第二章

使用numpy.array訪問圖像數據

改變一個特定像素的值：

• numpy.array提供的item()
  • item(x,y,id)
  • id為索引，B G R
• itemset()
  • itemset((x,y,id),val)
  • (x,y,索引，要設定的值)

操作通道：將指定通道所有值置0

import cv2 import numpy as np img = cv2.imread('1.png') img[:,:,1] = 0

視頻文件的讀/寫

• OpenCV提供了VideoCapture類和VideoWriter類支持各種格式的視頻文件(都支持AVI格式),到達視頻文件末尾前，可通過read()獲取新的幀，每一幀是一幅基于BGR格式的圖像

import cv2 videoCapture = cv2.VideoCapture('2_1.avi') fps = videoCapture.get(cv2.CAP_PROP_FPS)#幀速率 size = (int(videoCapture.get(cv2.CAP_PROP_FRAME_WIDTH)),int(videoCapture.get(cv2.CAP_PROP_FRAME_HEIGHT))) videoWriter = cv2.VideoWriter('2_1_copy.avi',cv2.VideoWriter_fourcc('I','4','2','0'),fps,size)sucess , frame = videoCapture.read() while sucess:videoWriter.write(frame)sucess , frame = videoCapture.read()

其中編解碼器的可用性常用選項：

• cv2.VideoWriter_fource('I','4','2','0'):該選項是一個未壓縮的YUV顏色編碼，4：2：0色度子采樣。這種編碼有很好地兼容性，但會產生較大文件,文件擴展名.avi (例子視頻145K，產生了12.4M文件)
• cv2.VideoWriter_fource(‘P’,‘I’,‘M’,‘1’): 生成.avi
• cv2.VideoWriter_fource(‘X’,‘V’,‘I’,‘D’):MPEG-4編碼類型，生成.avi(若希望視頻大小為平均值，推薦使用)
• cv2.VideoWriter_fource(‘F’,‘L’,‘V’,‘1’):該選項是一個Flash視頻，擴展名.flv

捕獲攝像頭的幀

import cv2cameraCapture = cv2.VideoCapture(0) #為了針對攝像頭創建合適的VideoWriter類， #要么對幀速率作出假設， #要么使用計時器測量 fps = 30 size = (int(cameraCapture.get(cv2.CAP_PROP_FRAME_WIDTH)),int(cameraCapture.get(cv2.CAP_PROP_FRAME_HEIGHT)))videoWriter = cv2.VideoWriter('2_1_catch_camera.avi',cv2.VideoWriter_fourcc('I','4','2','0'),fps,size)success , frame = cameraCapture.read() numFramesRemaining = 10 * fps - 1while success and numFramesRemaining > 0 :videoWriter.write(frame) #需要同步一組攝像頭，read()方法不適用，可用grab(),retrive()替代success , frame = cameraCapture.read()numFramesRemaining -= 1cameraCapture.release()

imshow在窗口顯示圖像

import cv2 import numpy as np img = cv2.imread('2_1.jpg') cv2.imshow('2_1.jpg',img) cv2.waitKey()#保證顯示視頻時窗口上幀可以一直更新 cv2.destroyAllWindows()

在窗口顯示攝像頭幀

• 任意窗口下都可以通過waitKey()獲取鍵盤輸入；
• 通過setMouseCallback()獲取鼠標輸入。

#實時顯示攝像頭幀#OpenCV不提供任何處理窗口事件的方法，單擊窗口關閉時，并不能關閉應用程序 import cv2 clicked = False #鼠標響應函數 """ flags: 代表鼠標的拖拽事件，以及鍵盤鼠標聯合事件 param:函數指針 標識了所響應的事件函數，相當于自定義了一個OnMouseAction()函數的ID。 """ def onMouse( event , x , y , flags , param ):global clickedif event == cv2.EVENT_LBUTTONUP:clicked = TruecameraCapture = cv2.VideoCapture(0) cv2.namedWindow('MyWindow')#鼠標回調函數，param為可選參數 """ C++中原型： void setMouseCallback(const string& winname, //圖像視窗名稱 MouseCallback onMouse, //鼠標響應函數，監視到鼠標操作后調用并處理相應動作 void* userdata = 0 //鼠標響應處理函數的ID，識別號，默認0 ); """ cv2.setMouseCallback('MyWindow',onMouse)print('Showing camera feed. Click window or press any key to stop.')sucess , frame = cameraCapture.read()#waitKey()參數為等待鍵盤觸發的時間，單位ms，返回值-1(表示沒有被按下) #某些系統waitKey()返回值可能比ASCII更大，可通過讀取返回值最后一個字節 #保證只提取ASCII碼： # keycode = cv2.waitKey(1) # if keycode != -1 : # keycode &= 0xFFwhile sucess and cv2.waitKey(1) == -1 and not clicked:cv2.imshow('MyWindow',frame)sucess , frame = cameraCapture.read()cv2.destroyWindow('MyWindow') cameraCapture.release()

Cameo-面向對象設計

• 使用多個I/O流
• 創建CaptureManager類和WindowManager類作為高級的I/O流接口

使用managers.CaptureManager提取視頻流

使用windowManager抽象窗口和鍵盤

監聽鍵盤和鼠標事件：進行截圖，截屏

運行時，攝像頭幀被鏡像，存儲圖片也被鏡像（默認設置鏡像為True）
managers.py

#managers.CaptureManager提取視頻流 import cv2 import numpy import timeclass CaptureManager(object):def __init__(self,capture,previewWindowManager = None,shouldMirrorPreview = False):self.previewWindowManager = previewWindowManagerself.shouldMirrorPreview = shouldMirrorPreviewself._capture = captureself._channel = 0self._enteredFrame = Falseself._frame = Noneself._mirroredframe = Noneself._imageFilename = Noneself._videoFilename = Noneself._videoEncoding = Noneself._videoWriter = Noneself._startTime = Noneself._framesElapsed = numpy.long(0)self._fpsEstimate = None@propertydef channel(self):return self._channel@channel.setterdef channel(self,value):if self._channel != value:self._channel = valueself._frame = None@propertydef frame(self):if self._enteredFrame and self._frame is None:_ , self._frame = self._capture.retrieve() #retrieve()是解碼并返回一個幀,grab是指向下一個幀(不需要當前幀時可跳過),read是grab和retrieve的結合return self._frame@propertydef isWritingImage(self):return self._imageFilename is not None@propertydef isWritingVideo(self):return self._videoFilename is not Nonedef enterFrame(self): #Capture the next frame#check previous frame was existed.# 檢查上一幀是否退出assert not self._enteredFrame , \'previous enterFrame() had no matching exitFrame()'if self._capture is not None :self._enteredFrame = self._capture.grab() #用grab()指向下一幀def exitFrame(self):#draw to the window ,write to files , release the frame.if self.frame is None:self._enteredFrame = Falsereturnif self._framesElapsed == 0 :self._startTime = time.time()else:timeElapsed = time.time() - self._startTimeself._fpsEstimate = self._framesElapsed / timeElapsedself._framesElapsed += 1#draw to the windowif self.previewWindowManager is not None:if self.shouldMirrorPreview:self._mirroredframe = numpy.fliplr(self._frame).copy()self.previewWindowManager.show(self._mirroredframe)else:self.previewWindowManager.show(self._frame)if self.isWritingImage:if self.shouldMirrorPreview:cv2.imwrite(self._imageFilename,self._mirroredframe)else :cv2.imwrite(self._imageFilename, self._frame)self._imageFilename = Noneself._writeVideoFrame()self._frame = Noneself._enteredFrame = Falsedef writeImage(self , filename):self._imageFilename = filenamedef startWritingVideo(self,filename,encoding=cv2.VideoWriter_fourcc('I','4','2','0')):self._videoFilename = filenameself._videoEncoding = encodingdef stopWritingVideo(self):self._videoFilename = Noneself._videoEncoding = Noneself._videoWriter = Nonedef _writeVideoFrame(self): #非公有if not self.isWritingVideo:returnif self._videoWriter is None:fps = self._capture.get(cv2.CAP_PROP_FPS)if fps == 0.0:if self._framesElapsed < 20 :returnelse:fps = self._fpsEstimatesize = (int(self._capture.get(cv2.CAP_PROP_FRAME_WIDTH)),int(self._capture.get(cv2.CAP_PROP_FRAME_HEIGHT)))self._videoWriter = cv2.VideoWriter(self._videoFilename,self._videoEncoding,fps,size)self._videoWriter.write(self._frame)#抽象窗口和鍵盤 class WindowManager(object):def __init__(self,windowName,keypressCallback = None,mousepressCallback = None):self.keypressCallback = keypressCallbackself.mousepressCallback = mousepressCallback #鼠標self._windowName = windowNameself._isWindowCreated = False@propertydef isWindowCreated(self):return self._isWindowCreateddef createWindow(self):cv2.namedWindow(self._windowName)self._isWindowCreated = Truecv2.setMouseCallback(self._windowName,self.mousepressCallback) #鼠標事件def show(self , frame):cv2.imshow(self._windowName,frame)def destoryWindow(self):cv2.destroyWindow(self._windowName)self._isWindowCreated = Falsedef processEvent(self):keycode = cv2.waitKey(1)if self.keypressCallback is not None and keycode != -1:keycode &= 0xFFself.keypressCallback(keycode)#python內置的@property裝飾器就是負責把一個方法變成屬性調用的

cameo.py

import cv2 from managers import WindowManager , CaptureManagerclass Cameo(object):def __init__(self):self._windowManager = WindowManager('cameo',self.onKeypress,self.onMouse)self._captureManager = CaptureManager(cv2.VideoCapture(0),self._windowManager,True)def run(self):self._windowManager.createWindow() #創建窗口while self._windowManager.isWindowCreated:self._captureManager.enterFrame() #檢查上一幀是否退出，并指向下一幀frame = self._captureManager.frame #獲取當前幀self._captureManager.exitFrame() #draw to the window ,write to files , release the frame.self._windowManager.processEvent() #處理鍵盤事件def onKeypress(self , keycode): #鍵盤響應"""Handle a keypress.space -> Take a screenshottab -> Start/Stop recording a screencastescape -> Quit"""if keycode == 32: #spaceself._captureManager.writeImage('screenshot.png')elif keycode == 9:#tabif not self._captureManager.isWritingVideo:self._captureManager.startWritingVideo('screencast.avi')else:self._captureManager.stopWritingVideo()elif keycode == 27:#escapeself._windowManager.destoryWindow()def onMouse(self , event, x, y, flags, param): #鼠標響應函數if event == cv2.EVENT_LBUTTONUP:self._windowManager.destoryWindow() if __name__=="__main__":Cameo().run()

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