android中利用Camera编写拍照应用
2013-07-19 21:24 1287 0
在安卓中使用拍照功能有两种方式，一是调用已有的拍照应用；二是使用android的Camera对象直接操作相机，自己写代码来实现拍照功能。
如果是采用Camera的方式，相当于自己写了个拍照程序。直接使用Camera的好处是拍照界面可以完全自定义，UI风格可以和自己应用保持一致，但也要麻烦一些。
下面来介绍开发一个拍照程序的步骤，实现方法参考了谷歌android开发的官方文档。
整个步骤大概分为三步：
1.启动相机，其实就是打开摄像头。
2.生成摄像的预览图像。
3.拍照
为了完成上述的三个步骤，我们至少需要建立以下三个类，并新建这三个类的对象，他们之间通过一定的关联，就能完成最基本的拍照功能。
Camera****对象：管理硬件camera的打开和关闭，触发拍照命令。
一个继承自的SurfaceView****相机图像预览类：因为官方文档把这个自定义的SurfaceView类命名为CameraPreview，在此我也这样命名。该类负责在打开相机的时候在activity中显示采集到的图像预览效果。该过程和UI主线程是异步处理的，因此使用SurfaceView。
拍照数据捕获类：当用户点击某个按钮开始拍照的时候，需要有相应的方法能在拍照完成之后将图像数据捕获并存储，该类通过实现Camera.PictureCallback接口来完成捕获过程。
先展示一下拍照的界面：
界面很简单，只有两个部分，上面是图像预览区，下面是拍照按钮，xml布局代码如下：
<LinearLayoutxmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<FrameLayout
android:id="@+id/camera_preview"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:layout_weight="1"
/>
<Button
android:id="@+id/button_capture"
android:text="Capture"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
/>
</LinearLayout>
预览区域我们采用的是FrameLayout,但是预览效果其实和FrameLayout没什么关系，能在FrameLayout中产生预览效果是因为我们往FrameLayout中添加了上面提到的一个继承自的SurfaceView相机图像预览类。添加该类到FrameLayout的代码在activity的onCreate方法中，下面贴出整个activity的代码：
package com.example.cameratest;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.Date;
importandroid.net.Uri;
import android.os.Bundle;
import android.os.Environment;
importandroid.provider.MediaStore;
import android.app.Activity;
import android.content.Context;
importandroid.content.Intent;
import android.hardware.Camera;
import android.hardware.Camera.PictureCallback;
import android.util.Log;
importandroid.view.Menu;
import android.view.Surface;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.View;
import android.widget.Button;
import android.widget.FrameLayout;
importandroid.widget.Toast;
public****class CameraActivity extends Activity {
publicstaticfinal****intMEDIA_TYPE_IMAGE = 1;
publicstaticfinal****intMEDIA_TYPE_VIDEO = 2;
protectedstaticfinal String TAG = null;
private Camera mCamera;
private CameraPreview mPreview;
@Override
public****void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.camera_activity);
// Create an instance ofCamera
mCamera = getCameraInstance();
// Create our Preview viewand set it as the content of our activity.
mPreview = new CameraPreview(this, mCamera);
FrameLayout preview = (FrameLayout) findViewById(R.id.camera_preview);
preview.addView(mPreview);
Button captureButton = (Button) findViewById(R.id.button_capture);
captureButton.setOnClickListener(new View.OnClickListener() {
@Override
public****void onClick(View v) {
// get an imagefrom the camera
mCamera.takePicture(null, null, mPicture);
}
});
}
/** A safe way to get an instance of theCamera object. */
public****static Camera getCameraInstance() {
Camera c = null;
try {
c = Camera.open(); // attempt to get a Camera instance
} catch (Exception e) {
// Camera is notavailable (in use or does not exist)
}
return c; // returns nullif camera is unavailable
}
/** A basic Camera preview class */
public****class CameraPreview extends SurfaceView implements
SurfaceHolder.Callback {
private SurfaceHolder mHolder;
private Camera mCamera;
public CameraPreview(Context context, Camera camera) {
super(context);
mCamera = camera;
// Install aSurfaceHolder.Callback so we get notified when the
// underlyingsurface is created and destroyed.
mHolder = getHolder();
mHolder.addCallback(this);
// deprecatedsetting, but required on Android versions prior to 3.0
mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
}
public****void surfaceCreated(SurfaceHolder holder) {
// The Surfacehas been created, now tell the camera where to draw
// the preview.
try {
mCamera.setPreviewDisplay(holder);
mCamera.setDisplayOrientation(90);
mCamera.startPreview();
} catch (IOException e) {
Log.d(TAG, "Errorsetting camera preview: " + e.getMessage());
}
}
public****void surfaceDestroyed(SurfaceHolder holder) {
// empty. Takecare of releasing the Camera preview in your
// activity.
}
public****void surfaceChanged(SurfaceHolder holder, int format, int w,
int h) {
// If yourpreview can change or rotate, take care of those events
// here.
// Make sure tostop the preview before resizing or reformatting it.
if (mHolder.getSurface() == null) {
// previewsurface does not exist
return;
}
// stop previewbefore making changes
try {
mCamera.stopPreview();
} catch (Exception e) {
// ignore: triedto stop a non-existent preview
}
// set previewsize and make any resize, rotate or
// reformattingchanges here
// start previewwith new settings
try {
mCamera.setPreviewDisplay(mHolder);
mCamera.setDisplayOrientation(90);
mCamera.startPreview();
} catch (Exception e) {
Log.d(TAG, "Errorstarting camera preview: " + e.getMessage());
}
}
}
private PictureCallback mPicture = new PictureCallback() {
@Override
public****void onPictureTaken(byte[] data, Camera camera) {
File pictureFile = getOutputMediaFile(MEDIA_TYPE_IMAGE);
if (pictureFile == null) {
return;
}
try {
FileOutputStream fos = new FileOutputStream(pictureFile);
fos.write(data);
fos.close();
Log.i("cameratest", "pictureFiledata=" + data.length);
} catch (FileNotFoundException e) {
Log.i("cameratest", "File notfound: " + e.getMessage());
} catch (IOException e) {
Log.i("cameratest", "Erroraccessing file: " + e.getMessage());
}
}
};
private****static File getOutputMediaFile(int type) {
// To be safe, you shouldcheck that the SDCard is mounted
// usingEnvironment.getExternalStorageState() before doing this.
File mediaStorageDir = new File(
Environment
.getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES),
"MyCameraApp");
// This location works bestif you want the created images to be shared
// between applications andpersist after your app has been uninstalled.
// Create the storagedirectory if it does not exist
if (!mediaStorageDir.exists()) {
if (!mediaStorageDir.mkdirs()) {
Log.d("MyCameraApp", "failed tocreate directory");
return****null;
}
}
// Create a media file name
String timeStamp = newSimpleDateFormat("yyyyMMdd_HHmmss")
.format(new Date());
File mediaFile;
if (type == MEDIA_TYPE_IMAGE) {
mediaFile = new File(mediaStorageDir.getPath() + File.separator
+ "IMG_" + timeStamp + ".jpg");
} else****if (type == MEDIA_TYPE_VIDEO) {
mediaFile = new File(mediaStorageDir.getPath() + File.separator
+ "VID_" + timeStamp + ".mp4");
} else {
return****null;
}
return mediaFile;
}
@Override
protected****void onStop() {
Log.i("cameratest", "onStop");
super.onStop(); // Always callthe superclass method first
}
@Override
public****void onPause() {
super.onPause(); // Alwayscall the superclass method first
Log.i("cameratest", "onPause");
// Release the Camera becausewe don't need it when paused
// and other activities mightneed to use it.
if (mCamera != null) {
mCamera.release();
mCamera = null;
}
}
}
代码解读：
获得一个Camera对象不是通过new的方式，而是通过Camera.open()返回一个Camera对象，这一过程必须要有异常处理，为此我们将整个过程封装在getCameraInstance函数中，getCameraInstance函数的代码如下：
/** A safeway to get an instance of the Camera object. */
public****static Camera getCameraInstance(){
Camera c = null;
try {
c = Camera.open(); // attempt to get a Camera instance
}
catch (Exception e){
// Camera is not available (in use or does not exist)
}
return c; // returns null if camera is unavailable
}
mCamera = getCameraInstance();
然后在activity的onCreate方法中我们将mCamera作为参数传递给CameraPreview的构造函数，建立了相机图像预览类Ca****meraPreview对象mPreview，将mPreview添加进FrameLayout容器中，如下：
mPreview = new CameraPreview(this, mCamera);
FrameLayout preview = (FrameLayout) findViewById(R.id.camera_preview);
preview.addView(mPreview);
Ca****meraPreview的实现在上面给出得的activity代码中可以找到，其实他就是一个实现了SurfaceHolder.Callback接口的SurfaceView。关于如何使用SurfaceView这里不做讲解，这里只说明为什么我们仅仅是新建了个继承自SurfaceView的Ca****meraPreview对象，然后将该对象放入布局文件中，就能实现预览效果。其中关键的代码是：
mHolder = getHolder();
mHolder.addCallback(this);
……
……
mCamera.setPreviewDisplay(mHolder);
将获得的mHolder和mCamera关联起来。非常简单吧，其实android系统已经为我们做了绝大多数的工作。
通过这几步，我们已经能够得到一个相机拍照的界面，并且带有预览功能，但是好像还缺少点什么，对，还去少触发拍照的动作。
点击如上图所示的captrue按钮即开始触发拍照行为，在这里其实是调用mCamera 的takePicture方法：
ButtoncaptureButton = (Button) findViewById(R.id.button_capture);
captureButton.setOnClickListener(newView.OnClickListener() {
@Override
publicvoid onClick(View v) {
//get an image from the camera
mCamera.takePicture(null,null, mPicture);
}
});
我们注意到mCamera.takePicture(null,null, mPicture);中mPicture这个变量还一直没有提到，他就是我们的拍照数据捕获类PictureCallback，它的任务很简单，就是在拍照完成后，处理图片，这里我们是将图片保存下来。onPictureTaken方法中可以通过data参数获得拍照的照片数据。我们将这些数据存储在公共目录的picture目录里面，就是存放相册图片的地方，获得这个存放地址的方法为getOutputMediaFile，具体的代码参见上面的activity的代码。
上面基本描述了拍照程序的实现流程，下面介绍一些细节。
1.默认情况下，拍照预览窗口的角度是横屏显示的。如果你的activity是竖屏状态，那么需要设置预览窗口的角度。本例子中我们将角度设置为90度，这样刚好能正确显示：
mCamera.setDisplayOrientation(90);
2.Camera不能同时被两个activity使用，否则会引起程序崩溃，所以需要在不需要camera的时候释放资源，一般我们是在onPause中释放。比如这个例子中：
@Override
public****void onPause() {
super.onPause(); // Alwayscall the superclass method first
// Release the Camera becausewe don't need it when paused
// and other activities mightneed to use it.
if (mCamera != null) {
mCamera.release();
mCamera = null;
}
}
