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遇到一个需求，背景变换不大，捕捉一个发光羽毛球的运动轨迹，涉及的核心方法就是帧间差分

看一下原始视频和最终效果

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这个发光的就是发光羽毛球，视频的背景变化不大，还算可控

感谢 https://uutool.cn/gif-edit/ https://www.freeconvert.com/zh/gif-compressor 提供的降帧压缩服务

帧间差分

frame_gray = cv.cvtColor(frame,cv.COLOR_BGR2GRAY)
frame_gauss = cv.GaussianBlur(frame_gray,(5,5),0)if background is None:background = frame_gausscontinue# diff = cv.absdiff(background,frame_gauss)diff = cv.subtract(frame_gauss,background)ret,thresh = cv.threshold(diff,120,255,cv.THRESH_BINARY)
# dilated = cv.dilate(thresh,es,iterations=3)
background = frame_gauss

打开视频流

对得到到的帧进行灰度化、gauss 模糊，一定程度上降低可能存在的轻微抖动，减少噪声

如果背景是绝对静止不对，background 直接使用第一帧就行，

也就不需要更新了

进行差分

# diff = cv.absdiff(background,frame_gauss)diff = cv.subtract(frame_gauss,background)ret,thresh = cv.threshold(diff,120,255,cv.THRESH_BINARY)

这里测试如果使用 absdiff() 接口会出现两个目标的情况，使用 subtract() 不会出现，效果如下右下

然后应用阈值得到二值图像，右上

然后找边界，根据面积范围筛选到唯一的运动目标

contours, hierarchy = cv.findContours(thresh.copy(), cv.RETR_EXTERNAL, cv.CHAIN_APPROX_SIMPLE)
for c in contours:if cv.contourArea(c) > 250: # for ballcontinueprint("frame {} detected, update tracking point".format({frame_id}))(x, y, w, h) = cv.boundingRect(c) tracking_point_list[frame_id] = (x+w//2, y+h//2)

将边界用矩形框保住，中心作为跟踪的目标点

绘制、写出

后续就是进行绘制，写出

绘制

使用 line() circle() rectangle() 基于保存的tracking_point_list 绘制即可。

略

写出

out = cv.VideoWriter('task1_result.avi',int(fourcc), fps, (int(w),int(h)),True)# ... 省去 frame 绘制out.write(frame)

VideoWriter(filename, fourcc, fps, frameSize, isColor)

filename 文件路径
fourcc 编码器
fps 码率
frameSize 帧的宽高，int 类型
isColor 是否为彩色

报错： Failed to load OpenH264 library: openh264-1.8.0-win64.dll

在这里 https://github.com/cisco/openh264/releases 下载后解压放到 python.exe 同级目录下即可

完整代码

import cv2 as cv 
import numpy as np
import timevideo = cv.VideoCapture('task1.avi')
if(video.isOpened()):n_frame = video.get(cv.CAP_PROP_FRAME_COUNT)w = video.get(cv.CAP_PROP_FRAME_WIDTH)h = video.get(cv.CAP_PROP_FRAME_HEIGHT)fps = video.get(cv.CAP_PROP_FPS)fourcc = video.get(cv.CAP_PROP_FOURCC)print("frams:{} w:{} h:{}".format(n_frame,w,h))out = cv.VideoWriter('task1_result.avi',int(fourcc), fps, (int(w),int(h)),True)es = cv.getStructuringElement(cv.MORPH_ELLIPSE, (9, 9))
frames = []
background = None
circle_r = 1
line_thick = 1
tracking_point = None
tracking_point_list = [None for i in range(int(n_frame))]frame_id = 0
while video.isOpened():ret, frame = video.read()if frame is None: breakframe_id += 1print("frame {}".format(frame_id))# cv.putText(frame, 'frame:{}'.format(frame_id), (50, 50), cv.FONT_HERSHEY_SIMPLEX, 1, (0, 0, 255), 2)frame_gray = cv.cvtColor(frame,cv.COLOR_BGR2GRAY)frame_gauss = cv.GaussianBlur(frame_gray,(5,5),0)if background is None:background = frame_gausscontinue# diff = cv.absdiff(background,frame_gauss)diff = cv.subtract(frame_gauss,background)ret,thresh = cv.threshold(diff,120,255,cv.THRESH_BINARY)# dilated = cv.dilate(thresh,es,iterations=3)background = frame_gauss# contours, hierarchy = cv.findContours(diff.copy(), cv.RETR_EXTERNAL, cv.CHAIN_APPROX_SIMPLE) contours, hierarchy = cv.findContours(thresh.copy(), cv.RETR_EXTERNAL, cv.CHAIN_APPROX_SIMPLE)for c in contours:if cv.contourArea(c) > 250: # for ballcontinueprint("frame {} detected, update tracking point".format({frame_id}))(x, y, w, h) = cv.boundingRect(c) tracking_point_list[frame_id] = (x+w//2, y+h//2)rect_l = 2i = 0while i  < len(tracking_point_list):if i == 0 or  tracking_point_list[i] is None:i += 1continuej = i + 1while((j < len(tracking_point_list)) and tracking_point_list[j] is None):j = j + 1if(j >= len(tracking_point_list)):breaknextPoint = tracking_point_list[j]cv.rectangle(frame, tracking_point_list[i], (tracking_point_list[i][0]+rect_l,tracking_point_list[i][1]+rect_l), (0, 0, 255), 2)# cv.circle(frame, tracking_point_list[i], 2, (0, 0, 255), 1)cv.line(frame, tracking_point_list[i], nextPoint, (0, 0, 255), line_thick)    cv.rectangle(frame, nextPoint, (nextPoint[0]+rect_l,nextPoint[1]+rect_l), (0, 0, 255), 2)i += 1out.write(frame)cv.imshow('raw',frame)# cv.imshow('contours', frame_gauss)cv.imshow('diff', diff)cv.imshow('thresh',thresh)if cv.waitKey(10) & 0xFF == 27 :break time.sleep(0.2)video.release()
cv.destroyAllWindows()