<Android Framework 之路>Android5.1 Camera Framework（二）

上一次讲解了一下CameraService的启动过程，今天梳理一下Camera预览的过程

StartPreview过程

首先，我们还是从应用层的使用入手

Camera.java (packages\apps\legacycamera\src\com\android\camera)

Thread mCameraPreviewThread = new Thread(new Runnable() {
public void run() {
initializeCapabilities(); //初始化参数
startPreview(); //启动预览
}
});

针对相机应用，采用了单独的线程来处理预览，猜测是为了加快预览显示的速度

private void startPreview() {
......
// If we're previewing already, stop the preview first (this will blank
// the screen).
if (mCameraState != PREVIEW_STOPPED) stopPreview();

setPreviewDisplay(mSurfaceHolder); //设置SurfaceHolder
setDisplayOrientation();       //设置显示方向
......
setCameraParameters(UPDATE_PARAM_ALL); //设置参数

// Inform the mainthread to go on the UI initialization.
if (mCameraPreviewThread != null) {
synchronized (mCameraPreviewThread) {
mCameraPreviewThread.notify();
}
}

try {
Log.v(TAG, "startPreview");
mCameraDevice.startPreview(); //启动预览，若失败，关闭Camera
} catch (Throwable ex) {
closeCamera();
throw new RuntimeException("startPreview failed", ex);
}
......
}

这里就是APP中启动预览的过程，过程必然会是

java -> JNI -> cpp，

然后通过Binder机制执行到CameraClient中的

CameraClient.cpp (av\services\camera\libcameraservice\api1)

status_t CameraClient::startPreview() {
LOG1("startPreview (pid %d)", getCallingPid());
return startCameraMode(CAMERA_PREVIEW_MODE);
}

这里传入的是CAMERA_PREVIEW_MODE，枚举类型是在CameraClient.h中定义的

// camera operation mode

enum camera_mode {

CAMERA_PREVIEW_MODE = 0, // frame automatically released

CAMERA_RECORDING_MODE = 1, // frame has to be explicitly released by releaseRecordingFrame()

};

第一种是针对普通的预览，第二种是针对录像

// start preview or recording
status_t CameraClient::startCameraMode(camera_mode mode) {
LOG1("startCameraMode(%d)", mode);
Mutex::Autolock lock(mLock);
status_t result = checkPidAndHardware();
if (result != NO_ERROR) return result;

switch(mode) {
case CAMERA_PREVIEW_MODE:
if (mSurface == 0 && mPreviewWindow == 0) {
LOG1("mSurface is not set yet.");
// still able to start preview in this case.
}
return startPreviewMode(); //开始预览模式
case CAMERA_RECORDING_MODE:
if (mSurface == 0 && mPreviewWindow == 0) {
ALOGE("mSurface or mPreviewWindow must be set before startRecordingMode.");
return INVALID_OPERATION;
}
return startRecordingMode(); //开始录像模式
default:
return UNKNOWN_ERROR;
}
}

这里我们走的是预览模式

status_t CameraClient::startPreviewMode() {
LOG1("startPreviewMode"); //LOG1,一直忘记说了，这是有log开关用过setprop可以使用
status_t result = NO_ERROR;

// if preview has been enabled, nothing needs to be done
if (mHardware->previewEnabled()) { //如果已经启动预览，不必重复
return NO_ERROR;
}

if (mPreviewWindow != 0) {
//适配显示窗口的大小
native_window_set_scaling_mode(mPreviewWindow.get(),
NATIVE_WINDOW_SCALING_MODE_SCALE_TO_WINDOW);
//调整帧数据的方向
native_window_set_buffers_transform(mPreviewWindow.get(),
mOrientation);
}
mHardware->setPreviewWindow(mPreviewWindow); //设置mPreviewWindow为显示窗口
result = mHardware->startPreview();   //HAL层启动预览

return result; //返回结果
}

这里面涉及到native_window_set_scaling_mode，native_window_set_buffers_transform，直接跟代码，看注释就可以理解，这部分涉及到显示的一些内容，这里暂时不做讲解，native_window_set_scaling_mode设置模式为NATIVE_WINDOW_SCALING_MODE_SCALE_TO_WINDOW，native_window_set_buffers_transform是用来调整方向。

这里有一个问题，mPreviewWindow是从何而来的呢？

还记得我们在应用层的startPreview()方法中会有这么一个过程

setPreviewDisplay(mSurfaceHolder);
setDisplayOrientation();

这里的setPreviewDisplay(mSurfaceHolder)，中间的过程大家可以自己跟一下，最后会执行到

status_t CameraClient::setPreviewTarget(
const sp<IGraphicBufferProducer>& bufferProducer) {
......
sp<IBinder> binder;
sp<ANativeWindow> window;
if (bufferProducer != 0) {
binder = bufferProducer->asBinder();
// Using controlledByApp flag to ensure that the buffer queue remains in
// async mode for the old camera API, where many applications depend
// on that behavior.
window = new Surface(bufferProducer, /*controlledByApp*/ true); //这个家伙
}
return setPreviewWindow(binder, window);
}

status_t CameraClient::setPreviewWindow(const sp<IBinder>& binder,
const sp<ANativeWindow>& window) {
Mutex::Autolock lock(mLock);
......
if (window != 0) {
result = native_window_api_connect(window.get(), NATIVE_WINDOW_API_CAMERA);
if (result != NO_ERROR) {
ALOGE("native_window_api_connect failed: %s (%d)", strerror(-result),
result);
return result;
}
}

// If preview has been already started, register preview buffers now.
if (mHardware->previewEnabled()) {
if (window != 0) {
native_window_set_scaling_mode(window.get(),
NATIVE_WINDOW_SCALING_MODE_SCALE_TO_WINDOW);
native_window_set_buffers_transform(window.get(), mOrientation);
result = mHardware->setPreviewWindow(window);
}
}
if (result == NO_ERROR) {
// Everything has succeeded.  Disconnect the old window and remember the
// new window.
disconnectWindow(mPreviewWindow);
mSurface = binder;
mPreviewWindow = window; //这里便是赋值的操作了，后面我们操作的mPreviewWindow
} else {
// Something went wrong after we connected to the new window, so
// disconnect here.
disconnectWindow(window);
}
return result;
}

是不是看的很眼熟，和startPreviewMode()的过程有点相似。这就是mPreviewWindow的赋值过程。

回到setPreviewMode()函数中，其中主要的过程是这两个

mHardware->setPreviewWindow(mPreviewWindow);

result = mHardware->startPreview();

这里都是HAL层的处理，将窗口传下去，然后启动预览，最后数据就可以投射到这个预览窗口上了。

我们继续往下看一下，

CameraHardwareInterface.h (av\services\camera\libcameraservice\device1)

/** Set the ANativeWindow to which preview frames are sent */
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);
}
return INVALID_OPERATION;
}

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

这是一个空壳，我们去看具体的实现，这里我们看下android5.1源码中Qcom的实现，由于针对HAL层的不同厂商有不同的处理方式，在这里我们就随便找个目录下的进行分析，旨在看流程，理解一些基础的内容，

QCamera2Hal.cpp （\device\asus\flo\camera\qcamera2\hal）

#include "QCamera2Factory.h"

static hw_module_t camera_common = {
tag: HARDWARE_MODULE_TAG,
module_api_version: CAMERA_MODULE_API_VERSION_1_0,
hal_api_version: HARDWARE_HAL_API_VERSION,
id: CAMERA_HARDWARE_MODULE_ID,
name: "QCamera Module",
author: "Qualcomm Innovation Center Inc",
methods: &qcamera::QCamera2Factory::mModuleMethods,
dso: NULL,
reserved:  {0},
};

camera_module_t HAL_MODULE_INFO_SYM = {
common: camera_common,
get_number_of_cameras: qcamera::QCamera2Factory::get_number_of_cameras,
get_camera_info: qcamera::QCamera2Factory::get_camera_info,
set_callbacks: NULL,
get_vendor_tag_ops: NULL,
open_legacy: NULL,
reserved: {0}
};

这里提一下HAL_MODULE_INFO_SYM这个东西，本身就是一个定义在hardware.h下的一个宏，看注释，意思很明显

define HAL_MODULE_INFO_SYM HMI
//.so中将一个符号HMI,获取此符号的地址，就获取到了对应的hw_module_t地址

HAL_MODULE_INFO_SYM，这个是HAL 编译生成的so的入口，CameraService会获取这个来操作so

camera_common是针对HAL规范定义的一些内容。

Camera的open指向的是&qcamera::QCamera2Factory::mModuleMethods中的open方法，如下

struct hw_module_methods_t QCamera2Factory::mModuleMethods = {
open: QCamera2Factory::camera_device_open,
};

这个方法中打开设备节点，我们可以看到HAL层中open的过程是很有讲究的，也不能这么说，应为HAL的处理方式基本上都是如此。

int QCamera2Factory::camera_device_open(
const struct hw_module_t *module, const char *id,
struct hw_device_t **hw_device)
{
if (module != &HAL_MODULE_INFO_SYM.common) {
ALOGE("Invalid module. Trying to open %p, expect %p",
module, &HAL_MODULE_INFO_SYM.common);
return INVALID_OPERATION;
}
if (!id) {
ALOGE("Invalid camera id");
return BAD_VALUE;
}
return gQCamera2Factory.cameraDeviceOpen(atoi(id), hw_device);
}

int QCamera2Factory::cameraDeviceOpen(int camera_id,
struct hw_device_t **hw_device)
{
int rc = NO_ERROR;
if (camera_id < 0 || camera_id >= mNumOfCameras)
return BAD_VALUE;
//到这里才是真正的HAL层的创建，可见HAL层的创建和open操作是相关的
QCamera2HardwareInterface *hw = new QCamera2HardwareInterface(camera_id);
if (!hw) {
ALOGE("Allocation of hardware interface failed");
return NO_MEMORY;
}
rc = hw->openCamera(hw_device);
if (rc != NO_ERROR) {
delete hw;
}
return rc;
}

之前在CameraService过程中提到的CameraHardwareInterface空壳就是为这个QCamera2HardwareInterface准备的，具体实现全部都在这个类中。

关于QCamera2HardwareInterface的内容我们在后面会讲到，这里暂且先放一下，接着上面的

mHardware->setPreviewWindow(mPreviewWindow);

result = mHardware->startPreview();

经过CameraHardwareInterface后

mDevice->ops->set_preview_window(mDevice, buf.get() ? &mHalPreviewWindow.nw : 0);

mDevice->ops->start_preview(mDevice);

然后经过QCamera2HardwareInterface中的mCameraOps函数指针对应表

set_preview_window: QCamera2HardwareInterface::set_preview_window

start_preview: QCamera2HardwareInterface::start_preview

所以会调用到

int QCamera2HardwareInterface::set_preview_window(struct camera_device *device,
struct preview_stream_ops *window)
{
int rc = NO_ERROR;
QCamera2HardwareInterface *hw =
reinterpret_cast<QCamera2HardwareInterface *>(device->priv);
if (!hw) {
ALOGE("%s: NULL camera device", __func__);
return BAD_VALUE;
}

hw->lockAPI();
rc = hw->processAPI(QCAMERA_SM_EVT_SET_PREVIEW_WINDOW, (void *)window);
if (rc == NO_ERROR) {
hw->waitAPIResult(QCAMERA_SM_EVT_SET_PREVIEW_WINDOW);
rc = hw->m_apiResult.status;
}
hw->unlockAPI();

return rc;
}

这里会经过一轮状态机，暂时先不讲，然后执行到

int QCamera2HardwareInterface::setPreviewWindow(

struct preview_stream_ops *window)

{

mPreviewWindow = window;

return NO_ERROR;

}

同理，startPreview也会执行到

int QCamera2HardwareInterface::startPreview()

{

int32_t rc = NO_ERROR;

ALOGD(“%s: E”, func);

// start preview stream

if (mParameters.isZSLMode() && mParameters.getRecordingHintValue() !=true) {

rc = startChannel(QCAMERA_CH_TYPE_ZSL);

} else {

rc = startChannel(QCAMERA_CH_TYPE_PREVIEW);

}

ALOGD(“%s: X”, func);

return rc;

}

这里开启通道，可以理解成数据通道，ZSL是之前没有的，所谓ZSL就是触发拍照后不停止预览。

这里看到会根据当前是都支持ZSL模式而进入不同的通道，我们这里就看QCAMERA_CH_TYPE_PREVIEW，startChannel

int32_t QCamera2HardwareInterface::startChannel(qcamera_ch_type_enum_t ch_type)
{
int32_t rc = UNKNOWN_ERROR;
if (m_channels[ch_type] != NULL) {
rc = m_channels[ch_type]->start();
}

return rc;
}

m_channels是不同的通道的实例的数组，这里如果没有PREVIEW的channel就直接return，岂不是无法启动预览，这个流程感觉有点不对劲。但是这整个过程跟下来也没有看到m_channels相关的初始化过程。

这个问题出在我刚才从CameraHardwareInterface跟到QCamera2HardwareInterface的时候跳过的一个内容—–状态机，在状态机中会执行一个preparePreview()的操作

int32_t QCamera2HardwareInterface::preparePreview()
{
int32_t rc = NO_ERROR;

if (mParameters.isZSLMode() && mParameters.getRecordingHintValue() !=true) {
rc = addChannel(QCAMERA_CH_TYPE_ZSL); //这里我们就添加了一个channel,当然这里是ZSL的
if (rc != NO_ERROR) {
return rc;
}
} else {
bool recordingHint = mParameters.getRecordingHintValue(); //recording
if(recordingHint) {
rc = addChannel(QCAMERA_CH_TYPE_SNAPSHOT);  //录像中是可以拍照的，需要snapshot channel
if (rc != NO_ERROR) {
return rc;
}

rc = addChannel(QCAMERA_CH_TYPE_VIDEO); //video channel
if (rc != NO_ERROR) {
delChannel(QCAMERA_CH_TYPE_SNAPSHOT);
return rc;
}
}

rc = addChannel(QCAMERA_CH_TYPE_PREVIEW); //添加preview  channel
if (rc != NO_ERROR) {
if (recordingHint) {
delChannel(QCAMERA_CH_TYPE_SNAPSHOT);
delChannel(QCAMERA_CH_TYPE_VIDEO);
}
return rc;
}

}

return rc;
}

在addchannel()的过程中会根据不同的channel类型创建不同的实例，这里我们直接看从addChannel()转到的addPreviewChannel()函数

int32_t QCamera2HardwareInterface::addPreviewChannel()
{
int32_t rc = NO_ERROR;
QCameraChannel *pChannel = NULL; //初始化一个QCameraChanel，后面要使用
if (m_channels[QCAMERA_CH_TYPE_PREVIEW] != NULL) {
// if we had preview channel before, delete it first
delete m_channels[QCAMERA_CH_TYPE_PREVIEW]; //如果之前preview channel存在，干掉
m_channels[QCAMERA_CH_TYPE_PREVIEW] = NULL;
}
pChannel = new QCameraChannel(mCameraHandle->camera_handle,
mCameraHandle->ops);
//new 一个新的channel
.....
// meta data stream always coexists with preview if applicable
rc = addStreamToChannel(pChannel, CAM_STREAM_TYPE_METADATA,
metadata_stream_cb_routine, this);
//添加metadata stream cb
if (rc != NO_ERROR) {
ALOGE("%s: add metadata stream failed, ret = %d", __func__, rc);
delete pChannel;
return rc;
}

if (isNoDisplayMode()) { //判断是否为不需要显示的模式
rc = addStreamToChannel(pChannel, CAM_STREAM_TYPE_PREVIEW,
nodisplay_preview_stream_cb_routine, this);
} else {
//这里添加preview stream cb到channel中
rc = addStreamToChannel(pChannel, CAM_STREAM_TYPE_PREVIEW,
preview_stream_cb_routine, this);
}
if (rc != NO_ERROR) {
ALOGE("%s: add preview stream failed, ret = %d", __func__, rc);
delete pChannel;
return rc;
}

m_channels[QCAMERA_CH_TYPE_PREVIEW] = pChannel; //维护m_channels数据
return rc;
}

这里注册的preview_stream_cb_routine回调，这之后的过程我们暂时先不去看，了解这部分之后，回到之前chanel 的start()，最后会执行到QCameraChannel::start()方法，这里往下的内容我们暂时不往下看，知道这个过程中会执行数据采集，然后返回给HAL层就行了，HAL针对底层返回的数据，我们在哪里获取，做什么对应的处理呢？找到之前注册的Callback.

QCamera2HWICallbacks.cpp (\device\asus\flo\camera\qcamera2\hal)

void QCamera2HardwareInterface::preview_stream_cb_routine(mm_camera_super_buf_t *super_frame,
QCameraStream * stream,
void *userdata)
{
ALOGD("[KPI Perf] %s : BEGIN", __func__);
int err = NO_ERROR;
QCamera2HardwareInterface *pme = (QCamera2HardwareInterface *)userdata;
QCameraGrallocMemory *memory = (QCameraGrallocMemory *)super_frame->bufs[0]->mem_info;
......
mm_camera_buf_def_t *frame = super_frame->bufs[0];
......
if (!pme->needProcessPreviewFrame()) {
ALOGE("%s: preview is not running, no need to process", __func__);
stream->bufDone(frame->buf_idx);
free(super_frame);
return;
}
if (pme->needDebugFps()) {
pme->debugShowPreviewFPS();
}
int idx = frame->buf_idx;
pme->dumpFrameToFile(frame->buffer, frame->frame_len,
frame->frame_idx, QCAMERA_DUMP_FRM_PREVIEW);
//这里的注释很明显，displayer buffer而这个buffer就是我们需要投射到屏幕上的数据
// Display the buffer.
int dequeuedIdx = memory->displayBuffer(idx); //这部分涉及到显示的过程，这里不做赘述
if (dequeuedIdx < 0 || dequeuedIdx >= memory->getCnt()) {
ALOGD("%s: Invalid dequeued buffer index %d from display",
__func__, dequeuedIdx);
} else {
// Return dequeued buffer back to driver
err = stream->bufDone(dequeuedIdx);
if ( err < 0) {
ALOGE("stream bufDone failed %d", err);
}
}
//针对上层设置的datacallback过程做些处理
// Handle preview data callback
if (pme->mDataCb != NULL && pme->msgTypeEnabledWithLock(CAMERA_MSG_PREVIEW_FRAME) > 0) {
......
qcamera_callback_argm_t cbArg;
memset(&cbArg, 0, sizeof(qcamera_callback_argm_t));
cbArg.cb_type = QCAMERA_DATA_CALLBACK;
cbArg.msg_type = CAMERA_MSG_PREVIEW_FRAME;
cbArg.data = data;
if ( previewMem ) {
cbArg.user_data = previewMem;
cbArg.release_cb = releaseCameraMemory;
}
cbArg.cookie = pme;
pme->m_cbNotifier.notifyCallback(cbArg); //封装完之后往上甩
}

free(super_frame);
ALOGD("[KPI Perf] %s : END", __func__);
return;
}

这就是在addPreviewChannel的过程中添加的preview stream callback，当然还有metadata的，暂时先看preview的这个。这里面作的操作就是显示预览数据到窗口中，然后对设置下面的preview callback做对应的callback处理.

讲到这里，Camera的预览过程基本上就结束了，关于底层如果采集数据以及HAL中一些其他的内容，在这里没有讲解，主要是要理解这个过程，之后再每一个过程中在往下学习。

本文中代码使用的是Android5.1原始代码，欢迎大家留言交流。

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