Changes between Version 2 and Version 3 of rfc11_fastidentify


Ignore:
Timestamp:
Apr 29, 2007, 2:29:15 PM (17 years ago)
Author:
warmerdam
Comment:

add various materia.

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  • rfc11_fastidentify

    v2 v3  
    4040}}}
    4141
     42Internally GDALIdentifyDriver() will do the following
     43
     44 1. A GDALOpenInfo structure will be initialized based on pszDatasource and papszDirFiles.
     45 2. It will iterate over all drivers similarly to GDALOpen().  For each driver it will use the pfnIdentify function if available, otherwise it will use the pfnOpen() method to establish if the driver supports the file.
     46 3. It will return the driver handle for the first driver to respond positively or NULL if none accept it.
     47
     48== Driver Changes ==
     49
     50In theory it is not necessary for any drivers to be modified, since GDALIdentifyDriver() will fallback to using the pfnOpen function to test.  But in practice, no optimization is achieved unless at least some drivers (hopefully those for which Open can be very expensive) are updated.  Part of the ongoing effort then is to implement identify functions for GDAL drivers.
     51
     52Generally speaking it should be easy to craft an identify function from the initial test logic in the open function.  For instance, the GeoTIFF driver might be changed like this:
     53
     54{{{
     55int GTiffDataset::Identify( GDALOpenInfo * poOpenInfo )
     56
     57{
     58/* -------------------------------------------------------------------- */
     59/*      We have a special hook for handling opening a specific          */
     60/*      directory of a TIFF file.                                       */
     61/* -------------------------------------------------------------------- */
     62    if( EQUALN(poOpenInfo->pszFilename,"GTIFF_DIR:",10) )
     63        return TRUE;
     64
     65/* -------------------------------------------------------------------- */
     66/*      First we check to see if the file has the expected header       */
     67/*      bytes.                                                          */   
     68/* -------------------------------------------------------------------- */
     69    if( poOpenInfo->nHeaderBytes < 2 )
     70        return FALSE;
     71
     72    if( (poOpenInfo->pabyHeader[0] != 'I' || poOpenInfo->pabyHeader[1] != 'I')
     73        && (poOpenInfo->pabyHeader[0] != 'M' || poOpenInfo->pabyHeader[1] != 'M'))
     74        return FALSE;
     75
     76    // We can't support BigTIFF files for now.
     77    if( poOpenInfo->pabyHeader[2] == 43 && poOpenInfo->pabyHeader[3] == 0 )
     78        return FALSE;
     79 
     80
     81    if( (poOpenInfo->pabyHeader[2] != 0x2A || poOpenInfo->pabyHeader[3] != 0)
     82        && (poOpenInfo->pabyHeader[3] != 0x2A || poOpenInfo->pabyHeader[2] != 0) )
     83        return FALSE;
     84
     85    return TRUE;
     86}
     87}}}
     88
     89The open might then be modified to use the identify function to avoid duplicating the test logic.
     90
     91{{{
     92GDALDataset *GTiffDataset::Open( GDALOpenInfo * poOpenInfo )
     93
     94{
     95    TIFF        *hTIFF;
     96
     97    if( !Identify( poOpenInfo ) )
     98        return NULL;
     99
     100/* -------------------------------------------------------------------- */
     101/*      We have a special hook for handling opening a specific          */
     102/*      directory of a TIFF file.                                       */
     103/* -------------------------------------------------------------------- */
     104    if( EQUALN(poOpenInfo->pszFilename,"GTIFF_DIR:",10) )
     105        return OpenDir( poOpenInfo->pszFilename );
     106
     107    GTiffOneTimeInit();
     108...
     109}}}
     110
     111Drivers which require header files such as the EHdr driver might implement Identify() like this:
     112
     113{{{
     114int EHdrDataset::Identify( GDALOpenInfo * poOpenInfo )
     115
     116{
     117    int         i, bSelectedHDR;
     118    const char  *pszHDRFilename;
     119   
     120/* -------------------------------------------------------------------- */
     121/*      We assume the user is pointing to the binary (ie. .bil) file.   */
     122/* -------------------------------------------------------------------- */
     123    if( poOpenInfo->nHeaderBytes < 2 )
     124        return FALSE;
     125
     126/* -------------------------------------------------------------------- */
     127/*      Now we need to tear apart the filename to form a .HDR           */
     128/*      filename.                                                       */
     129/* -------------------------------------------------------------------- */
     130    CPLString osBasename = CPLGetBasename( poOpenInfo->pszFilename );
     131    pszHDRFilename = CPLFormCIFilename( "", osBasename, "hdr" );
     132
     133    if( CSLFindString( poOpenInfo->papszSiblingFiles, pszHDRFilename) )
     134        return TRUE;
     135    else
     136        return FALSE;
     137}
     138}}}
     139
     140During the initial implementation a variety of drivers will be updated, including the following.  As well some performance and file system activity logging will be done to identify drivers that are currently expensive.
     141
     142 * HFA
     143 * GTiff
     144 * JPEG
     145 * PNG
     146 * GIF
     147 * HDF4
     148 * DTED
     149 * USGS DEM
     150 * MrSID
     151 * JP2KAK
     152 * ECW
     153 * EHdr
     154