Android平台上从Camera的jpegquality菜单研究JNI接口(三)

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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