数字图像处理笔记（一）：利用OpenCV的Cameo框架搭建实验环境

1 - 引言

Python的应用程序可以通过面向对象的方法来实现，OpenCV提供了一个Cameo框架可以捕捉电脑的摄像头。我们可以通过编写添加框架里的类和方法来对摄像头捕捉到的画面进行图像处理和实验，是一个很好的学习方法，下面让我们来搭建一下这个Cameo框架

2 - 使用managers.CaptureManager提取视频流

无论图像来自视频文件还是摄像头，OpenCV都可以获取、显示和记录图像流，但是每种情况都有一些需要特殊考虑的地方。CaptureManager类对一些差异进行了抽象，并提供了更高级的接口从获取流中分配图像，再将图像分到一个或多个输出中（如图像文件、视频文件或窗口）

建立managers.py文件

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


# 增加要导入的包、构造函数和属性值
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._imageFilename = None
        self._videoFilename = None
        self._videoEncoding = None
        self._videoWriter = None

        self._startTime = None
        self._framesElapsed = int(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()
        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):
        assert not self._enteredFrame, 'previous enterFrame() had no matching exitFram()'
        if self._capture is not None:
            self._enteredFrame = self._capture.grab()

    def exitFrame(self):
        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
        if self.previewWindowManager is not None:
            if self.shouldMirrorPreview:
                mirroredFrame = numpy.fliplr(self._frame).copy()
                self.previewWindowManager.show(mirroredFrame)
            else:
                self.previewWindowManager.show(self._frame)
        if self.isWritingImage:
            cv2.imwrite(self._imageFilename, self._frame)
            self._imageFilename = None
        self._writerVideoFrame()
        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 _writerVideoFrame(self):
        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)

# 使用managers.WindowManager抽象窗口和键盘
class WindowManager(object):
    # 实现支持键盘事件
    def __init__(self, windowName, keypressCallback = None):
        self.keypressCallback = keypressCallback
        self._windowName = windowName
        self._isWindowCreated = False
    @property
    def isWindowCreated(self):
        return self._isWindowCreated
    def createWindow(self):
        cv2.namedWindow(self._windowName)
        self._isWindowCreated = True
    def show(self, frame):
        cv2.imshow(self._windowName, frame)
    def destoryWindow(self):
        cv2.destroyWindow(self._windowName)
        self._isWindowCreated = False
    def processEvents(self):
        keycode = cv2.waitKey(1)
        if self.keypressCallback is not None and keycode != -1:
            keycode &= 0xFF
            self.keypressCallback(keycode)

3 - 使用manager.WindowManager抽象窗口和键盘

对于一个应用程序，我们还需要能够实现人机交互，通过manager.WindowManager抽象窗口和键盘，我们可以通过键盘来实现人机交互
创建一个cameo.py文件

import cv2
from managers import WindowManager, CaptureManager
class Cameo(object):
    def __init__(self):
        self._windowManager = WindowManager('Cameo', self.onKeypress)
        self._captureManager = CaptureManager(cv2.VideoCapture(0), self._windowManager, True)
    def run(self):
        self._windowManager.createWindow()
        while self._windowManager.isWindowCreated:
            self._captureManager.enterFrame()
            frame = self._captureManager.enterFrame
            self._captureManager.exitFrame()
            self._windowManager.processEvents()
    def onKeypress(self,keycode):
        if keycode == 32:  # space
            self._captureManager.writeImage('cameo/screenshot.png')
        elif keycode == 9:  # tab
            if not self._captureManager.isWritingVideo:
                self._captureManager.startWritingVideo('cameo/screenshot.avi')
            else:
                self._captureManager.stopWritingVideo()
        elif keycode == 27:    # escape
            self._windowManager.destoryWindow()
if __name__ == "__main__":
    Cameo().run()

3 - 实现效果

至此，我们已经搭建好了一个Cameo最原始的框架，可以捕捉摄像头并且通过键盘实现截图和录制视频，在之后的学习中，我们会慢慢的扩充这个框架来学习图像处理的各种算法

按下空格键后保存摄像头捕捉到的图片

按下ESC退出