12.4 调用QGIS空间分析算法(native,qgis,gdal)

前言

介绍在C++中如何调用各个平台(GDAL/QGIS/GRASS等)提供的算法，包含C++实现的算法和Python实现的算法
说明：文章中的示例代码均来自开源项目qgis_cpp_api_apps中子项目ll_qgis_process

算法相关的类

QgsProcessingRegistry

Registry for various processing components, including providers, algorithms and various parameters and outputs.

注册QgsProcessingProvider QgsProcessingAlgorithm等
QgsProcessingProvider

An algorithm provider is a set of related algorithms, typically from the same external application or related to a common area of analysis.

包含一组算法如native qgis gdal
QgsProcessingAlgorithm

Abstract base class for processing algorithms.

代表一个算法
算法运行的两个必要信息
QgsProcessingContext

Contains information about the context in which a processing algorithm is executed.

Contextual information includes settings such as the associated project, and expression context.

可以设置算法运行的关联信息如project crs等等
QgsProcessingFeedback

Base class for providing feedback from a processing algorithm.This base class implementation silently ignores all feedback reported by algorithms. Subclasses of QgsProcessingFeedback can be used to log this feedback or report it to users via the GUI.

算法运行的反馈信息

算法分类

常见的有native qgis gdal

native

native是以C++写的，以native:addxyfields为例

qgis

qgis是以python写的，以qgis:basicstatisticsforfields为例

gdal

gdal同样是python代码,以gdal:aspect为例

C++中如何调用python算法

在C++中调用python算法需要加载python解释器，在qgis中有一个lib库libqgispython.so实现了这个功能
python实现

添加libqgispython.so

以下分步骤说明增加python支持

在CMakeLists.txt增加qgispython

if (WITH_BINDINGS)
target_link_libraries(ll_qgis_process qgispython)
add_definitions(-DWITH_BINDINGS)
endif()
main中增加初始化

QgsProviderRegistry::instance( QgsApplication::pluginPath() );
MainWIndows initialize 初始化python

#ifdef WITH_BINDINGS
std::unique_ptr MainWindow::loadPythonSupport()
{
QString pythonlibName( QStringLiteral( "qgispython" ) );
#if defined(Q_OS_UNIX) && !defined(Q_OS_ANDROID)
pythonlibName.prepend( QgsApplication::libraryPath() );
#endif
#ifdef MINGW32
pythonlibName.prepend( "lib" );
#endif
QString version = QStringLiteral( "%1.%2.%3" ).arg( Qgis::versionInt() / 10000 ).arg( Qgis::versionInt() / 100 % 100 ).arg( Qgis::versionInt() % 100 );
QgsDebugMsgLevel( QStringLiteral( "load library %1 (%2)" ).arg( pythonlibName, version ), 1 );
QLibrary pythonlib( pythonlibName, version );
// It's necessary to set these two load hints, otherwise Python library won't work correctly
// see http://lists.kde.org/?l=pykde&m=117190116820758&w=2
pythonlib.setLoadHints( QLibrary::ResolveAllSymbolsHint | QLibrary::ExportExternalSymbolsHint );
if ( !pythonlib.load() )
{
pythonlib.setFileName( pythonlibName );
if ( !pythonlib.load() )
{
std::cerr << QStringLiteral( "Couldn't load Python support library: %1\n" ).arg( pythonlib.errorString() ).toLocal8Bit().constData();
return nullptr;
}
}
复制代码
```
 typedef QgsPythonUtils*( *inst )();
 inst pythonlib_inst = reinterpret_cast< inst >( cast_to_fptr( pythonlib.resolve( "instance" ) ) );
 if ( !pythonlib_inst )
 {
   //using stderr on purpose because we want end users to see this [TS]
   std::cerr << "Couldn't resolve Python support library's instance() symbol.\n";
   return nullptr;
 }

 std::unique_ptr< QgsPythonUtils > pythonUtils( pythonlib_inst() );
 if ( pythonUtils )
 {
   pythonUtils->initPython( nullptr, false );
 }

 return pythonUtils;
```
}
#endif
initPython函数为libqgispython.so库中的函数

void QgsPythonUtilsImpl::initPython( QgisInterface *interface, const bool installErrorHook, const QString &faultHandlerLogPath )
{
init();
if ( !checkSystemImports() )
{
exitPython();
return;
}

if ( !faultHandlerLogPath.isEmpty() )
{
runString( QStringLiteral( "import faulthandler" ) );
QString escapedPath = faultHandlerLogPath;
escapedPath.replace( '\', QLatin1String( "\\" ) );
escapedPath.replace( ''', QLatin1String( "\'" ) );
runString( QStringLiteral( "qgis.utils.__qgis_fault_handler_file_path='%1'" ).arg( escapedPath ) );
runString( QStringLiteral( "qgis.utils.__qgis_fault_handler_file=open('%1', 'wt')" ).arg( escapedPath ) );
runString( QStringLiteral( "faulthandler.enable(file=qgis.utils.__qgis_fault_handler_file)" ) );
mFaultHandlerLogPath = faultHandlerLogPath;
}

if ( interface )
{
// initialize 'iface' object
runString( QStringLiteral( "qgis.utils.initInterface(%1)" ).arg( reinterpret_cast< quint64 >( interface ) ) );
}

if ( !checkQgisUser() )
{
exitPython();
return;
}
doCustomImports();
if ( installErrorHook )
QgsPythonUtilsImpl::installErrorHook();
finish();
}

其中runString函数为执行python语句如
runString( QStringLiteral( "qgis.utils.initInterface(%1)" ).arg( reinterpret_cast< quint64 >( interface ) ) );
qgis.utils.initInterface即python代码

测试qgis:basicstatisticsforfields算法

QGis中调用

toolbox中打开算法，设置输入、输出
运行算法可以看到结果

二次开发调用

先查看算法帮助信息，可以使用qgsprocess 或者本文提供的界面化程序

设置参数可以从帮助信息中看到
运行算法完整代码如下

if(id.compare("qgis:basicstatisticsforfields") == 0)
{
//添加测试图层
QString filename = QStringLiteral("maps/shapefile/myplaces.shp");
QVariantMap conf;
conf.insert(QStringLiteral("INPUT_LAYER"),filename);//直接用路径
conf.insert(QStringLiteral("FIELD_NAME"),QStringLiteral("name"));
QgsProcessingOutputLayerDefinition value( "TEMPORARY_OUTPUT" );
conf.insert(QStringLiteral("OUTPUT_HTML_FILE"),value);
auto algorithm = QgsApplication::processingRegistry()->createAlgorithmById(id,conf);
QgsProcessingContext *context = new QgsProcessingContext;
context->setProject(QgsProject::instance());
QgsProcessingFeedback *feedback = new QgsProcessingFeedback(false);
QVariantMap runResults = algorithm->run(conf,*context,feedback);
QString htmlFilePath = runResults["OUTPUT_HTML_FILE"].toString();
statusBar()->showMessage(QStringLiteral("统计结果文件路径: %1").arg(htmlFilePath), 10000);
QDesktopServices::openUrl(QUrl::fromLocalFile(htmlFilePath));
}
效果如下图

测试gdal:aspect算法

QGis中调用

toolbox中打开算法，设置输入、输出
运行算法生成对应的坡向图层

二次开发中调用

先查看算法帮助信息，可以使用qgsprocess 或者本文提供的界面化程序
根据帮助信息确定参数parameters outputs等

运行算法即可完整代码如下

else if(id.compare("gdal:aspect") == 0)
{
QString filename = QStringLiteral("maps/raster/3420C_2010_327_RGB_LATLNG.tif");
QFileInfo ff(filename);
QgsRasterLayer* layer = (QgsRasterLayer*)mApp->addRasterLayer(filename,ff.baseName());

复制代码

      QVariantMap conf;
      conf.insert(QStringLiteral("INPUT"), layer->id());
      conf.insert(QStringLiteral("BAND"), 1);

      QgsProcessingOutputLayerDefinition value( "TEMPORARY_OUTPUT" );
      conf.insert(QStringLiteral("OUTPUT"), value);

      auto algorithm = QgsApplication::processingRegistry()->createAlgorithmById(id,conf);
      QgsProcessingContext *context = new QgsProcessingContext;
      context->setProject(QgsProject::instance());
      QgsProcessingFeedback *feedback = new QgsProcessingFeedback(false);

      QVariantMap runResults = algorithm->run(conf,*context,feedback);

      QString tempFilename = runResults["OUTPUT"].toString();
      QFileInfo t(tempFilename);
      QgsRasterLayer* tempLayer = (QgsRasterLayer*)mApp->addRasterLayer(tempFilename,t.baseName());
  }

效果如下

总结

介绍了如何在C++程序中调用python算法