先来一句话，看了这么多GDAL的源代码，并不喜欢其C风格的烙印太重，还是更喜欢boost风格的简洁的现代C++风格。不过为了更好地应用GDAL，更深的定制它，还是需要将源代码看到底。因为GDAL毕竟是一个很好的图像处理的解决方案。复用它，可以省掉很多人年的工作。
GDALOpen函数代码：注释值得一看
/************************************************************************/
/* GDALOpen() */
/************************************************************************/
/**
* rief Open a raster file as a GDALDataset.
*
* This function will try to open the passed file, or virtual dataset
* name by invoking the Open method of each registered GDALDriver in turn.
* The first successful open will result in a returned dataset. If all
* drivers fail then NULL is returned and an error is issued.
*
* Several recommandations :
* <ul>
* <li>If you open a dataset object with GA_Update access, it is not recommanded
* to open a new dataset on the same underlying file.</li>
* <li>The returned dataset should only be accessed by one thread at a time. If you
* want to use it from different threads, you must add all necessary code (mutexes, etc.)
* to avoid concurrent use of the object. (Some drivers, such as GeoTIFF, maintain internal
* state variables that are updated each time a new block is read, thus preventing concurrent
* use.) </li>
* </ul>
*
* sa GDALOpenShared()
*
* @param pszFilename the name of the file to access. In the case of
* exotic drivers this may not refer to a physical file, but instead contain
* information for the driver on how to access a dataset. It should be in UTF8
* encoding.
*
* @param eAccess the desired access, either GA_Update or GA_ReadOnly. Many
* drivers support only read only access.
*
* @return A GDALDatasetH handle or NULL on failure. For C++ applications
* this handle can be cast to a GDALDataset *.
*/
GDALDatasetH CPL_STDCALL
GDALOpen( const char * pszFilename, GDALAccess eAccess )
{
return GDALOpenInternal(pszFilename, eAccess, NULL);
}
注释提示了几个地方：
1. 会依次调用每个已经注册的driver的open函数，第一个成功的会返回Dataset。这个依次应该是按照注册顺序，先注册的driver先被调用。
2. Dataset对象不是线程安全的，使用者自己注意维护多线程环境下的安全性。
3. 返回NULL代表打开失败
GDALDatasetH实际上是个void* , 又是C的玩法。逃过了编译器类型检查。
/** Opaque type used for the C bindings of the C++ GDALDataset class */ typedef void *GDALDatasetH;
实际上就是CDALDataset* 指针。
真正实现代码在下面的函数里面：
/* The drivers listed in papszAllowedDrivers can be in any order */
/* Only the order of registration will be taken into account */
GDALDatasetH GDALOpenInternal( const char * pszFilename, GDALAccess eAccess,
const char* const * papszAllowedDrivers)
{
VALIDATE_POINTER1( pszFilename, "GDALOpen", NULL );
int iDriver;
GDALDriverManager *poDM = GetGDALDriverManager();
GDALOpenInfo oOpenInfo( pszFilename, eAccess );
CPLLocaleC oLocaleForcer;
CPLErrorReset();
CPLAssert( NULL != poDM );
for( iDriver = 0; iDriver < poDM->GetDriverCount(); iDriver++ )
{
GDALDriver *poDriver = poDM->GetDriver( iDriver );
GDALDataset *poDS;
if (papszAllowedDrivers != NULL &&
CSLFindString((char**)papszAllowedDrivers, GDALGetDriverShortName(poDriver)) == -1)
continue;
if ( poDriver->pfnOpen == NULL )
continue;
poDS = poDriver->pfnOpen( &oOpenInfo );
if( poDS != NULL )
{
if( strlen(poDS->GetDescription()) == 0 )
poDS->SetDescription( oOpenInfo.pszFilename );
if( poDS->poDriver == NULL )
poDS->poDriver = poDriver;
if( CPLGetPID() != GDALGetResponsiblePIDForCurrentThread() )
CPLDebug( "GDAL", "GDALOpen(%s, this=%p) succeeds as %s (pid=%d, responsiblePID=%d).",
pszFilename, poDS, poDriver->GetDescription(),
(int)CPLGetPID(), (int)GDALGetResponsiblePIDForCurrentThread() );
else
CPLDebug( "GDAL", "GDALOpen(%s, this=%p) succeeds as %s.",
pszFilename, poDS, poDriver->GetDescription() );
return (GDALDatasetH) poDS;
}
if( CPLGetLastErrorNo() != 0 )
return NULL;
}
if( oOpenInfo.bStatOK )
CPLError( CE_Failure, CPLE_OpenFailed,
"`%s' not recognised as a supported file format.
",
pszFilename );
else
CPLError( CE_Failure, CPLE_OpenFailed,
"`%s' does not exist in the file system,
"
"and is not recognised as a supported dataset name.
",
pszFilename );
return NULL;
}
这段就和注释说的一样，遍历driver，依次尝试打开文件。在我的GeoTiff driver中，open方法会调用geotiff.cpp文件的Open方法：
/************************************************************************/
/* Open() */
/************************************************************************/
GDALDataset *GTiffDataset::Open( GDALOpenInfo * poOpenInfo )
{
TIFF *hTIFF;
int bAllowRGBAInterface = TRUE;
const char *pszFilename = poOpenInfo->pszFilename;
/* -------------------------------------------------------------------- */
/* Check if it looks like a TIFF file. */
/* -------------------------------------------------------------------- */
if (!Identify(poOpenInfo))
return NULL;
if( EQUALN(pszFilename,"GTIFF_RAW:", strlen("GTIFF_RAW:")) )
{
bAllowRGBAInterface = FALSE;
pszFilename += strlen("GTIFF_RAW:");
}
/* -------------------------------------------------------------------- */
/* We have a special hook for handling opening a specific */
/* directory of a TIFF file. */
/* -------------------------------------------------------------------- */
if( EQUALN(pszFilename,"GTIFF_DIR:",strlen("GTIFF_DIR:")) )
return OpenDir( poOpenInfo );
if (!GTiffOneTimeInit())
return NULL;
/* -------------------------------------------------------------------- */
/* Try opening the dataset. */
/* -------------------------------------------------------------------- */
if( poOpenInfo->eAccess == GA_ReadOnly )
hTIFF = VSI_TIFFOpen( pszFilename, "r" );
else
hTIFF = VSI_TIFFOpen( pszFilename, "r+" );
if( hTIFF == NULL )
return( NULL );
/* -------------------------------------------------------------------- */
/* Create a corresponding GDALDataset. */
/* -------------------------------------------------------------------- */
GTiffDataset *poDS;
poDS = new GTiffDataset();
poDS->SetDescription( pszFilename );
poDS->osFilename = pszFilename;
poDS->poActiveDS = poDS;
if( poDS->OpenOffset( hTIFF, &(poDS->poActiveDS),
TIFFCurrentDirOffset(hTIFF), TRUE,
poOpenInfo->eAccess,
bAllowRGBAInterface, TRUE,
poOpenInfo->papszSiblingFiles) != CE_None )
{
delete poDS;
return NULL;
}
/* -------------------------------------------------------------------- */
/* Initialize any PAM information. */
/* -------------------------------------------------------------------- */
poDS->TryLoadXML();
poDS->ApplyPamInfo();
int i;
for(i=1;i<=poDS->nBands;i++)
{
GTiffRasterBand* poBand = (GTiffRasterBand*) poDS->GetRasterBand(i);
/* Load scale, offset and unittype from PAM if available */
if (!poBand->bHaveOffsetScale)
{
poBand->dfScale = poBand->GDALPamRasterBand::GetScale(&poBand->bHaveOffsetScale);
poBand->dfOffset = poBand->GDALPamRasterBand::GetOffset();
}
if (poBand->osUnitType.size() == 0)
{
const char* pszUnitType = poBand->GDALPamRasterBand::GetUnitType();
if (pszUnitType)
poBand->osUnitType = pszUnitType;
}
}
poDS->bMetadataChanged = FALSE;
poDS->bGeoTIFFInfoChanged = FALSE;
/* -------------------------------------------------------------------- */
/* Check for external overviews. */
/* -------------------------------------------------------------------- */
poDS->oOvManager.Initialize( poDS, pszFilename );
return poDS;
}
这里主要看poDs->OpenOffset函数，它负责读取tiff文件的基本信息，以便后面快速读取。因为这么大的文件，显然不可能一次读进内存来。
/************************************************************************/
/* OpenOffset() */
/* */
/* Initialize the GTiffDataset based on a passed in file */
/* handle, and directory offset to utilize. This is called for */
/* full res, and overview pages. */
/************************************************************************/
CPLErr GTiffDataset::OpenOffset( TIFF *hTIFFIn,
GTiffDataset **ppoActiveDSRef,
toff_t nDirOffsetIn,
int bBaseIn, GDALAccess eAccess,
int bAllowRGBAInterface,
int bReadGeoTransform,
char** papszSiblingFiles )
该函数的定义在geotiff.cpp文件中，非常长，所以这里就不列代码了。
如果要完整的将GeoTiff整个加载过程分析透，需要更多的篇幅。我以后会不断的修改已经有的文章，使得其更准确，并增加新的文章，描述更多的细节。
欢迎讨论。