Android4.2.2的preview的数据流和控制流以及最终的预览显示

 本文均属自己阅读源码的点滴总结,转账请注明出处谢谢。

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Android源码版本Version:4.2.2; 硬件平台 全志A31

step1:之前在讲到CameraService处的setPreviewWindow中传入一个窗口给HAL

    status_t setPreviewWindow(const sp<ANativeWindow>& buf)
    {
        ALOGV("%s(%s) buf %p", __FUNCTION__, mName.string(), buf.get());

        if (mDevice->ops->set_preview_window) {
            mPreviewWindow = buf;
            mHalPreviewWindow.user = this;
            ALOGV("%s &mHalPreviewWindow %p mHalPreviewWindow.user %p", __FUNCTION__,
                    &mHalPreviewWindow, mHalPreviewWindow.user);
            return mDevice->ops->set_preview_window(mDevice,
                    buf.get() ? &mHalPreviewWindow.nw : 0);//调用底层硬件hal接口
        }
        return INVALID_OPERATION;
    }

传入给HAL的参数buf为一个Surface,还有一个变量是关于预览窗口的数据流操作nw

    struct camera_preview_window {
        struct preview_stream_ops nw;
        void *user;
    };

该变量的初始如下:这些函数接口看上去很熟悉,的确在SurfaceFlinger中,客户端的Surface也就是通过这些接口来向SurfaceFlinger申请图形缓存并处理图形缓存显示的,只是之前的操作都交个了OpenGL ES的eglswapbuf()来对这个本地窗口进行如下的dequeueBuffer和enqueuebuffer的操作而已。而在Camera的预览中,这些操作将手动完成。

    void initHalPreviewWindow()
    {
        mHalPreviewWindow.nw.cancel_buffer = __cancel_buffer;
        mHalPreviewWindow.nw.lock_buffer = __lock_buffer;
        mHalPreviewWindow.nw.dequeue_buffer = __dequeue_buffer;
        mHalPreviewWindow.nw.enqueue_buffer = __enqueue_buffer;
        mHalPreviewWindow.nw.set_buffer_count = __set_buffer_count;
        mHalPreviewWindow.nw.set_buffers_geometry = __set_buffers_geometry;
        mHalPreviewWindow.nw.set_crop = __set_crop;
        mHalPreviewWindow.nw.set_timestamp = __set_timestamp;
        mHalPreviewWindow.nw.set_usage = __set_usage;
        mHalPreviewWindow.nw.set_swap_interval = __set_swap_interval;

        mHalPreviewWindow.nw.get_min_undequeued_buffer_count =
                __get_min_undequeued_buffer_count;
    }

step2.继续前面的preview的处理操作,在CameraService处的CameraClinet已经调用了CameraHardwareInterface的startPreview函数,实际就是操作HAL处的Camera设备如下

    status_t startPreview()
    {
        ALOGV("%s(%s)", __FUNCTION__, mName.string());
        if (mDevice->ops->start_preview)
            return mDevice->ops->start_preview(mDevice);
        return INVALID_OPERATION;
    }


step3.进入HAL来看Preview的处理

    
status_t CameraHardware::doStartPreview(){
...........
res = camera_dev->startDevice(mCaptureWidth, mCaptureHeight, org_fmt, video_hint);//启动设备
......
}

调用V4L2设备来启动视频流的采集,startDevice()函数更好的解释了预览的启动也就是视频采集的启动。

status_t V4L2CameraDevice::startDevice(int width,
                                       int height,
                                       uint32_t pix_fmt,
                                       bool video_hint)
{
	LOGD("%s, wxh: %dx%d, fmt: %d", __FUNCTION__, width, height, pix_fmt);
	
	Mutex::Autolock locker(&mObjectLock);
	
	if (!isConnected()) 
	{
		LOGE("%s: camera device is not connected.", __FUNCTION__);
		return EINVAL;
	}
	
	if (isStarted()) 
	{
		LOGE("%s: camera device is already started.", __FUNCTION__);
		return EINVAL;
	}

	// VE encoder need this format
	mVideoFormat = pix_fmt;
	mCurrentV4l2buf = NULL;

	mVideoHint = video_hint;
	mCanBeDisconnected = false;

	// set capture mode and fps
	// CHECK_NO_ERROR(v4l2setCaptureParams());	// do not check this error
	v4l2setCaptureParams();
	
	// set v4l2 device parameters, it maybe change the value of mFrameWidth and mFrameHeight.
	CHECK_NO_ERROR(v4l2SetVideoParams(width, height, pix_fmt));
	
	// v4l2 request buffers
	int buf_cnt = (mTakePictureState == TAKE_PICTURE_NORMAL) ? 1 : NB_BUFFER;
	CHECK_NO_ERROR(v4l2ReqBufs(&buf_cnt));//buf申请
	mBufferCnt = buf_cnt;

	// v4l2 query buffers
	CHECK_NO_ERROR(v4l2QueryBuf());//buffer的query,完成mmap等操作
	
	// stream on the v4l2 device
	CHECK_NO_ERROR(v4l2StartStreaming());//启动视频流采集

	mCameraDeviceState = STATE_STARTED;

	mContinuousPictureAfter = 1000000 / 10;
	mFaceDectectAfter = 1000000 / 15;
	mPreviewAfter = 1000000 / 24;
	
    return NO_ERROR;
}

这个是完全参考了V4L2的视频采集处理流程:

1.v4l2setCaptureParams()设置采集的相关参数;

2.v4l2QueryBuf():获取内核图像缓存的信息,并将所有的内核图像缓存映射到当前的进程中来。方便用户空间的处理

3.v4l2StartStreaming():开启V4L2的视频采集流程。

step4: 图像采集线程bool V4L2CameraDevice::captureThread();

该函数的内容比较复杂,但核心是 ret = getPreviewFrame(&buf),获取当前一帧图像:

int V4L2CameraDevice::getPreviewFrame(v4l2_buffer *buf)
{
	int ret = UNKNOWN_ERROR;
	
	buf->type   = V4L2_BUF_TYPE_VIDEO_CAPTURE; 
    buf->memory = V4L2_MEMORY_MMAP; 
 
    ret = ioctl(mCameraFd, VIDIOC_DQBUF, buf); //获取一帧数据
    if (ret < 0) 
	{ 
        LOGW("GetPreviewFrame: VIDIOC_DQBUF Failed, %s", strerror(errno)); 
        return __LINE__; 			// can not return false
    }

	return OK;
}

调用了典型的VIDIOC_DQBUF命令,出列一帧图形缓存,提取到用户空间供显示。


当前的平台通过定义一个V4L2BUF_t结构体来表示当前采集到的一帧图像,分别记录到Y和C所在的物理地址和用户空间的虚拟地址。虚拟地址是对内核采集缓存的映射

typedef struct V4L2BUF_t
{
	unsigned int	addrPhyY;		// physical Y address of this frame
	unsigned int	addrPhyC;		// physical Y address of this frame
	unsigned int	addrVirY;		// virtual Y address of this frame
	unsigned int	addrVirC;		// virtual Y address of this frame
	unsigned int	width;
	unsigned int	height;
	int 			index;			// DQUE id number
	long long		timeStamp;		// time stamp of this frame
	RECT_t			crop_rect;
	int				format;
	void*           overlay_info;
	
	// thumb 
	unsigned char	isThumbAvailable;
	unsigned char	thumbUsedForPreview;
	unsigned char	thumbUsedForPhoto;
	unsigned char	thumbUsedForVideo;
	unsigned int	thumbAddrPhyY;		// physical Y address of thumb buffer
	unsigned int	thumbAddrVirY;		// virtual Y address of thumb buffer
	unsigned int	thumbWidth;
	unsigned int	thumbHeight;
	RECT_t			thumb_crop_rect;
	int 			thumbFormat;
	
	int 			refCnt; 		// used for releasing this frame
	unsigned int	bytesused;      // used by compressed source
}V4L2BUF_t;

来看看该结构体的初始化代码:

	V4L2BUF_t v4l2_buf;
	if (mVideoFormat != V4L2_PIX_FMT_YUYV
		&& mCaptureFormat == V4L2_PIX_FMT_YUYV)
	{
		v4l2_buf.addrPhyY		= mVideoBuffer.buf_phy_addr[buf.index]; 
		v4l2_buf.addrVirY		= mVideoBuffer.buf_vir_addr[buf.index]; 
	}
	else
	{
		v4l2_buf.addrPhyY		= buf.m.offset & 0x0fffffff;//内核物理地址
		v4l2_buf.addrVirY		= (unsigned int)mMapMem.mem[buf.index];//虚拟地址
	}
	v4l2_buf.index				= buf.index;//内部采集缓存的索引
	v4l2_buf.timeStamp			= mCurFrameTimestamp;
	v4l2_buf.width				= mFrameWidth;
	v4l2_buf.height				= mFrameHeight;
	v4l2_buf.crop_rect.left		= mRectCrop.left;
	v4l2_buf.crop_rect.top		= mRectCrop.top;
	v4l2_buf.crop_rect.width	= mRectCrop.right - mRectCrop.left + 1;
	v4l2_buf.crop_rect.height	= mRectCrop.bottom - mRectCrop.top + 1;
	v4l2_buf.format				= mVideoFormat;

addrPhy和addrViry分别记录到Y和C所在的物理地址和用户空间的虚拟地址。而这个地址都是通过当前Buf的index直接设置的,为什么?因为内核的图像缓存区的mmap操作将每一个缓存,以其Index分别逐一的映射到了用户空间,并记录缓存的物理和虚拟地址,而这主要是方便后续图像的显示而已。

 

step5:bool V4L2CameraDevice::previewThread()//预览线程
获得了一帧数据必须通知预览线程进行图像的显示,采集线程和显示线程之间通过pthread_cond_wait(&mPreviewCond, &mPreviewMutex);进程间锁进行等待。

bool V4L2CameraDevice::previewThread()//预览线程
{
	V4L2BUF_t * pbuf = (V4L2BUF_t *)OSAL_Dequeue(&mQueueBufferPreview);//获取预览帧buffer信息
	if (pbuf == NULL)
	{
		// LOGV("picture queue no buffer, sleep...");
		pthread_mutex_lock(&mPreviewMutex);
		pthread_cond_wait(&mPreviewCond, &mPreviewMutex);//等待
		pthread_mutex_unlock(&mPreviewMutex);
		return true;
	}

	Mutex::Autolock locker(&mObjectLock);
	if (mMapMem.mem[pbuf->index] == NULL
		|| pbuf->addrPhyY == 0)
	{
		LOGV("preview buffer have been released...");
		return true;
	}

	// callback
	mCallbackNotifier->onNextFrameAvailable((void*)pbuf, mUseHwEncoder);//回调采集到帧数据

	// preview
	if (isPreviewTime())//预览
	{
		mPreviewWindow->onNextFrameAvailable((void*)pbuf);//帧可以显示
	}

	// LOGD("preview id : %d", pbuf->index);

	releasePreviewFrame(pbuf->index);

	return true;
}

预览线程主要做了两件事,一是完成图像缓存数据的回调供最最上层的使用;另一件当然是送显。

 

step6:预览线程如何显示?

bool PreviewWindow::onNextFrameAvailable(const void* frame)//使用本地窗口surface 进行初始化
{
    int res;
    Mutex::Autolock locker(&mObjectLock);

	V4L2BUF_t * pv4l2_buf = (V4L2BUF_t *)frame;//一帧图像所在的地址信息
......
        res = mPreviewWindow->set_buffers_geometry(mPreviewWindow,
                                                   mPreviewFrameWidth,
                                                   mPreviewFrameHeight,
					           format);//设置本地窗口的buffer的几何熟悉
......
    res = mPreviewWindow->dequeue_buffer(mPreviewWindow, &buffer, &stride);//申请SF进行bufferqueue的图形缓存操作。返回当前进程地址到buffer
..................
    res = grbuffer_mapper.lock(*buffer, GRALLOC_USAGE_SW_WRITE_OFTEN, rect, &img);//把映射回来的buffer信息中的地址放到img中,用来填充
.............
	mPreviewWindow->enqueue_buffer(mPreviewWindow, buffer);//交由surfaceFlinger去做显示
............
}

上述代码实时了本地窗口图像向SurfaceFlinger的投递,为何这么说,看下面的分析:

1.PreviewWindow类里的mPreviewWindow成员变量是什么?

这个是从应用端的setPreviewDisplay()设置过来的,传入到HAL的地方在CameraHardwareInterface的initialize函数里:

            return mDevice->ops->set_preview_window(mDevice,
                    buf.get() ? &mHalPreviewWindow.nw : 0);//调用底层硬件hal接口
        }

nw的操作在step1里面已经有说明了,初始化相关的一些操作。

2.以dequeue_buffer为例:

    static int __dequeue_buffer(struct preview_stream_ops* w,
                                buffer_handle_t** buffer, int *stride)
    {
        int rc;
        ANativeWindow *a = anw(w);
        ANativeWindowBuffer* anb;
        rc = native_window_dequeue_buffer_and_wait(a, &anb);
        if (!rc) {
            *buffer = &anb->handle;
            *stride = anb->stride;
        }
        return rc;
    }

调用到本地的窗口,通过w获得ANativeWindow对象,来看看该宏的实现:

    static ANativeWindow *__to_anw(void *user)
    {
        CameraHardwareInterface *__this =
                reinterpret_cast<CameraHardwareInterface *>(user);
        return __this->mPreviewWindow.get();
    }
#define anw(n) __to_anw(((struct camera_preview_window *)n)->user)

首先获取user对象为CameraHardwareInterface对象,通过它获得之前初始化的Surface对象即成员变量mPreviewWindow(属于本地窗口ANativeWindow类)。

 

3.本地窗口的操作

static inline int native_window_dequeue_buffer_and_wait(ANativeWindow *anw,
        struct ANativeWindowBuffer** anb) {
    return anw->dequeueBuffer_DEPRECATED(anw, anb);
}

上述的过程其实是调用应用层创建的Surface对象,该对象已经完全打包传递给了CameraService,来进行绘图和渲染的处理。如下所示:BpCamera

   // pass the buffered Surface to the camera service
    status_t setPreviewDisplay(const sp<Surface>& surface)
    {
        ALOGV("setPreviewDisplay");
        Parcel data, reply;
        data.writeInterfaceToken(ICamera::getInterfaceDescriptor());
        Surface::writeToParcel(surface, &data);//数据打包
        remote()->transact(SET_PREVIEW_DISPLAY, data, &reply);
        return reply.readInt32();
    }

BnCamera处,内部实现了新建一个CameraService处的Surface,但是都是用客户端处的参数来初始化的。即两者再不同进程中,但所包含的信息完全一样。

        case SET_PREVIEW_DISPLAY: {
            ALOGV("SET_PREVIEW_DISPLAY");
            CHECK_INTERFACE(ICamera, data, reply);
            sp<Surface> surface = Surface::readFromParcel(data);
            reply->writeInt32(setPreviewDisplay(surface));//设置sueface
            return NO_ERROR;
        } break;
Surface::Surface(const Parcel& parcel, const sp<IBinder>& ref)
    : SurfaceTextureClient()
{
    mSurface = interface_cast<ISurface>(ref);
    sp<IBinder> st_binder(parcel.readStrongBinder());
    sp<ISurfaceTexture> st;
    if (st_binder != NULL) {
        st = interface_cast<ISurfaceTexture>(st_binder);
    } else if (mSurface != NULL) {
        st = mSurface->getSurfaceTexture();
    }

    mIdentity   = parcel.readInt32();
    init(st);
}

这里的Surface建立是通过mSurface来完成和SurfaceFlinger的通信的,因为之前Camera客户端处的Surface是和SurfaceFLinger进行Binder通信,现在要将原先的Bpxxx相关的写入到CameraService进一步和SurfaceFlinger做后续的Binder通信处理,如queueBuffer()处理中和SurfaceFlinger的Bufferqueue的通信等。

4.故anw->dequeueBuffer的函数就和之前的从Android Bootanimation理解SurfaceFlinger的客户端建立完全对应起来,而且完全一样,只是Bootanimation进程创建的Surface交给OpenGL Es来进行底层的比如dequeue(缓存申请,填充当前的buffer)和enqueue(入列渲染)的绘图操作而已,见Android4.2.2 SurfaceFlinger之图形缓存区申请与分配dequeueBuffer一文。具体的绘图就不在这里说明了。通过该方法已经和SurfaceFlinger建立起连接,最终交由其进行显示。


 

 

 

 

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