GDAL/OGR创建DXF文件中多图层的方法

在使用GDAL/OGR创建DXF，有时会将点、线、面数据分到到各自的图层中，或者同类别的要素、对象写到DXF文件中不同的图层。如下图：



有人认为新建多个OGRLayer就可以将类别分开，这种观点是错误的，OGRLayer和DXF中的图层不是一个概念。

在写DXF文件时，不支持创建多个ORGLayer图层，一个OGRDXFWriterDS对应一个OGRDXFWriterLayer在new OGRDXFWriterLayer时，OGRDXFWriterLayer构造函数中，会创建DXF标准字段也就是OGRFieldDefn，DXF不支持额外的新建字段：标准字段新建代码（在OGRDXFWriterLayer.cpp中第58-83行）：

// 图层字段，设置此字段，将要素分类到，所设置的图层中，不设置默认为0层
OGRFieldDefn  oLayerField( "Layer", OFTString );
poFeatureDefn->AddFieldDefn( &oLayerField );
// 好像未被使用，不管它
OGRFieldDefn  oClassField( "SubClasses", OFTString );
poFeatureDefn->AddFieldDefn( &oClassField );
// 好像未被使用，也不管它
OGRFieldDefn  oExtendedField( "ExtendedEntity", OFTString );
poFeatureDefn->AddFieldDefn( &oExtendedField );
// 线类型字段
OGRFieldDefn  oLinetypeField( "Linetype", OFTString );
poFeatureDefn->AddFieldDefn( &oLinetypeField );
// 好像未被使用，也不管它
OGRFieldDefn  oEntityHandleField( "EntityHandle", OFTString );
poFeatureDefn->AddFieldDefn( &oEntityHandleField );
//文本字段 好像未被使用，也不管它
OGRFieldDefn  oTextField( "Text", OFTString );
poFeatureDefn->AddFieldDefn( &oTextField );
// 块名称 写块时设置此字段
OGRFieldDefn  oBlockField( "BlockName", OFTString );
poFeatureDefn->AddFieldDefn( &oBlockField );
// 块缩放 写块时设置此字段
OGRFieldDefn  oScaleField( "BlockScale", OFTRealList );
poFeatureDefn->AddFieldDefn( &oScaleField );
// 块角度 写块时设置此字段
OGRFieldDefn  oBlockAngleField( "BlockAngle", OFTReal );
poFeatureDefn->AddFieldDefn( &oBlockAngleField );

下面我们做个试验，看以下创建DXF图层的方法，代码如下：

#include "ogrsf_frmts.h"
#include "stdio.h"
#include <iostream>
#include <string>
using namespace std;

int main()
{
const char *pszDriverName = "DXF";
OGRSFDriver *poDriver;

RegisterOGRDXF();

poDriver = OGRSFDriverRegistrar::GetRegistrar()->GetDriverByName(
pszDriverName );
if( poDriver == NULL )
{
printf( "%s driver not available.\n", pszDriverName );
exit( 1 );
}

OGRDataSource *poDS;
// 模板文件名称，文件在GDAL中data下，拷贝到与cpp同目录下
// 也可以设置GDAL_DATA路径
// 按下设置，也可以设置一个正确的相对路径和绝对路径
char **papszOptions = (char **)CPLCalloc(sizeof(char *),3);
papszOptions[0] = "HEADER=header.dxf";
papszOptions[1] = "TRAILER=trailer.dxf";
poDS = poDriver->CreateDataSource( "out.dxf", papszOptions );
CPLFree(papszOptions);
if( poDS == NULL )
{
printf( "Creation of output file failed.\n" );
exit( 1 );
}

OGRLayer *poLayer;

poLayer = poDS->CreateLayer( "hatch_out", NULL, wkbUnknown, NULL );
if( poLayer == NULL )
{
printf( "Layer creation failed.\n" );
exit( 1 );
}

OGRFeature *poFeature = OGRFeature::CreateFeature( poLayer->GetLayerDefn() );

// 生成一个正方形,放置在"正方形"图层
OGRLinearRing oSquare;
oSquare.addPoint(0.0,0.0);
oSquare.addPoint(1.0,0.0);
oSquare.addPoint(1.0,1.0);
oSquare.addPoint(0.0,1.0);
oSquare.closeRings();
poFeature->SetField( "Layer", "正方形" );
poFeature->SetGeometry( &oSquare );
if( poLayer->CreateFeature( poFeature ) != OGRERR_NONE )
{
printf( "Failed to create feature in dxffile.\n" );
exit( 1 );
}

// 生成一个三角形,放置在"三角形"图层
poFeature = OGRFeature::CreateFeature( poLayer->GetLayerDefn() );
OGRLinearRing oTriangle;
oTriangle.addPoint(2.0,2.0);
oTriangle.addPoint(3.0,2.0);
oTriangle.addPoint(3.0,3.0);
oTriangle.closeRings();
poFeature->SetField( "Layer", "三角形" );
poFeature->SetGeometry( &oTriangle );
if( poLayer->CreateFeature( poFeature ) != OGRERR_NONE )
{
printf( "Failed to create feature in dxffile.\n" );
exit( 1 );
}

// 生成一个菱形,放置在"三角形"图层
poFeature = OGRFeature::CreateFeature( poLayer->GetLayerDefn() );
OGRLinearRing oRhombus;
oRhombus.addPoint(4.0,3.0);
oRhombus.addPoint(5.0,4.0);
oRhombus.addPoint(4.0,5.0);
oRhombus.addPoint(3.0,4.0);
oRhombus.closeRings();
poFeature->SetField( "Layer", "三角形" );
poFeature->SetGeometry( &oRhombus );
if( poLayer->CreateFeature( poFeature ) != OGRERR_NONE )
{
printf( "Failed to create feature in dxffile.\n" );
exit( 1 );
}

// 生成一个多段线,不设置图层，将默认层
poFeature = OGRFeature::CreateFeature( poLayer->GetLayerDefn() );
OGRLinearRing oLinearRing;
oLinearRing.addPoint(5.0,5.0);
oLinearRing.addPoint(6.0,4.0);
oLinearRing.addPoint(7.0,5.0);
oLinearRing.addPoint(8.0,4.0);
poFeature->SetGeometry( &oLinearRing );
if( poLayer->CreateFeature( poFeature ) != OGRERR_NONE )
{
printf( "Failed to create feature in dxffile.\n" );
exit( 1 );
}

OGRFeature::DestroyFeature( poFeature );

OGRDataSource::DestroyDataSource( poDS );
}