为什么将opencv.NewWindow（）移到子函数中时程序立即终止？

Question

I'm still trying to wrap my head around the Go language and I've just come across some rather confusing behavior. Here is a working version of my code:

Note, you will need to install OpenCV (package libopencv-dev in Ubuntu) and go-opencv (go get github.com/lazywei/go-opencv/opencv) if you want to execute these examples.

Working:

package main

import (
    "fmt"
    "github.com/lazywei/go-opencv/opencv"
)

func main() {
    win := opencv.NewWindow("Go-OpenCV Webcam")
    defer win.Destroy()

    frames := GetFrameGenerator()
    go DisplayFrames(win, frames)

    opencv.WaitKey(0)
}

func GetFrameGenerator() chan *opencv.IplImage {
    frames := make(chan *opencv.IplImage)

    cap := opencv.NewCameraCapture(0)
    if cap == nil {
        panic("cannot open camera")
    }

    go func() {
        defer cap.Release()
        for {
            if cap.GrabFrame() {
                img := cap.RetrieveFrame(1)
                if img != nil {
                    frames <- img
                } else {
                    fmt.Println("Image ins nil")
                }
            }
        }
    }()

    return frames
}

func DisplayFrames(win *opencv.Window, frames <-chan *opencv.IplImage) {
    for fr := range frames {
        win.ShowImage(fr)
    }
}

Not Working:

package main

import (
    "fmt"
    "github.com/lazywei/go-opencv/opencv"
)

func main() {

    frames := GetFrameGenerator()
    go DisplayFrames(frames)

    opencv.WaitKey(0)
}

func GetFrameGenerator() chan *opencv.IplImage {
    frames := make(chan *opencv.IplImage)

    cap := opencv.NewCameraCapture(0)
    if cap == nil {
        panic("cannot open camera")
    }

    go func() {
        defer cap.Release()
        for {
            if cap.GrabFrame() {
                img := cap.RetrieveFrame(1)
                if img != nil {
                    frames <- img
                } else {
                    fmt.Println("Image ins nil")
                }
            }
        }
    }()

    return frames
}

func DisplayFrames(frames <-chan *opencv.IplImage) {
    win := opencv.NewWindow("Go-OpenCV Webcam")
    defer win.Destroy()
    for fr := range frames {
        win.ShowImage(fr)
    }
}

Intuitively, it seems as though the program should run until it is forcibly stopped because

• the call to opencv.WaitKey(0) should just sit there and wait for some sort of keypress
• the anonymous goroutine in GetFrameGenerator should just keep chugging away, synchronizing with DisplayFrames on each frame.
• The OpenCV window instance is tied to the lifetime of DisplayFrames' call stack, which is in turn tied to frames generator being unclosed.

So what gives? What am I missing, here?

Answer 1

I don't have opencv installed so I could not test, but something like this should do the trick:

package main

import (
        "fmt"
        "github.com/lazywei/go-opencv/opencv"
)

func main() {

        frames := GetFrameGenerator()
        ready := make(chan struct{})
        defer close(ready)
        go DisplayFrames(frames, ready)
        <-ready
        opencv.WaitKey(0)
}

func GetFrameGenerator() chan *opencv.IplImage {
        frames := make(chan *opencv.IplImage)

        cap := opencv.NewCameraCapture(0)
        if cap == nil {
                panic("cannot open camera")
        }

        go func() {
                defer cap.Release()
                for {
                        if cap.GrabFrame() {
                                img := cap.RetrieveFrame(1)
                                if img != nil {
                                        frames <- img
                                } else {
                                        fmt.Println("Image ins nil")
                                }
                        }
                }
        }()

        return frames
}

func DisplayFrames(frames <-chan *opencv.IplImage, ready <-chan struct{}) {
        win := opencv.NewWindow("Go-OpenCV Webcam")
        defer win.Destroy()
        ready <- struct{}{}

        for fr := range frames {
                win.ShowImage(fr)
        }
}

opencv.WaitKey() will block only if opencv.NewWindow() has been called. As you are calling it in a goroutine, you need to add synchronization to make sure it has been done.