如何使用Python和PyObjC捕获Apple iSight的帧？

Question

我正在尝试用Python（版本2.7或2.6）和PyObjC（版本2.2）从Macbook Pro内置的Apple iSight摄像头捕捉一帧图像。

作为起点，我参考了这个旧的StackOverflow问题。为了确认这个方法是否合理，我还对照了苹果的MyRecorder示例，看起来这个问题是基于那个示例的。不幸的是，我的脚本没有成功。

我有几个主要问题：

• 我是不是正确地初始化了摄像头？
• 我是不是正确地启动了事件循环？
• 我是不是还需要做其他的设置？

在下面的示例脚本中，预期的操作是，在调用startImageCapture()之后，我应该开始从CaptureDelegate打印“得到了一个帧...”的消息。然而，摄像头的指示灯从未亮起，委托的回调也从未执行。

而且，在startImageCapture()过程中没有出现任何失败，所有函数都声称成功，并且成功找到了iSight设备。在pdb中分析会话对象显示它有有效的输入和输出对象，输出对象分配了委托，设备没有被其他进程使用，并且在调用startRunning()后会话被标记为正在运行。

以下是代码：

#!/usr/bin/env python2.7

import sys
import os
import time
import objc
import QTKit
import AppKit
from Foundation import NSObject
from Foundation import NSTimer
from PyObjCTools import AppHelper
objc.setVerbose(True)

class CaptureDelegate(NSObject):
    def captureOutput_didOutputVideoFrame_withSampleBuffer_fromConnection_(self, captureOutput, 
                                                                           videoFrame, sampleBuffer, 
                                                                           connection):
        # This should get called for every captured frame
        print "Got a frame: %s" % videoFrame

class QuitClass(NSObject):
    def quitMainLoop_(self, aTimer):
        # Just stop the main loop.
        print "Quitting main loop."
        AppHelper.stopEventLoop()


def startImageCapture():
    error = None

    # Create a QT Capture session
    session = QTKit.QTCaptureSession.alloc().init()

    # Find iSight device and open it
    dev = QTKit.QTCaptureDevice.defaultInputDeviceWithMediaType_(QTKit.QTMediaTypeVideo)
    print "Device: %s" % dev
    if not dev.open_(error):
        print "Couldn't open capture device."
        return

    # Create an input instance with the device we found and add to session
    input = QTKit.QTCaptureDeviceInput.alloc().initWithDevice_(dev)
    if not session.addInput_error_(input, error):
        print "Couldn't add input device."
        return

    # Create an output instance with a delegate for callbacks and add to session
    output = QTKit.QTCaptureDecompressedVideoOutput.alloc().init()
    delegate = CaptureDelegate.alloc().init()
    output.setDelegate_(delegate)
    if not session.addOutput_error_(output, error):
        print "Failed to add output delegate."
        return

    # Start the capture
    print "Initiating capture..."
    session.startRunning()


def main():
    # Open camera and start capturing frames
    startImageCapture()

    # Setup a timer to quit in 10 seconds (hack for now)
    quitInst = QuitClass.alloc().init()
    NSTimer.scheduledTimerWithTimeInterval_target_selector_userInfo_repeats_(10.0, 
                                                                             quitInst, 
                                                                             'quitMainLoop:', 
                                                                             None, 
                                                                             False)
    # Start Cocoa's main event loop
    AppHelper.runConsoleEventLoop(installInterrupt=True)

    print "After event loop"


if __name__ == "__main__":
    main()

感谢你提供的任何帮助！

Answer 1

QTKit已经不再被推荐使用，而PyObjC这个库依赖性很大（而且如果你想在HomeBrew上安装它，似乎会有点麻烦）。另外，PyObjC也没有包含大部分的AVFoundation功能，所以我创建了一个简单的Python摄像头扩展，它使用AVFoundation来录制视频或拍照，不需要其他依赖（为了大多数用户方便，我把Cython的中间文件也提交上去了，这样就不需要安装Cython了）。

你可以这样来构建它：

pip install -e git+https://github.com/dashesy/pyavfcam.git

然后我们可以用它来拍照：

import pyavfcam

# Open the default video source
cam = pyavfcam.AVFCam(sinks='image')
frame = cam.snap_picture('test.jpg')  # frame is a memory buffer np.asarray(frame) can retrieve

这与这个问题没有直接关系，但如果你对AVFCam类进行子类化，重写的方法会在调用时返回结果。

Answer 2

好的，我花了一整天深入研究PyObjC，终于让它工作了。

为了以后参考，问题中的代码之所以不管用，是因为变量的作用域和垃圾回收。session这个变量在超出作用域后就被删除了，而这发生在事件处理器运行之前。我们需要做点什么来保持它的存在，这样它才不会在有时间运行之前就被释放掉。

把所有东西放到一个类里面，并把session设为类变量后，回调函数就开始正常工作了。此外，下面的代码展示了如何将帧的像素数据转换成位图格式，并通过Cocoa的调用保存它，同时也演示了如何将它复制回Python的视角，作为一个缓冲区或字符串。

下面的脚本将捕获一帧画面。

#!/usr/bin/env python2.7
#
# camera.py -- by Trevor Bentley (02/04/2011)
# 
# This work is licensed under a Creative Commons Attribution 3.0 Unported License.
#
# Run from the command line on an Apple laptop running OS X 10.6, this script will
# take a single frame capture using the built-in iSight camera and save it to disk
# using three methods.
#

import sys
import os
import time
import objc
import QTKit
from AppKit import *
from Foundation import NSObject
from Foundation import NSTimer
from PyObjCTools import AppHelper

class NSImageTest(NSObject):
    def init(self):
        self = super(NSImageTest, self).init()
        if self is None:
            return None

        self.session = None
        self.running = True

        return self

    def captureOutput_didOutputVideoFrame_withSampleBuffer_fromConnection_(self, captureOutput, 
                                                                           videoFrame, sampleBuffer, 
                                                                           connection):
        self.session.stopRunning() # I just want one frame

        # Get a bitmap representation of the frame using CoreImage and Cocoa calls
        ciimage = CIImage.imageWithCVImageBuffer_(videoFrame)
        rep = NSCIImageRep.imageRepWithCIImage_(ciimage)
        bitrep = NSBitmapImageRep.alloc().initWithCIImage_(ciimage)
        bitdata = bitrep.representationUsingType_properties_(NSBMPFileType, objc.NULL)

        # Save image to disk using Cocoa
        t0 = time.time()
        bitdata.writeToFile_atomically_("grab.bmp", False)
        t1 = time.time()
        print "Cocoa saved in %.5f seconds" % (t1-t0)

        # Save a read-only buffer of image to disk using Python
        t0 = time.time()
        bitbuf = bitdata.bytes()
        f = open("python.bmp", "w")
        f.write(bitbuf)
        f.close()
        t1 = time.time()
        print "Python saved buffer in %.5f seconds" % (t1-t0)

        # Save a string-copy of the buffer to disk using Python
        t0 = time.time()
        bitbufstr = str(bitbuf)
        f = open("python2.bmp", "w")
        f.write(bitbufstr)
        f.close()
        t1 = time.time()
        print "Python saved string in %.5f seconds" % (t1-t0)

        # Will exit on next execution of quitMainLoop_()
        self.running = False

    def quitMainLoop_(self, aTimer):
        # Stop the main loop after one frame is captured.  Call rapidly from timer.
        if not self.running:
            AppHelper.stopEventLoop()

    def startImageCapture(self, aTimer):
        error = None
        print "Finding camera"

        # Create a QT Capture session
        self.session = QTKit.QTCaptureSession.alloc().init()

        # Find iSight device and open it
        dev = QTKit.QTCaptureDevice.defaultInputDeviceWithMediaType_(QTKit.QTMediaTypeVideo)
        print "Device: %s" % dev
        if not dev.open_(error):
            print "Couldn't open capture device."
            return

        # Create an input instance with the device we found and add to session
        input = QTKit.QTCaptureDeviceInput.alloc().initWithDevice_(dev)
        if not self.session.addInput_error_(input, error):
            print "Couldn't add input device."
            return

        # Create an output instance with a delegate for callbacks and add to session
        output = QTKit.QTCaptureDecompressedVideoOutput.alloc().init()
        output.setDelegate_(self)
        if not self.session.addOutput_error_(output, error):
            print "Failed to add output delegate."
            return

        # Start the capture
        print "Initiating capture..."
        self.session.startRunning()


    def main(self):
        # Callback that quits after a frame is captured
        NSTimer.scheduledTimerWithTimeInterval_target_selector_userInfo_repeats_(0.1, 
                                                                                 self, 
                                                                                 'quitMainLoop:', 
                                                                                 None, 
                                                                                 True)

        # Turn on the camera and start the capture
        self.startImageCapture(None)

        # Start Cocoa's main event loop
        AppHelper.runConsoleEventLoop(installInterrupt=True)

        print "Frame capture completed."

if __name__ == "__main__":
    test = NSImageTest.alloc().init()
    test.main()