OpenCV3计算机视觉：Python实现 读书笔记-第二章

第二章

使用numpy.array访问图像数据

改变一个特定像素的值：

• numpy.array提供的item()
  • item(x,y,id)
  • id为索引，B G R
• itemset()
  • itemset((x,y,id),val)
  • (x,y,索引，要设定的值)

操作通道：将指定通道所有值置0

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

视频文件的读/写

• OpenCV提供了VideoCapture类和VideoWriter类支持各种格式的视频文件(都支持AVI格式),到达视频文件末尾前，可通过read()获取新的帧，每一帧是一幅基于BGR格式的图像

import cv2
videoCapture = cv2.VideoCapture('2_1.avi')
fps = videoCapture.get(cv2.CAP_PROP_FPS)#帧速率
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()

其中编解码器的可用性常用选项：

• cv2.VideoWriter_fource('I','4','2','0'):该选项是一个未压缩的YUV颜色编码，4：2：0色度子采样。这种编码有很好地兼容性，但会产生较大文件,文件扩展名.avi (例子视频145K，产生了12.4M文件)
• cv2.VideoWriter_fource(‘P’,‘I’,‘M’,‘1’): 生成.avi
• cv2.VideoWriter_fource(‘X’,‘V’,‘I’,‘D’):MPEG-4编码类型，生成.avi(若希望视频大小为平均值，推荐使用)
• cv2.VideoWriter_fource(‘F’,‘L’,‘V’,‘1’):该选项是一个Flash视频，扩展名.flv

捕获摄像头的帧

import cv2

cameraCapture = cv2.VideoCapture(0)
#为了针对摄像头创建合适的VideoWriter类，
#要么对帧速率作出假设，
#要么使用计时器测量
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 - 1

while success and numFramesRemaining > 0 :
    videoWriter.write(frame)
#需要同步一组摄像头，read()方法不适用，可用grab(),retrive()替代
    success , frame = cameraCapture.read()
    numFramesRemaining -= 1

cameraCapture.release()

imshow在窗口显示图像

import cv2
import  numpy as np
img = cv2.imread('2_1.jpg')
cv2.imshow('2_1.jpg',img)
cv2.waitKey()#保证显示视频时窗口上帧可以一直更新
cv2.destroyAllWindows()

在窗口显示摄像头帧

• 任意窗口下都可以通过waitKey()获取键盘输入；
• 通过setMouseCallback()获取鼠标输入。

#实时显示摄像头帧

#OpenCV不提供任何处理窗口事件的方法，单击窗口关闭时，并不能关闭应用程序
import cv2
clicked = False
#鼠标响应函数
"""
flags: 代表鼠标的拖拽事件，以及键盘鼠标联合事件
param:函数指针 标识了所响应的事件函数，相当于自定义了一个OnMouseAction()函数的ID。
"""
def onMouse( event , x , y , flags , param ):
    global clicked
    if event == cv2.EVENT_LBUTTONUP:
        clicked = True


cameraCapture = cv2.VideoCapture(0)
cv2.namedWindow('MyWindow')

#鼠标回调函数，param为可选参数
"""
C++中原型：
void setMouseCallback(const string& winname,     //图像视窗名称
MouseCallback onMouse,     //鼠标响应函数，监视到鼠标操作后调用并处理相应动作
void* userdata = 0        //鼠标响应处理函数的ID，识别号，默认0
);
"""
cv2.setMouseCallback('MyWindow',onMouse)

print('Showing camera feed. Click window or press any key to stop.')

sucess , frame = cameraCapture.read()

#waitKey()参数为等待键盘触发的时间，单位ms，返回值-1(表示没有被按下)
#某些系统waitKey()返回值可能比ASCII更大，可通过读取返回值最后一个字节
#保证只提取ASCII码：
# keycode = cv2.waitKey(1)
# if keycode != -1 :
#     keycode &= 0xFF

while sucess and cv2.waitKey(1) == -1 and not clicked:
    cv2.imshow('MyWindow',frame)
    sucess , frame = cameraCapture.read()

cv2.destroyWindow('MyWindow')
cameraCapture.release()

Cameo-面向对象设计

• 使用多个I/O流
• 创建CaptureManager类和WindowManager类作为高级的I/O流接口

使用managers.CaptureManager提取视频流

使用windowManager抽象窗口和键盘

监听键盘和鼠标事件：进行截图，截屏

运行时，摄像头帧被镜像，存储图片也被镜像（默认设置镜像为True）
managers.py

#managers.CaptureManager提取视频流
import cv2
import numpy
import time

class CaptureManager(object):

    def __init__(self,capture,previewWindowManager = None,shouldMirrorPreview = False):

        self.previewWindowManager = previewWindowManager
        self.shouldMirrorPreview = shouldMirrorPreview

        self._capture = capture
        self._channel = 0
        self._enteredFrame = False
        self._frame = None
        self._mirroredframe = None
        self._imageFilename = None
        self._videoFilename = None
        self._videoEncoding = None
        self._videoWriter = None

        self._startTime = None
        self._framesElapsed = numpy.long(0)
        self._fpsEstimate = None

    @property
    def channel(self):
        return self._channel

    @channel.setter
    def channel(self,value):
        if self._channel != value:
            self._channel = value
            self._frame = None

    @property
    def frame(self):
        if self._enteredFrame and self._frame is None:
            _ , self._frame = self._capture.retrieve() #retrieve()是解码并返回一个帧,grab是指向下一个帧(不需要当前帧时可跳过),read是grab和retrieve的结合
        return self._frame

    @property
    def isWritingImage(self):
        return self._imageFilename is not None

    @property
    def isWritingVideo(self):
        return  self._videoFilename is not None

    def enterFrame(self): #Capture the next frame
        #check previous frame was existed.
        # 检查上一帧是否退出
        assert not self._enteredFrame , \
        'previous enterFrame() had no matching exitFrame()'

        if self._capture is not None :
            self._enteredFrame = self._capture.grab() #用grab()指向下一帧
    def exitFrame(self):
        #draw to the window ,write to files , release the frame.
        if self.frame is None:
            self._enteredFrame = False
            return

        if self._framesElapsed == 0 :
            self._startTime = time.time()
        else:
            timeElapsed = time.time() - self._startTime
            self._fpsEstimate = self._framesElapsed / timeElapsed
            self._framesElapsed += 1
        #draw to the window
        if 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 = None

        self._writeVideoFrame()
        self._frame = None
        self._enteredFrame = False

    def writeImage(self , filename):
        self._imageFilename = filename
    def startWritingVideo(self,filename,encoding=cv2.VideoWriter_fourcc('I','4','2','0')):
        self._videoFilename = filename
        self._videoEncoding = encoding
    def stopWritingVideo(self):
        self._videoFilename = None
        self._videoEncoding = None
        self._videoWriter = None
    def _writeVideoFrame(self): #非公有
        if not self.isWritingVideo:
            return
        if self._videoWriter is None:
            fps = self._capture.get(cv2.CAP_PROP_FPS)
            if fps == 0.0:
                if self._framesElapsed < 20 :
                    return
                else:
                    fps = self._fpsEstimate
            size = (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)



#抽象窗口和键盘
class WindowManager(object):
    def __init__(self,windowName,keypressCallback = None,mousepressCallback = None):
        self.keypressCallback = keypressCallback
        self.mousepressCallback = mousepressCallback #鼠标
        self._windowName = windowName
        self._isWindowCreated = False

    @property
    def isWindowCreated(self):
        return self._isWindowCreated

    def createWindow(self):
        cv2.namedWindow(self._windowName)
        self._isWindowCreated = True
        cv2.setMouseCallback(self._windowName,self.mousepressCallback) #鼠标事件

    def show(self , frame):
        cv2.imshow(self._windowName,frame)

    def destoryWindow(self):
        cv2.destroyWindow(self._windowName)
        self._isWindowCreated = False

    def processEvent(self):
        keycode = cv2.waitKey(1)
        if self.keypressCallback is not None and keycode != -1:
            keycode &= 0xFF
            self.keypressCallback(keycode)


#python内置的@property装饰器就是负责把一个方法变成属性调用的

cameo.py

import cv2
from managers import WindowManager , CaptureManager

class 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() #创建窗口

        while self._windowManager.isWindowCreated:
            self._captureManager.enterFrame()     #检查上一帧是否退出，并指向下一帧
            frame = self._captureManager.frame    #获取当前帧

            self._captureManager.exitFrame()      #draw to the window ,write to files , release the frame.
            self._windowManager.processEvent()    #处理键盘事件


    def onKeypress(self , keycode): #键盘响应
        """
        Handle a keypress.
            space  -> Take a screenshot
            tab    -> Start/Stop recording a screencast
            escape -> Quit
        """
        if keycode == 32: #space
            self._captureManager.writeImage('screenshot.png')
        elif keycode == 9:#tab
            if not self._captureManager.isWritingVideo:
                self._captureManager.startWritingVideo('screencast.avi')
            else:
                self._captureManager.stopWritingVideo()
        elif keycode == 27:#escape
            self._windowManager.destoryWindow()

    def onMouse(self , event, x, y, flags, param): #鼠标响应函数
        if event == cv2.EVENT_LBUTTONUP:
            self._windowManager.destoryWindow()
if __name__=="__main__":
    Cameo().run()