玩转Android Camera开发(五):基于Google自带算法实时检测人脸并绘制人脸框(网络首发，附完整demo)
2016-07-28 10:43
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http://blog.csdn.net/yanzi1225627/article/details/38098729/
本文主要介绍使用Google自带的FaceDetectionListener进行人脸检测，并将检测到的人脸用矩形框绘制出来。本文代码基于PlayCameraV1.0.0，在Camera的open和preview流程上进行了改动。原先是放在单独线程里，这次我又把它放到Surfaceview的生命周期里进行打开和开预览。
首先要反省下，去年就推出了静态图片的人脸检测demo，当时许诺一周内推出Camera预览实时检测并绘制的demo，结果拖到现在才整。哎，屌丝一天又一天，蹉跎啊。在demo制作过程中还是遇到了一些麻烦的，第一个问题是检测到人脸rect默认是以预览界面为坐标系，这个坐标系是经过变换的，中心点为(0,
0),左上顶点坐标是(-1000, -1000)，右下顶点是(1000, 1000).也就是说不管预览预览Surfaceview多大，检测出来的rect的坐标始终对应的是在这个变换坐标系。而Android里默认的view的坐标系是，左上顶点为(0,
0)，横为x轴，竖为y轴。这就需要把rect坐标变换下。另一个难点是，这个人脸检测必须在camera开启后进行start，如果一旦拍照或停预览，则需要再次激活。激活时需要加个延迟，否则的话就不起作用了。
另外，仍要交代下，在预览界面实时检测人脸并绘制(基于Google自带算法)，还是有两个思路的。一是在PreviewCallback里的onPreviewFrame里得到yuv数据后，转成rgb后再转成Bitmap，然后利用静态图片的人脸检测流程，即利用FaceDetector类进行检测。另一个思路是，直接实现FaceDetectionListener接口，这样在onFaceDetection()里就得到检测到的人脸Face[]
faces数据了。这里只需控制何时start，何时stop即可，这都是android标准接口。毫无疑问，这种方法是上选。从Android4.0后android源码里的camera app都是用的这个接口进行人脸检测。下面上源码:
考虑到下次准备介绍JNI里用opencv检测人脸，为此杂家新建了一个包org.yanzi.mode里面准备放所有的关于图像的东西。新建文件GoogleFaceDetect.java实现FaceDetectionListener，在构造函数里传进来一个Handler，将检测到的人脸数据发给Activity，经Activity中转再刷新UI.
[java] view
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<span style="font-family:Comic Sans MS;font-size:18px;">package org.yanzi.mode;
import org.yanzi.util.EventUtil;
import android.content.Context;
import android.hardware.Camera;
import android.hardware.Camera.Face;
import android.hardware.Camera.FaceDetectionListener;
import android.os.Handler;
import android.os.Message;
import android.util.Log;
public class GoogleFaceDetect implements FaceDetectionListener {
private static final String TAG = "YanZi";
private Context mContext;
private Handler mHander;
public GoogleFaceDetect(Context c, Handler handler){
mContext = c;
mHander = handler;
}
@Override
public void onFaceDetection(Face[] faces, Camera camera) {
// TODO Auto-generated method stub
Log.i(TAG, "onFaceDetection...");
if(faces != null){
Message m = mHander.obtainMessage();
m.what = EventUtil.UPDATE_FACE_RECT;
m.obj = faces;
m.sendToTarget();
}
}
/* private Rect getPropUIFaceRect(Rect r){
Log.i(TAG, "人脸检测 = " + r.flattenToString());
Matrix m = new Matrix();
boolean mirror = false;
m.setScale(mirror ? -1 : 1, 1);
Point p = DisplayUtil.getScreenMetrics(mContext);
int uiWidth = p.x;
int uiHeight = p.y;
m.postScale(uiWidth/2000f, uiHeight/2000f);
int leftNew = (r.left + 1000)*uiWidth/2000;
int topNew = (r.top + 1000)*uiHeight/2000;
int rightNew = (r.right + 1000)*uiWidth/2000;
int bottomNew = (r.bottom + 1000)*uiHeight/2000;
return new Rect(leftNew, topNew, rightNew, bottomNew);
}*/
}
</span>
上面代码注释掉的一部分是我最初想自己写矩阵变换算法的过程，一番努力感觉变换后坐标还是有问题，后来参考Android4.0里的Camera APP源码才解决.这个变换转移到了FaceView里。
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<span style="font-family:Comic Sans MS;font-size:18px;">package org.yanzi.ui;
import org.yanzi.camera.CameraInterface;
import org.yanzi.playcamera.R;
import org.yanzi.util.Util;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Matrix;
import android.graphics.Paint;
import android.graphics.Paint.Style;
import android.graphics.RectF;
import android.graphics.drawable.Drawable;
import android.hardware.Camera.CameraInfo;
import android.hardware.Camera.Face;
import android.util.AttributeSet;
import android.widget.ImageView;
public class FaceView extends ImageView {
private static final String TAG = "YanZi";
private Context mContext;
private Paint mLinePaint;
private Face[] mFaces;
private Matrix mMatrix = new Matrix();
private RectF mRect = new RectF();
private Drawable mFaceIndicator = null;
public FaceView(Context context, AttributeSet attrs) {
super(context, attrs);
// TODO Auto-generated constructor stub
initPaint();
mContext = context;
mFaceIndicator = getResources().getDrawable(R.drawable.ic_face_find_2);
}
public void setFaces(Face[] faces){
this.mFaces = faces;
invalidate();
}
public void clearFaces(){
mFaces = null;
invalidate();
}
@Override
protected void onDraw(Canvas canvas) {
// TODO Auto-generated method stub
if(mFaces == null || mFaces.length < 1){
return;
}
boolean isMirror = false;
int Id = CameraInterface.getInstance().getCameraId();
if(Id == CameraInfo.CAMERA_FACING_BACK){
isMirror = false; //后置Camera无需mirror
}else if(Id == CameraInfo.CAMERA_FACING_FRONT){
isMirror = true; //前置Camera需要mirror
}
Util.prepareMatrix(mMatrix, isMirror, 90, getWidth(), getHeight());
canvas.save();
mMatrix.postRotate(0); //Matrix.postRotate默认是顺时针
canvas.rotate(-0); //Canvas.rotate()默认是逆时针
for(int i = 0; i< mFaces.length; i++){
mRect.set(mFaces[i].rect);
mMatrix.mapRect(mRect);
mFaceIndicator.setBounds(Math.round(mRect.left), Math.round(mRect.top),
Math.round(mRect.right), Math.round(mRect.bottom));
mFaceIndicator.draw(canvas);
// canvas.drawRect(mRect, mLinePaint);
}
canvas.restore();
super.onDraw(canvas);
}
private void initPaint(){
mLinePaint = new Paint(Paint.ANTI_ALIAS_FLAG);
// int color = Color.rgb(0, 150, 255);
int color = Color.rgb(98, 212, 68);
// mLinePaint.setColor(Color.RED);
mLinePaint.setColor(color);
mLinePaint.setStyle(Style.STROKE);
mLinePaint.setStrokeWidth(5f);
mLinePaint.setAlpha(180);
}
}
</span>
注意事项有两个：
1.就是Rect变换问题，通过Util.prepareMatrix(mMatrix, isMirror, 90, getWidth(), getHeight());进行变换，为了解决人脸检测坐标系和实际绘制坐标系不一致问题。第三个参数90，是因为前手摄像头都设置了mCamera.setDisplayOrientation(90);
接下来的Matrix和canvas两个旋转我传的都是0，所以此demo只能在手机0、90、180、270四个标准角度下得到的人脸坐标是正确的。其他情况下，需要将OrientationEventListener得到的角度传过来。为了简单，我这块就么写，OrientationEventListener的用法参见我的前文，后续将再推出一个demo。
最终是通过mMatrix.mapRect(mRect);来将mRect变换成UI坐标系的人脸Rect.
Util.prepareMatrix()代码如下：
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<span style="font-family:Comic Sans MS;font-size:18px;">package org.yanzi.util;
import android.graphics.Matrix;
public class Util {
public static void prepareMatrix(Matrix matrix, boolean mirror, int displayOrientation,
int viewWidth, int viewHeight) {
// Need mirror for front camera.
matrix.setScale(mirror ? -1 : 1, 1);
// This is the value for android.hardware.Camera.setDisplayOrientation.
matrix.postRotate(displayOrientation);
// Camera driver coordinates range from (-1000, -1000) to (1000, 1000).
// UI coordinates range from (0, 0) to (width, height).
matrix.postScale(viewWidth / 2000f, viewHeight / 2000f);
matrix.postTranslate(viewWidth / 2f, viewHeight / 2f);
}
}
</span>
2.得到实际UI里的人脸rect怎么画的问题。之前都是通过paint直接画，但实际上也可以通过Drawable.draw(canvas)来画。后者的好处是将一个图片画上去，而通过paint绘制基础图行如Rect、Circle比较方面。代码里把两种方法的代码都写了，供大家参考。
本次将这两个流程放到了Surfaceview的两个生命周期里，因为之前放在单独Thread还是会有一些问题。如个别手机上，Surfaceview创建的很慢，这时的SurfaceHolder还没准备好，结果Camera已经走到开预览了，导致黑屏问题。
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<span style="font-family:Comic Sans MS;font-size:18px;">@Override
public void surfaceCreated(SurfaceHolder holder) {
// TODO Auto-generated method stub
Log.i(TAG, "surfaceCreated...");
CameraInterface.getInstance().doOpenCamera(null, CameraInfo.CAMERA_FACING_BACK);
}
@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width,
int height) {
// TODO Auto-generated method stub
Log.i(TAG, "surfaceChanged...");
CameraInterface.getInstance().doStartPreview(mSurfaceHolder, 1.333f);
}</span>
若要开启人脸检测，必须要在Camera已经startPreview完毕之后。本文暂时采用在onCreate里延迟1.5s开启人脸检测，1.5s基本上camera已经开预览了。后续准备将Handler传到Surfaceview里，在开预览后通过Handler通知Activity已经开启预览了。
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自定义的MainHandler:
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<span style="font-family:Comic Sans MS;font-size:18px;"> private class MainHandler extends Handler{
@Override
public void handleMessage(Message msg) {
// TODO Auto-generated method stub
switch (msg.what){
case EventUtil.UPDATE_FACE_RECT:
Face[] faces = (Face[]) msg.obj;
faceView.setFaces(faces);
break;
case EventUtil.CAMERA_HAS_STARTED_PREVIEW:
startGoogleFaceDetect();
break;
}
super.handleMessage(msg);
}
}</span>
在onCreate里：
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<span style="font-family:Comic Sans MS;font-size:18px;"> protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_camera);
initUI();
initViewParams();
mMainHandler = new MainHandler();
googleFaceDetect = new GoogleFaceDetect(getApplicationContext(), mMainHandler);
shutterBtn.setOnClickListener(new BtnListeners());
switchBtn.setOnClickListener(new BtnListeners());
mMainHandler.sendEmptyMessageDelayed(EventUtil.CAMERA_HAS_STARTED_PREVIEW, 1500);
}</span>
这里写了两个重要的方法分别是开始检测和停止检测:
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<span style="font-family:Comic Sans MS;font-size:18px;">private void startGoogleFaceDetect(){
Camera.Parameters params = CameraInterface.getInstance().getCameraParams();
if(params.getMaxNumDetectedFaces() > 0){
if(faceView != null){
faceView.clearFaces();
faceView.setVisibility(View.VISIBLE);
}
CameraInterface.getInstance().getCameraDevice().setFaceDetectionListener(googleFaceDetect);
CameraInterface.getInstance().getCameraDevice().startFaceDetection();
}
}
private void stopGoogleFaceDetect(){
Camera.Parameters params = CameraInterface.getInstance().getCameraParams();
if(params.getMaxNumDetectedFaces() > 0){
CameraInterface.getInstance().getCameraDevice().setFaceDetectionListener(null);
CameraInterface.getInstance().getCameraDevice().stopFaceDetection();
faceView.clearFaces();
}
}</span>
先来看下官方对startFaceDetection()一段注释:
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<span style="font-family:Comic Sans MS;font-size:18px;"> /**
* Starts the face detection. This should be called after preview is started.
* The camera will notify {@link FaceDetectionListener} of the detected
* faces in the preview frame. The detected faces may be the same as the
* previous ones. Applications should call {@link #stopFaceDetection} to
* stop the face detection. This method is supported if {@link
* Parameters#getMaxNumDetectedFaces()} returns a number larger than 0.
* If the face detection has started, apps should not call this again.
*
* <p>When the face detection is running, {@link Parameters#setWhiteBalance(String)},
* {@link Parameters#setFocusAreas(List)}, and {@link Parameters#setMeteringAreas(List)}
* have no effect. The camera uses the detected faces to do auto-white balance,
* auto exposure, and autofocus.
*
* <p>If the apps call {@link #autoFocus(AutoFocusCallback)}, the camera
* will stop sending face callbacks. The last face callback indicates the
* areas used to do autofocus. After focus completes, face detection will
* resume sending face callbacks. If the apps call {@link
* #cancelAutoFocus()}, the face callbacks will also resume.</p>
*
* <p>After calling {@link #takePicture(Camera.ShutterCallback, Camera.PictureCallback,
* Camera.PictureCallback)} or {@link #stopPreview()}, and then resuming
* preview with {@link #startPreview()}, the apps should call this method
* again to resume face detection.</p>
*
* @throws IllegalArgumentException if the face detection is unsupported.
* @throws RuntimeException if the method fails or the face detection is
* already running.
* @see FaceDetectionListener
* @see #stopFaceDetection()
* @see Parameters#getMaxNumDetectedFaces()
*/</span>
相信大家都能看懂，杂家就不一句一句翻了。关键信息是，在调用takePicture和stopPreview时，必须重新start来恢复人脸检测。而在拍照前是不需要手动stop的。经杂家测试，手动stop反而会坏事。另外就是takePicture之后(实际上camera做了stopPreview和startPreview)，不能立即startFaceDetection()，如果立即做是没有效果的，必须加个延时。
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<span style="font-family:Comic Sans MS;font-size:18px;"> private void takePicture(){
CameraInterface.getInstance().doTakePicture();
mMainHandler.sendEmptyMessageDelayed(EventUtil.CAMERA_HAS_STARTED_PREVIEW, 1500);
}</span>
第二个问题是在Camera切换之后，Camera的实例发生了变化。必须调用stopFaceDetection()，在此之前调用setFaceDetectionListener(null)将其监听置为null。再切换过来重新预览后，再次start。
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<span style="font-family:Comic Sans MS;font-size:18px;"> private void switchCamera(){
stopGoogleFaceDetect();
int newId = (CameraInterface.getInstance().getCameraId() + 1)%2;
CameraInterface.getInstance().doStopCamera();
CameraInterface.getInstance().doOpenCamera(null, newId);
CameraInterface.getInstance().doStartPreview(surfaceView.getSurfaceHolder(), previewRate);
startGoogleFaceDetect();
}</span>
其他代码变化不大，杂家就不一一贴出来了，想看的请看源码。下面上效果图:
下图为预览界面，拍照图片和切换图片直接换成了Android4.4原生的，原来的实在太丑了。
下图为直接把Camera对着电视剧的检测效果:
再来一张，对着电脑里的图片：
很多人质疑google自带检测算法效果太弱，但就测试结果来看，人家做的已经很牛逼了。待下次推出opencv的demo后用同样的预览画面来对比分析。其实大多人抱怨的是google只提供了检测，没有提供识别和认证。不过，它要啥都提供，那杂家就没饭吃了。但可以预见，不久的将来该来的总会来。
补充说明:有的手机上尽管是4.0以上的版本，但可能仍然不支持这种人脸检测接口。个别手机后置camera支持良好，但前置Camera不支持，如中兴的Geek，切到前置Camera后直接报Camera Server died，ICamera died。这种问题只能骂手机厂家，不要喷我的代码哈。
源码下载链接：http://download.csdn.net/detail/yanzi1225627/7674929
--------------------本文系原创，转载请注明作者:yanzi1225627
本文主要介绍使用Google自带的FaceDetectionListener进行人脸检测，并将检测到的人脸用矩形框绘制出来。本文代码基于PlayCameraV1.0.0，在Camera的open和preview流程上进行了改动。原先是放在单独线程里，这次我又把它放到Surfaceview的生命周期里进行打开和开预览。
首先要反省下，去年就推出了静态图片的人脸检测demo，当时许诺一周内推出Camera预览实时检测并绘制的demo，结果拖到现在才整。哎，屌丝一天又一天，蹉跎啊。在demo制作过程中还是遇到了一些麻烦的，第一个问题是检测到人脸rect默认是以预览界面为坐标系，这个坐标系是经过变换的，中心点为(0,
0),左上顶点坐标是(-1000, -1000)，右下顶点是(1000, 1000).也就是说不管预览预览Surfaceview多大，检测出来的rect的坐标始终对应的是在这个变换坐标系。而Android里默认的view的坐标系是，左上顶点为(0,
0)，横为x轴，竖为y轴。这就需要把rect坐标变换下。另一个难点是，这个人脸检测必须在camera开启后进行start，如果一旦拍照或停预览，则需要再次激活。激活时需要加个延迟，否则的话就不起作用了。
另外，仍要交代下，在预览界面实时检测人脸并绘制(基于Google自带算法)，还是有两个思路的。一是在PreviewCallback里的onPreviewFrame里得到yuv数据后，转成rgb后再转成Bitmap，然后利用静态图片的人脸检测流程，即利用FaceDetector类进行检测。另一个思路是，直接实现FaceDetectionListener接口，这样在onFaceDetection()里就得到检测到的人脸Face[]
faces数据了。这里只需控制何时start，何时stop即可，这都是android标准接口。毫无疑问，这种方法是上选。从Android4.0后android源码里的camera app都是用的这个接口进行人脸检测。下面上源码:
一、GoogleFaceDetect.java
考虑到下次准备介绍JNI里用opencv检测人脸，为此杂家新建了一个包org.yanzi.mode里面准备放所有的关于图像的东西。新建文件GoogleFaceDetect.java实现FaceDetectionListener，在构造函数里传进来一个Handler，将检测到的人脸数据发给Activity，经Activity中转再刷新UI.[java] view
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print?
<span style="font-family:Comic Sans MS;font-size:18px;">package org.yanzi.mode;
import org.yanzi.util.EventUtil;
import android.content.Context;
import android.hardware.Camera;
import android.hardware.Camera.Face;
import android.hardware.Camera.FaceDetectionListener;
import android.os.Handler;
import android.os.Message;
import android.util.Log;
public class GoogleFaceDetect implements FaceDetectionListener {
private static final String TAG = "YanZi";
private Context mContext;
private Handler mHander;
public GoogleFaceDetect(Context c, Handler handler){
mContext = c;
mHander = handler;
}
@Override
public void onFaceDetection(Face[] faces, Camera camera) {
// TODO Auto-generated method stub
Log.i(TAG, "onFaceDetection...");
if(faces != null){
Message m = mHander.obtainMessage();
m.what = EventUtil.UPDATE_FACE_RECT;
m.obj = faces;
m.sendToTarget();
}
}
/* private Rect getPropUIFaceRect(Rect r){
Log.i(TAG, "人脸检测 = " + r.flattenToString());
Matrix m = new Matrix();
boolean mirror = false;
m.setScale(mirror ? -1 : 1, 1);
Point p = DisplayUtil.getScreenMetrics(mContext);
int uiWidth = p.x;
int uiHeight = p.y;
m.postScale(uiWidth/2000f, uiHeight/2000f);
int leftNew = (r.left + 1000)*uiWidth/2000;
int topNew = (r.top + 1000)*uiHeight/2000;
int rightNew = (r.right + 1000)*uiWidth/2000;
int bottomNew = (r.bottom + 1000)*uiHeight/2000;
return new Rect(leftNew, topNew, rightNew, bottomNew);
}*/
}
</span>
上面代码注释掉的一部分是我最初想自己写矩阵变换算法的过程，一番努力感觉变换后坐标还是有问题，后来参考Android4.0里的Camera APP源码才解决.这个变换转移到了FaceView里。
二、FaceView.java
这个类继承ImageView，用来将Face[] 数据的rect取出来，变换后刷新到UI上。
[java] viewplain copy
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<span style="font-family:Comic Sans MS;font-size:18px;">package org.yanzi.ui;
import org.yanzi.camera.CameraInterface;
import org.yanzi.playcamera.R;
import org.yanzi.util.Util;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Matrix;
import android.graphics.Paint;
import android.graphics.Paint.Style;
import android.graphics.RectF;
import android.graphics.drawable.Drawable;
import android.hardware.Camera.CameraInfo;
import android.hardware.Camera.Face;
import android.util.AttributeSet;
import android.widget.ImageView;
public class FaceView extends ImageView {
private static final String TAG = "YanZi";
private Context mContext;
private Paint mLinePaint;
private Face[] mFaces;
private Matrix mMatrix = new Matrix();
private RectF mRect = new RectF();
private Drawable mFaceIndicator = null;
public FaceView(Context context, AttributeSet attrs) {
super(context, attrs);
// TODO Auto-generated constructor stub
initPaint();
mContext = context;
mFaceIndicator = getResources().getDrawable(R.drawable.ic_face_find_2);
}
public void setFaces(Face[] faces){
this.mFaces = faces;
invalidate();
}
public void clearFaces(){
mFaces = null;
invalidate();
}
@Override
protected void onDraw(Canvas canvas) {
// TODO Auto-generated method stub
if(mFaces == null || mFaces.length < 1){
return;
}
boolean isMirror = false;
int Id = CameraInterface.getInstance().getCameraId();
if(Id == CameraInfo.CAMERA_FACING_BACK){
isMirror = false; //后置Camera无需mirror
}else if(Id == CameraInfo.CAMERA_FACING_FRONT){
isMirror = true; //前置Camera需要mirror
}
Util.prepareMatrix(mMatrix, isMirror, 90, getWidth(), getHeight());
canvas.save();
mMatrix.postRotate(0); //Matrix.postRotate默认是顺时针
canvas.rotate(-0); //Canvas.rotate()默认是逆时针
for(int i = 0; i< mFaces.length; i++){
mRect.set(mFaces[i].rect);
mMatrix.mapRect(mRect);
mFaceIndicator.setBounds(Math.round(mRect.left), Math.round(mRect.top),
Math.round(mRect.right), Math.round(mRect.bottom));
mFaceIndicator.draw(canvas);
// canvas.drawRect(mRect, mLinePaint);
}
canvas.restore();
super.onDraw(canvas);
}
private void initPaint(){
mLinePaint = new Paint(Paint.ANTI_ALIAS_FLAG);
// int color = Color.rgb(0, 150, 255);
int color = Color.rgb(98, 212, 68);
// mLinePaint.setColor(Color.RED);
mLinePaint.setColor(color);
mLinePaint.setStyle(Style.STROKE);
mLinePaint.setStrokeWidth(5f);
mLinePaint.setAlpha(180);
}
}
</span>
注意事项有两个：
1.就是Rect变换问题，通过Util.prepareMatrix(mMatrix, isMirror, 90, getWidth(), getHeight());进行变换，为了解决人脸检测坐标系和实际绘制坐标系不一致问题。第三个参数90，是因为前手摄像头都设置了mCamera.setDisplayOrientation(90);
接下来的Matrix和canvas两个旋转我传的都是0，所以此demo只能在手机0、90、180、270四个标准角度下得到的人脸坐标是正确的。其他情况下，需要将OrientationEventListener得到的角度传过来。为了简单，我这块就么写，OrientationEventListener的用法参见我的前文，后续将再推出一个demo。
最终是通过mMatrix.mapRect(mRect);来将mRect变换成UI坐标系的人脸Rect.
Util.prepareMatrix()代码如下：
[java] view
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<span style="font-family:Comic Sans MS;font-size:18px;">package org.yanzi.util;
import android.graphics.Matrix;
public class Util {
public static void prepareMatrix(Matrix matrix, boolean mirror, int displayOrientation,
int viewWidth, int viewHeight) {
// Need mirror for front camera.
matrix.setScale(mirror ? -1 : 1, 1);
// This is the value for android.hardware.Camera.setDisplayOrientation.
matrix.postRotate(displayOrientation);
// Camera driver coordinates range from (-1000, -1000) to (1000, 1000).
// UI coordinates range from (0, 0) to (width, height).
matrix.postScale(viewWidth / 2000f, viewHeight / 2000f);
matrix.postTranslate(viewWidth / 2f, viewHeight / 2f);
}
}
</span>
2.得到实际UI里的人脸rect怎么画的问题。之前都是通过paint直接画，但实际上也可以通过Drawable.draw(canvas)来画。后者的好处是将一个图片画上去，而通过paint绘制基础图行如Rect、Circle比较方面。代码里把两种方法的代码都写了，供大家参考。
三.何时打开Camera，何时开预览?
本次将这两个流程放到了Surfaceview的两个生命周期里，因为之前放在单独Thread还是会有一些问题。如个别手机上，Surfaceview创建的很慢，这时的SurfaceHolder还没准备好，结果Camera已经走到开预览了，导致黑屏问题。[java] view
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<span style="font-family:Comic Sans MS;font-size:18px;">@Override
public void surfaceCreated(SurfaceHolder holder) {
// TODO Auto-generated method stub
Log.i(TAG, "surfaceCreated...");
CameraInterface.getInstance().doOpenCamera(null, CameraInfo.CAMERA_FACING_BACK);
}
@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width,
int height) {
// TODO Auto-generated method stub
Log.i(TAG, "surfaceChanged...");
CameraInterface.getInstance().doStartPreview(mSurfaceHolder, 1.333f);
}</span>
四.何时注册并开始人脸检测?
若要开启人脸检测，必须要在Camera已经startPreview完毕之后。本文暂时采用在onCreate里延迟1.5s开启人脸检测，1.5s基本上camera已经开预览了。后续准备将Handler传到Surfaceview里，在开预览后通过Handler通知Activity已经开启预览了。[java] view
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自定义的MainHandler:
[java] view
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<span style="font-family:Comic Sans MS;font-size:18px;"> private class MainHandler extends Handler{
@Override
public void handleMessage(Message msg) {
// TODO Auto-generated method stub
switch (msg.what){
case EventUtil.UPDATE_FACE_RECT:
Face[] faces = (Face[]) msg.obj;
faceView.setFaces(faces);
break;
case EventUtil.CAMERA_HAS_STARTED_PREVIEW:
startGoogleFaceDetect();
break;
}
super.handleMessage(msg);
}
}</span>
在onCreate里：
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<span style="font-family:Comic Sans MS;font-size:18px;"> protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_camera);
initUI();
initViewParams();
mMainHandler = new MainHandler();
googleFaceDetect = new GoogleFaceDetect(getApplicationContext(), mMainHandler);
shutterBtn.setOnClickListener(new BtnListeners());
switchBtn.setOnClickListener(new BtnListeners());
mMainHandler.sendEmptyMessageDelayed(EventUtil.CAMERA_HAS_STARTED_PREVIEW, 1500);
}</span>
这里写了两个重要的方法分别是开始检测和停止检测:
[java] view
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<span style="font-family:Comic Sans MS;font-size:18px;">private void startGoogleFaceDetect(){
Camera.Parameters params = CameraInterface.getInstance().getCameraParams();
if(params.getMaxNumDetectedFaces() > 0){
if(faceView != null){
faceView.clearFaces();
faceView.setVisibility(View.VISIBLE);
}
CameraInterface.getInstance().getCameraDevice().setFaceDetectionListener(googleFaceDetect);
CameraInterface.getInstance().getCameraDevice().startFaceDetection();
}
}
private void stopGoogleFaceDetect(){
Camera.Parameters params = CameraInterface.getInstance().getCameraParams();
if(params.getMaxNumDetectedFaces() > 0){
CameraInterface.getInstance().getCameraDevice().setFaceDetectionListener(null);
CameraInterface.getInstance().getCameraDevice().stopFaceDetection();
faceView.clearFaces();
}
}</span>
五.人脸检测如何和拍照及前后摄像头切换协调同步?
先来看下官方对startFaceDetection()一段注释:[plain] view
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<span style="font-family:Comic Sans MS;font-size:18px;"> /**
* Starts the face detection. This should be called after preview is started.
* The camera will notify {@link FaceDetectionListener} of the detected
* faces in the preview frame. The detected faces may be the same as the
* previous ones. Applications should call {@link #stopFaceDetection} to
* stop the face detection. This method is supported if {@link
* Parameters#getMaxNumDetectedFaces()} returns a number larger than 0.
* If the face detection has started, apps should not call this again.
*
* <p>When the face detection is running, {@link Parameters#setWhiteBalance(String)},
* {@link Parameters#setFocusAreas(List)}, and {@link Parameters#setMeteringAreas(List)}
* have no effect. The camera uses the detected faces to do auto-white balance,
* auto exposure, and autofocus.
*
* <p>If the apps call {@link #autoFocus(AutoFocusCallback)}, the camera
* will stop sending face callbacks. The last face callback indicates the
* areas used to do autofocus. After focus completes, face detection will
* resume sending face callbacks. If the apps call {@link
* #cancelAutoFocus()}, the face callbacks will also resume.</p>
*
* <p>After calling {@link #takePicture(Camera.ShutterCallback, Camera.PictureCallback,
* Camera.PictureCallback)} or {@link #stopPreview()}, and then resuming
* preview with {@link #startPreview()}, the apps should call this method
* again to resume face detection.</p>
*
* @throws IllegalArgumentException if the face detection is unsupported.
* @throws RuntimeException if the method fails or the face detection is
* already running.
* @see FaceDetectionListener
* @see #stopFaceDetection()
* @see Parameters#getMaxNumDetectedFaces()
*/</span>
相信大家都能看懂，杂家就不一句一句翻了。关键信息是，在调用takePicture和stopPreview时，必须重新start来恢复人脸检测。而在拍照前是不需要手动stop的。经杂家测试，手动stop反而会坏事。另外就是takePicture之后(实际上camera做了stopPreview和startPreview)，不能立即startFaceDetection()，如果立即做是没有效果的，必须加个延时。
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<span style="font-family:Comic Sans MS;font-size:18px;"> private void takePicture(){
CameraInterface.getInstance().doTakePicture();
mMainHandler.sendEmptyMessageDelayed(EventUtil.CAMERA_HAS_STARTED_PREVIEW, 1500);
}</span>
第二个问题是在Camera切换之后，Camera的实例发生了变化。必须调用stopFaceDetection()，在此之前调用setFaceDetectionListener(null)将其监听置为null。再切换过来重新预览后，再次start。
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<span style="font-family:Comic Sans MS;font-size:18px;"> private void switchCamera(){
stopGoogleFaceDetect();
int newId = (CameraInterface.getInstance().getCameraId() + 1)%2;
CameraInterface.getInstance().doStopCamera();
CameraInterface.getInstance().doOpenCamera(null, newId);
CameraInterface.getInstance().doStartPreview(surfaceView.getSurfaceHolder(), previewRate);
startGoogleFaceDetect();
}</span>
其他代码变化不大，杂家就不一一贴出来了，想看的请看源码。下面上效果图:
下图为预览界面，拍照图片和切换图片直接换成了Android4.4原生的，原来的实在太丑了。
下图为直接把Camera对着电视剧的检测效果:
再来一张，对着电脑里的图片：
很多人质疑google自带检测算法效果太弱，但就测试结果来看，人家做的已经很牛逼了。待下次推出opencv的demo后用同样的预览画面来对比分析。其实大多人抱怨的是google只提供了检测，没有提供识别和认证。不过，它要啥都提供，那杂家就没饭吃了。但可以预见，不久的将来该来的总会来。
补充说明:有的手机上尽管是4.0以上的版本，但可能仍然不支持这种人脸检测接口。个别手机后置camera支持良好，但前置Camera不支持，如中兴的Geek，切到前置Camera后直接报Camera Server died，ICamera died。这种问题只能骂手机厂家，不要喷我的代码哈。
源码下载链接：http://download.csdn.net/detail/yanzi1225627/7674929
--------------------本文系原创，转载请注明作者:yanzi1225627
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