
运行结果如下：
利用鼠标回调函数画瞄准星
具体操作说明：
鼠标移动时，以鼠标当前位置为圆心动态显示瞄准星
通过鼠标滚轮控制瞄准星的大小变化
使用+, -号键控制鼠标滚轮操作时瞄准星半径的变化步长
相关代码如下：
# -*- coding: utf-8 -*-
import cv2
WIN_NAME = 'draw_crosshair'
class DrawCrosshair(object):
def __init__(self, image, color, center, radius, thickness=1):
self.original_image = image
self.image_for_show = image.copy()
self.color = color
self.center = center
self.radius = radius
self.thichness = thickness
self.increment = 5
def increase_radius(self):
self.radius += self.increment
def decrease_radius(self):
self.radius -= self.increment
self.radius = max(self.radius, 0)
def increase_increment(self):
self.increment += 1
def decrease_increment(self):
self.increment -= 1
self.increment = max(self.increment, 1)
def reset_image(self):
"""
reset image_for_show using original image
"""
self.image_for_show = self.original_image.copy()
def draw_circle(self):
cv2.circle(self.image_for_show,
center=self.center,
radius=self.radius,
color=self.color,
thickness=self.thichness,
lineType=cv2.LINE_AA)
def draw_crossline(self):
pt_left = (self.center[0] - self.radius, self.center[1])
pt_right = (self.center[0] + self.radius, self.center[1])
pt_top = (self.center[0], self.center[1] - self.radius)
pt_bottom = (self.center[0], self.center[1] + self.radius)
cv2.line(self.image_for_show, pt_left, pt_right,
self.color, self.thichness)
cv2.line(self.image_for_show, pt_top, pt_bottom,
self.color, self.thichness)
def draw(self):
self.reset_image()
self.draw_circle()
self.draw_crossline()
def onmouse_draw_rect(event, x, y, flags, draw_crosshair):
if event == cv2.EVENT_MOUSEWHEEL and flags > 0:
draw_crosshair.increase_radius()
if event == cv2.EVENT_MOUSEWHEEL and flags < 0:
draw_crosshair.decrease_radius()
draw_crosshair.center = (x, y)
draw_crosshair.draw()
if __name__ == '__main__':
# image = np.zeros((512, 512, 3), np.uint8)
image = cv2.imread('luka.jpg')
draw_crosshair = DrawCrosshair(image,
color=(0, 255, 0),
center=(256, 256),
radius=100,
thickness=2)
cv2.namedWindow(WIN_NAME, 1)
cv2.setMouseCallback(WIN_NAME, onmouse_draw_rect, draw_crosshair)
while True:
cv2.imshow(WIN_NAME, draw_crosshair.image_for_show)
key = cv2.waitKey(30)
if key == 27: # ESC
break
elif key == ord('+'):
draw_crosshair.increase_increment()
elif key == ord('-'):
draw_crosshair.decrease_increment()
cv2.destroyAllWindows()
最终效果如图所示，借助瞄准星辅助，可以更精准地定位Luka的眼睛中心。同理，在进行人脸关键点标注时，该功能同样有助于更精确地确定眼睛中心位置。
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