Android平台上从Camera的jpegquality菜单研究JNI接口

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Jpeg quality菜单与picture size菜单等有些区别，在该菜单中，调用了JNI接口。第一次接触JNI接口，所以写个小文档，做个记录。

一、应用程序层介绍

1、 jpeg quality菜单项及其初始化

首先，介绍一下jpeg quality菜单及其菜单项。在文件

/packages/apps/Camera/res/xml/camera_preferences.xml中，有如下的code：

<PreferenceGroup xmlns:camera=

"http://schemas.android.com/apk/res/com.android.camera">

……

<ListPreference camera:key="pref_camera_jpegquality_key"

amera:defaultValue="@string/pref_camera_jpegquality_default" camera:title="@string/pref_camera_jpegquality_title" camera:entries="@array/pref_camera_jpegquality_entries" camera:entryValues="@array/pref_camera_jpegquality_entryvalues" />

…

</PreferenceGroup>

Jpeg quality菜单与picture size同样都属于preferenceGroup，但是它们的初始化方式却是不一样的。在文件/packages/apps/Camera/src/com/android/camera/CameraSetting.java中，有一个函数private void initPreference(PreferenceGroup group)用来初始化picture size等菜单，但是其并没有对jpeg quality菜单做初始化。在另一个函数upgradePreferences()中，对jpeg quality菜单做了初始化。其具体定义为：

public static void upgradePreferences(SharedPreferences pref) {

int version;

try {

version = pref.getInt(KEY_VERSION, 0);

} catch (Exception ex) {

version = 0;

}

if (version == CURRENT_VERSION) return;

SharedPreferences.Editor editor = pref.edit();

if (version == 0) {

// We won't use the preference which change in version 1.

// So, just upgrade to version 1 directly

version = 1;

}

if (version == 1) {

// Change jpeg quality {65,75,85} to {normal,fine,superfine}

String quality = pref.getString(KEY_JPEG_QUALITY, "85");

if (quality.equals("65")) {

quality = "normal";

} else if (quality.equals("75")) {

quality = "fine";

} else {

quality = "superfine";

}

editor.putString(KEY_JPEG_QUALITY, quality);

version = 2;

}

if (version == 2) {

editor.putString(KEY_RECORD_LOCATION,

pref.getBoolean(KEY_RECORD_LOCATION, false)

? RecordLocationPreference.VALUE_ON

: RecordLocationPreference.VALUE_NONE);

version = 3;

}

if (version == 3) {

// Just use video quality to replace it and

// ignore the current settings.

editor.remove("pref_camera_videoquality_key");

editor.remove("pref_camera_video_duration_key");

}

editor.putInt(KEY_VERSION, CURRENT_VERSION);

editor.commit();

}

该函数在文件/packages/apps/Camera/src/com/android/camera/Camera.java中被调用，其具体code为：

public void onCreate(Bundle icicle) {

super.onCreate(icicle);

setContentView(R.layout.camera);

mSurfaceView = (SurfaceView) findViewById(R.id.camera_preview);

mPreferences = PreferenceManager.getDefaultSharedPreferences(this);

CameraSettings.upgradePreferences(mPreferences);

mQuickCapture = getQuickCaptureSettings();

// comment out -- unused now.

//mQuickCapture = getQuickCaptureSettings();

……

在文件 /packages/apps/Camera/res/values/arrays.xml中有如下的code：

- <!--

 Camera Preferences JPEG quality dialog box entries

-->

<string name="pref_camera_jpegquality_entry_superfine">Super fine</string>

<string name="pref_camera_jpegquality_entry_normal">Normal</string>

由此，我们可以知道jpeg quality菜单有三项，分别为Normal，Fine和SuperFine。

下面我们看看jpeg quality菜单的更新。

2、 jpeg quality菜单的更新

在文件/packages/apps/Camera/src/com/android/camera/Camera.java有个函数

updateCameraParametersPreference()，它负责更新camera应用程序的菜单。其定义如下：

private void updateCameraParametersPreference() {

// Set picture size.

……

// Set the preview frame aspect ratio according to the picture size.

……

// Set a preview size that is closest to the viewfinder height and has

// the right aspect ratio.

……

// Since change scene mode may change supported values,

// Set scene mode first,

……

// Set JPEG quality.

String jpegQuality = mPreferences.getString(

CameraSettings.KEY_JPEG_QUALITY,

getString(R.string.pref_camera_jpegquality_default));

mParameters.setJpegQuality(JpegEncodingQualityMappings.getQualityNumber(jpegQuality));

// Set color effect parameter.

……

}

通过String jpegQuality = mPreferences.getString()，我们获取当前jpeg quality菜单所选中的项对应的字符串，然后通过mParameters.setJpegQuality()去设置该值，底层可以得知菜单中所选中的项对应的值。mParameters.setJpegQuality()的定义在文件

/framework/base/core/java/android/hardware/camera.java中，其具体code：

/**

* Sets Jpeg quality of captured picture.

* @param quality the JPEG quality of captured picture. The range is 1

* to 100, with 100 being the best.

public void setJpegQuality(int quality) {

set(KEY_JPEG_QUALITY, quality);

}

其中，KEY_JPEG_QUALITY定义为：

private static final String KEY_JPEG_QUALITY = "jpeg-quality";

由注释，我们可以知道这里设置的值在[1，100]之间。通过mPreferences.getString()，我们获取的是字符串，比如normal, fine和superfine,而mParameters.setJpegQuality()所设置的字符串却应该是[0，100]之间的值，这个转换就需要

JpegEncodingQualityMappings.getQualityNumber(jpegQuality)来实现了。

JpegEncodingQualityMappings是一个类，定义在文件

/packages/apps/Camera/src/com/android/camera/Camera.java中，具体定义为：

* Provide a mapping for Jpeg encoding quality levels

* from String representation to numeric representation.

class JpegEncodingQualityMappings {

private static final String TAG = "JpegEncodingQualityMappings";

private static final int DEFAULT_QUALITY = 85;

private static HashMap<String, Integer> mHashMap =

new HashMap<String, Integer>();

static {

mHashMap.put("normal", CameraProfile.QUALITY_LOW);

mHashMap.put("fine", CameraProfile.QUALITY_MEDIUM);

mHashMap.put("superfine", CameraProfile.QUALITY_HIGH);

}

// Retrieve and return the Jpeg encoding quality number

// for the given quality level.

public static int getQualityNumber(String jpegQuality) {

Integer quality = mHashMap.get(jpegQuality);

if (quality == null) {

Log.w(TAG, "Unknown Jpeg quality: " + jpegQuality);

return DEFAULT_QUALITY;

}

return CameraProfile.getJpegEncodingQualityParameter(quality.intValue());

}

很清楚，getQualityNumber(jpegQuality)的参数如果没在normal，fine和superfine之间就返回DEFAULT_QUALITY，默认的数值；如果在，则调用

CameraProfile.getJpegEncodingQualityParameter(quality.intValue())。这个函数定义在文件

/framework/base/media/java/android/media/CameraProfile.java中。

在该文件中，定义了：

public static final int QUALITY_LOW = 0;

public static final int QUALITY_MEDIUM = 1;

public static final int QUALITY_HIGH = 2;

这表示jpeg的quality分为3个等级，它们做为getJpegEncodingQualityParameter()的参数。

/**

* Returns a pre-defined still image capture (jpeg) quality level

* used for the given quality level in the Camera application.

* @param quality The target quality level

public static int getJpegEncodingQualityParameter(int quality) {

if (quality < QUALITY_LOW || quality > QUALITY_HIGH) {

throw new IllegalArgumentException("Unsupported quality level: " + quality);

}

return sJpegEncodingQualityParameters[quality];

}

sJpegEncodingQualityParameters的定义为：

* Cache the Jpeg encoding quality parameters

private static final int[] sJpegEncodingQualityParameters;

数组sJpegEncodingQualityParameters中的值如何得来呢？接着往下看。

3、 Jpeg quality参数的获取

在文件CameraProfile.java中，有下面的code：

static {

System.loadLibrary("media_jni");

native_init();

sJpegEncodingQualityParameters = getImageEncodingQualityLevels();

}

private static int[] getImageEncodingQualityLevels() {

int nLevels = native_get_num_image_encoding_quality_levels();

if (nLevels != QUALITY_HIGH + 1) {

throw new RuntimeException("Unexpected Jpeg encoding quality levels " + nLevels);

}

int[] levels = new int[nLevels];

for (int i = 0; i < nLevels; ++i) {

levels[i] = native_get_image_encoding_quality_level(i);

}

Arrays.sort(levels); // Lower quality level ALWAYS comes before higher one

return levels;

}

// Methods implemented by JNI

private static native final void native_init();

private static native final int native_get_num_image_encoding_quality_levels();

private static native final int native_get_image_encoding_quality_level(int index);

}

存储jpeg quality参数的数组sJpegEncodingQualityParameters由函数

getImageEncodingQualityLevels()产生，在该函数中调用了JNI接口

native_get_num_image_encoding_quality_levels()获取了quality等级的数目，然后又通过native_get_image_encoding_quality_level(i)获取每个等级对应的值，并通过Arrays.sort(levels)对这些值进行了排序，最好返回给数组

sJpegEncodingQualityParameters。

这里动态加载了库media_jni，并且调用了三个JNI接口，下面我们对JNI层的相关code做以介绍。

二、 JNI层介绍

1、 JNI接口注册

在文件/framework/base/media/java/android/media/CameraProfile.java中，code

System.loadLibrary("media_jni");动态加载了库media_jni，同时会调用文件

/framework/base/media/jni/android_media_MediaPlayer.cpp中的函数JNI_OnLoad()。它负责注册android里的JNI接口。其具体code如下：

jint JNI_OnLoad(JavaVM* vm, void* reserved)

{

JNIEnv* env = NULL;

jint result = -1;

if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {

LOGE("ERROR: GetEnv failed/n");

goto bail;

}

assert(env != NULL);

if (register_android_media_MediaPlayer(env) < 0) {

LOGE("ERROR: MediaPlayer native registration failed/n");

goto bail;

}

if (register_android_media_MediaRecorder(env) < 0) {

LOGE("ERROR: MediaRecorder native registration failed/n");

goto bail;

}

if (register_android_media_MediaPhone(env) < 0) {

LOGE("ERROR: MediaPhone native registration failed/n");

goto bail;

}

if (register_android_media_MediaScanner(env) < 0) {

LOGE("ERROR: MediaScanner native registration failed/n");

goto bail;

}

if (register_android_media_MediaMetadataRetriever(env) < 0) {

LOGE("ERROR: MediaMetadataRetriever native registration failed/n");

goto bail;

}

#ifndef NO_OPENCORE

if (register_android_media_AmrInputStream(env) < 0) {

LOGE("ERROR: AmrInputStream native registration failed/n");

goto bail;

}

#endif

if (register_android_media_ResampleInputStream(env) < 0) {

LOGE("ERROR: ResampleInputStream native registration failed/n");

goto bail;

}

if (register_android_media_MediaProfiles(env) < 0) {

LOGE("ERROR: MediaProfiles native registration failed");

goto bail;

}

/* success -- return valid version number */

result = JNI_VERSION_1_4;

bail:

return result;

}

该函数注册了media相关的各大模块的JNI接口，其中包括MediaPlayer，MediaRecorder，MediaPhone等，当然也包括MediaProfiles。

函数register_android_media_MediaProfiles()被定义在文件

/framework/base/media/jni/android_media_MediaProfiles.cpp中。其具体code为：

int register_android_media_MediaProfiles(JNIEnv *env)

{

int ret1 = AndroidRuntime::registerNativeMethods(env,

kEncoderCapabilitiesClassPathName,

gMethodsForEncoderCapabilitiesClass,

NELEM(gMethodsForEncoderCapabilitiesClass));

int ret2 = AndroidRuntime::registerNativeMethods(env,

kCamcorderProfileClassPathName,

gMethodsForCamcorderProfileClass,

NELEM(gMethodsForCamcorderProfileClass));

int ret3 = AndroidRuntime::registerNativeMethods(env,

kDecoderCapabilitiesClassPathName,

gMethodsForDecoderCapabilitiesClass,

NELEM(gMethodsForDecoderCapabilitiesClass));

int ret4 = AndroidRuntime::registerNativeMethods(env,

kCameraProfileClassPathName,

gMethodsForCameraProfileClass,

NELEM(gMethodsForCameraProfileClass));

// Success if all return values from above are 0

return (ret1 || ret2 || ret3 || ret4);

}

与cameraprofiles相关的是ret4对应的函数，其中，kCameraProfileClassPathName定义为：

static const char* const kCameraProfileClassPathName =

"android/media/CameraProfile";

这代表什么意思，尚未完全清楚，暂时不写了。

数组gMethodsForCameraProfileClass包含了CameraProfile中所定义的所有JNI接口，其具体定义为：

static JNINativeMethod gMethodsForCameraProfileClass[] = {

{"native_init", "()V", (void *)android_media_MediaProfiles_native_init},

{"native_get_num_image_encoding_quality_levels",

"()I", (void *)android_media_MediaProfiles_native_get_num_image_encoding_quality_levels},

{"native_get_image_encoding_quality_level","(I)I", (void *)android_media_MediaProfiles_native_get_image_encoding_quality_level},

};

2、 JNI接口实现

上面注册的几个JNI接口的具体定义也在文件

/framework/base/media/jni/android_media_MediaProfiles.cpp中，其具体code 为：

MediaProfiles *sProfiles = NULL;

// This function is called from a static block in MediaProfiles.java class,

// which won't run until the first time an instance of this class is used.

static void

android_media_MediaProfiles_native_init(JNIEnv *env)

{

LOGV("native_init");

Mutex::Autolock lock(sLock);

if (sProfiles == NULL) {

sProfiles = MediaProfiles::getInstance();

}

static jint

android_media_MediaProfiles_native_get_num_image_encoding_quality_levels(JNIEnv *env, jobject thiz)

{

LOGV("native_get_num_image_encoding_quality_levels");

return sProfiles->getImageEncodingQualityLevels().size();

}

static jint

android_media_MediaProfiles_native_get_image_encoding_quality_level(JNIEnv *env, jobject thiz, jint index)

{

LOGV("native_get_image_encoding_quality_level");

Vector<int> levels = sProfiles->getImageEncodingQualityLevels();

if (index < 0 || index >= levels.size()) {

jniThrowException(env, "java/lang/IllegalArgumentException", "out of array boundary");

return -1;

}

return static_cast<jint>(levels[index]);

}

这里调用MediaProfiles的getInstance()和getImageEncodingQualityLevels()，这些函数如何实现，需要看看libmedia层的介绍了。

三、Libmedia层介绍

在文件/framework/base/media/libmedia/MediaProfiles.cpp中实现了MediaProfiles的getInstance()和getImageEncodingQualityLevels()，其code如下：

/*static*/ MediaProfiles*

MediaProfiles::getInstance()

{

LOGV("getInstance");

Mutex::Autolock lock(sLock);

if (!sIsInitialized) {

char value[PROPERTY_VALUE_MAX];

if (property_get("media.settings.xml", value, NULL) <= 0) {

const char *defaultXmlFile = "/etc/media_profiles.xml";

FILE *fp = fopen(defaultXmlFile, "r");

if (fp == NULL) {

LOGW("could not find media config xml file");

sInstance = createDefaultInstance();

} else {

fclose(fp); // close the file first.

sInstance = createInstanceFromXmlFile(defaultXmlFile);

}

} else {

sInstance = createInstanceFromXmlFile(value);

}

return sInstance;

}

该函数中判断，如果"/etc/media_profiles.xml"存在就调用函数

createInstanceFromXmlFile()去解析该xml文件，否则调用createDefaultInstance()创建默认的数值。

函数createDefaultInstance()的实现如下：

/*static*/ MediaProfiles*

MediaProfiles::createDefaultInstance()

{

MediaProfiles *profiles = new MediaProfiles;

createDefaultCamcorderProfiles(profiles);

createDefaultVideoEncoders(profiles);

createDefaultAudioEncoders(profiles);

createDefaultVideoDecoders(profiles);

createDefaultAudioDecoders(profiles);

createDefaultEncoderOutputFileFormats(profiles);

createDefaultImageEncodingQualityLevels(profiles);

sIsInitialized = true;

return profiles;

}

函数createDefaultImageEncodingQualityLevels()实现如下：

/*static*/ void

MediaProfiles::createDefaultImageEncodingQualityLevels(MediaProfiles *profiles)

{

profiles->mImageEncodingQualityLevels.add(70);

profiles->mImageEncodingQualityLevels.add(80);

profiles->mImageEncodingQualityLevels.add(90);

}

由此，可以看出，默认情况下，会将70，80，90做为jpeg quality的三个quality等级对应的数值。

如果xml文件存在，就需要看看函数createInstanceFromXmlFile()了，其定义为：

/*static*/ MediaProfiles*

MediaProfiles::createInstanceFromXmlFile(const char *xml)

{

FILE *fp = NULL;

CHECK((fp = fopen(xml, "r")));

XML_Parser parser = ::XML_ParserCreate(NULL);

CHECK(parser != NULL);

MediaProfiles *profiles = new MediaProfiles();

::XML_SetUserData(parser, profiles);

::XML_SetElementHandler(parser, startElementHandler, NULL);

FIXME:

expat is not compiled with -DXML_DTD. We don't have DTD parsing support.

if (!::XML_SetParamEntityParsing(parser, XML_PARAM_ENTITY_PARSING_ALWAYS)) {

LOGE("failed to enable DTD support in the xml file");

return UNKNOWN_ERROR;

}

const int BUFF_SIZE = 512;

for (;;) {

void *buff = ::XML_GetBuffer(parser, BUFF_SIZE);

if (buff == NULL) {

LOGE("failed to in call to XML_GetBuffer()");

delete profiles;

profiles = NULL;

goto exit;

}

int bytes_read = ::fread(buff, 1, BUFF_SIZE, fp);

if (bytes_read < 0) {

LOGE("failed in call to read");

delete profiles;

profiles = NULL;

goto exit;

}

CHECK(::XML_ParseBuffer(parser, bytes_read, bytes_read == 0));

if (bytes_read == 0) break; // done parsing the xml file

}

exit:

::XML_ParserFree(parser);

::fclose(fp);

if (profiles) {

sIsInitialized = true;

}

return profiles;

}

函数getImageEncodingQualityLevels()的定义为：

Vector<int> MediaProfiles::getImageEncodingQualityLevels() const

{

return mImageEncodingQualityLevels; // copy out

}

只需返回存储了jpeg quality数值的数组mImageEncodingQualityLevels就行了。

四、小结

通过研究jpeg quality的菜单，简单了解了JNI接口的注册和实现流程。虽然里面还有不少不大清楚地地方，但此记录可以帮助日后使用JNI接口时使用。

E-mail：wxiaozhe@163.com

QQ：1226062415
Date：2011/5/14
Blog：http://blog.csdn.net/wxzking

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