GDAL Virtual Format Tutorial

Introduction

The VRT driver is a format driver for GDAL that allows a virtual GDAL dataset to be composed from other GDAL datasets with repositioning, and algorithms potentially applied as well as various kinds of metadata altered or added. VRT descriptions of datasets can be saved in an XML format normally given the extension .vrt.

An example of a simple .vrt file referring to a 512x512 dataset with one band loaded from utm.tif might look like this:

<VRTDataset rasterXSize="512" rasterYSize="512">
  <GeoTransform>440720.0, 60.0, 0.0, 3751320.0, 0.0, -60.0</GeoTransform>
  <VRTRasterBand dataType="Byte" band="1">
    <ColorInterp>Gray</ColorInterp>
    <SimpleSource>
      <SourceFilename relativeToVRT="1">utm.tif</SourceFilename>
      <SourceBand>1</SourceBand>
      <SrcRect xOff="0" yOff="0" xSize="512" ySize="512"/>
      <DstRect xOff="0" yOff="0" xSize="512" ySize="512"/>
    </SimpleSource>
  </VRTRasterBand>
</VRTDataset>

Many aspects of the VRT file are a direct XML encoding of the GDAL Data Model which should be reviewed for understanding of the semantics of various elements.

VRT files can be produced by translating to VRT format. The resulting file can then be edited to modify mappings, add metadata or other purposes. VRT files can also be produced programmatically by various means.

This tutorial will cover the .vrt file format (suitable for users editing .vrt files), and how .vrt files may be created and manipulated programmatically for developers.

.vrt Format

Virtual files stored on disk are kept in an XML format with the following elements.

.vrt Descriptions for Raw Files

So far we have described how to derive new virtual datasets from existing files supports by GDAL. However, it is also common to need to utilize raw binary raster files for which the regular layout of the data is known but for which no format specific driver exists. This can be accomplished by writing a .vrt file describing the raw file.

For example, the following .vrt describes a raw raster file containing floating point complex pixels in a file called l2p3hhsso.img. The image data starts from the first byte (ImageOffset=0). The byte offset between pixels is 8 (PixelOffset=8), the size of a CFloat32. The byte offset from the start of one line to the start of the next is 9376 bytes (LineOffset=9376) which is the width (1172) times the size of a pixel (8).

<VRTDataset rasterXSize="1172" rasterYSize="1864">
  <VRTRasterBand dataType="CFloat32" band="1" subClass="VRTRawRasterBand">
    <SourceFilename relativetoVRT="1">l2p3hhsso.img</SourceFilename>
    <ImageOffset>0</ImageOffset>
    <PixelOffset>8</PixelOffset>
    <LineOffset>9376</LineOffset>
    <ByteOrder>MSB</ByteOrder>
  </VRTRasterBand>
</VRTDataset>

Some things to note are that the VRTRasterBand has a subClass specifier of "VRTRawRasterBand". Also, the VRTRasterBand contains a number of previously unseen elements but no "source" information. VRTRawRasterBands may never have sources (ie. SimpleSource), but should contain the following elements in addition to all the normal "metadata" elements previously described which are still supported.

A few other notes:

Another example, in this case a 400x300 RGB pixel interleaved image.

<VRTDataset rasterXSize="400" rasterYSize="300">
  <VRTRasterBand dataType="Byte" band="1" subClass="VRTRawRasterBand">
    <ColorInterp>Red</ColorInterp>
    <SourceFilename relativetoVRT="1">rgb.raw</SourceFilename>
    <ImageOffset>0</ImageOffset>
    <PixelOffset>3</PixelOffset>
    <LineOffset>1200</LineOffset>
  </VRTRasterBand>
  <VRTRasterBand dataType="Byte" band="2" subClass="VRTRawRasterBand">
    <ColorInterp>Green</ColorInterp>
    <SourceFilename relativetoVRT="1">rgb.raw</SourceFilename>
    <ImageOffset>1</ImageOffset>
    <PixelOffset>3</PixelOffset>
    <LineOffset>1200</LineOffset>
  </VRTRasterBand>
  <VRTRasterBand dataType="Byte" band="3" subClass="VRTRawRasterBand">
    <ColorInterp>Blue</ColorInterp>
    <SourceFilename relativetoVRT="1">rgb.raw</SourceFilename>
    <ImageOffset>2</ImageOffset>
    <PixelOffset>3</PixelOffset>
    <LineOffset>1200</LineOffset>
  </VRTRasterBand>
</VRTDataset>

Programatic Creation of VRT Datasets

The VRT driver supports several methods of creating VRT datasets. As of GDAL 1.2.0 the vrtdataset.h include file should be installed with the core GDAL include files, allowing direct access to the VRT classes. However, even without that most capabilities remain available through standard GDAL interfaces.

To create a VRT dataset that is a clone of an existing dataset use the CreateCopy() method. For example to clone utm.tif into a wrk.vrt file in C++ the following could be used:

  GDALDriver *poDriver = (GDALDriver *) GDALGetDriverByName( "VRT" );
  GDALDataset *poSrcDS, *poVRTDS;

  poSrcDS = (GDALDataset *) GDALOpenShared( "utm.tif", GA_ReadOnly );

  poVRTDS = 
    poDriver->CreateCopy( "wrk.vrt", poSrcDS, FALSE, NULL, NULL, NULL );
  delete poVRTDS;
  delete poSrcDS;

To create a virtual copy of a dataset with some attributes added or changed such as metadata or coordinate system that are often hard to change on other formats, you might do the following. In this case, the virtual dataset is created "in memory" only by virtual of creating it with an empty filename, and then used as a modified source to pass to a CreateCopy() written out in TIFF format.

  poVRTDS = poDriver->CreateCopy( "", poSrcDS, FALSE, NULL, NULL, NULL );

  poVRTDS->SetMetadataItem( "SourceAgency", "United States Geological Survey");
  poVRTDS->SetMetadataItem( "SourceDate", "July 21, 2003" );

  poVRTDS->GetRasterBand( 1 )->SetNoDataValue( -999.0 );

  GDALDriver *poTIFFDriver = (GDALDriver *) GDALGetDriverByName( "GTiff" );
  GDALDataset *poTiffDS;

  poTiffDS = 
    poTIFFDriver->CreateCopy( "wrk.tif", poVRTDS, FALSE, NULL, NULL, NULL );
  delete poTiffDS;

In this example a virtual dataset is created with the Create() method, and adding bands and sources programmatically, but still via the "generic" API. A special attribute of VRT datasets is that sources can be added to the bands by passing the XML describing the source into SetMetadata() on the special domain target "new_vrt_sources". The domain target "vrt_sources" may also be used, in which case any existing sources will be discarded before adding the new ones. In this example we construct a simple averaging filter source instead of using the simple source.

  // construct XML for simple 3x3 average filter kernel source.
  const char *pszFilterSourceXML  =
"<KernelFilteredSource>"
"  <SourceFilename>utm.tif</SourceFilename>1<SourceBand>1</SourceBand>"
"  <Kernel>"
"    <Size>3</Size>"
"    <Coefs>0.111 0.111 0.111 0.111 0.111 0.111 0.111 0.111 0.111</Coefs>"
"  </Kernel>"
"</KernelFilteredSource>";

  // Create the virtual dataset. 
  poVRTDS = poDriver->Create( "", 512, 512, 1, GDT_Byte, NULL );
  poVRTDS->GetRasterBand(1)->SetMetadataItem("source_0",pszFilterSourceXML",
                                             "new_vrt_sources");

A more general form of this that will produce a 3x3 average filtered clone of any input datasource might look like the following. In this case we deliberately set the filtered datasource as in the "vrt_sources" domain to override the SimpleSource created by the CreateCopy() method. The fact that we used CreateCopy() ensures that all the other metadata, georeferencing and so forth is preserved from the source dataset ... the only thing we are changing is the data source for each band.

  int   nBand;
  GDALDriver *poDriver = (GDALDriver *) GDALGetDriverByName( "VRT" );
  GDALDataset *poSrcDS, *poVRTDS;

  poSrcDS = (GDALDataset *) GDALOpenShared( pszSourceFilename, GA_ReadOnly );

  poVRTDS = poDriver->CreateCopy( "", poSrcDS, FALSE, NULL, NULL, NULL );

  for( nBand = 1; nBand <= poVRTDS->GetRasterCount(); nBand++ )
  {
      char szFilterSourceXML[10000];

      GDALRasterBand *poBand = poVRTDS->GetRasterBand( nBand );

      sprintf( szFilterSourceXML, 
        "<KernelFilteredSource>"
        "  <SourceFilename>%s</SourceFilename>1<SourceBand>%d</SourceBand>"
        "  <Kernel>"
        "    <Size>3</Size>"
        "    <Coefs>0.111 0.111 0.111 0.111 0.111 0.111 0.111 0.111 0.111</Coefs>"
        "  </Kernel>"
        "</KernelFilteredSource>", 
        pszSourceFilename, nBand );
        
      poBand->SetMetadataItem( "source_0", szFilterSourceXML, "vrt_sources" );
  }

Using Derived Bands

A specialized type of band is a 'derived' band which derives its pixel information from its source bands. With this type of band you must also specify a pixel function, which has the responsibility of generating the output raster. Pixel functions are created by an application and then registered with GDAL using a unique key.

Using derived bands you can create VRT datasets that manipulate bands on the fly without having to create new band files on disk. For example, you might want to generate a band using four source bands from a nine band input dataset (x0, x3, x4, and x8):

  band_value = sqrt((x3*x3+x4*x4)/(x0*x8));

You could write the pixel function to compute this value and then register it with GDAL with the name "MyFirstFunction". Then, the following VRT XML could be used to display this derived band:

<VRTDataset rasterXSize="1000" rasterYSize="1000">
  <VRTRasterBand dataType="Float32" band="1" subClass="VRTDerivedRasterBand">>
    <Description>Magnitude</Description>
    <PixelFunctionType>MyFirstFunction</PixelFunctionType>
    <SimpleSource>
      <SourceFilename relativeToVRT="1">nine_band.dat</SourceFilename>
      <SourceBand>1</SourceBand>
      <SrcRect xOff="0" yOff="0" xSize="1000" ySize="1000"/>
      <DstRect xOff="0" yOff="0" xSize="1000" ySize="1000"/>
    </SimpleSource>
    <SimpleSource>
      <SourceFilename relativeToVRT="1">nine_band.dat</SourceFilename>
      <SourceBand>4</SourceBand>
      <SrcRect xOff="0" yOff="0" xSize="1000" ySize="1000"/>
      <DstRect xOff="0" yOff="0" xSize="1000" ySize="1000"/>
    </SimpleSource>
    <SimpleSource>
      <SourceFilename relativeToVRT="1">nine_band.dat</SourceFilename>
      <SourceBand>5</SourceBand>
      <SrcRect xOff="0" yOff="0" xSize="1000" ySize="1000"/>
      <DstRect xOff="0" yOff="0" xSize="1000" ySize="1000"/>
    </SimpleSource>
    <SimpleSource>
      <SourceFilename relativeToVRT="1">nine_band.dat</SourceFilename>
      <SourceBand>9</SourceBand>
      <SrcRect xOff="0" yOff="0" xSize="1000" ySize="1000"/>
      <DstRect xOff="0" yOff="0" xSize="1000" ySize="1000"/>
    </SimpleSource>
  </VRTRasterBand>
</VRTDataset>

In addition to the subclass specification (VRTDerivedRasterBand) and the PixelFunctionType value, there is another new parameter that can come in handy: SourceTransferType. Typically the source rasters are obtained using the data type of the derived band. There might be times, however, when you want the pixel function to have access to higher resolution source data than the data type being generated. For example, you might have a derived band of type "Float", which takes a single source of type "CFloat32" or "CFloat64", and returns the imaginary portion. To accomplish this, set the SourceTransferType to "CFloat64". Otherwise the source would be converted to "Float" prior to calling the pixel function, and the imaginary portion would be lost.

<VRTDataset rasterXSize="1000" rasterYSize="1000">
  <VRTRasterBand dataType="Float32" band="1" subClass="VRTDerivedRasterBand">>
    <Description>Magnitude</Description>
    <PixelFunctionType>MyFirstFunction</PixelFunctionType>
    <SourceTransferType>"CFloat64"</SourceTransferType>
    ...

Writing Pixel Functions

To register this function with GDAL (prior to accessing any VRT datasets with derived bands that use this function), an application calls GDALAddDerivedBandPixelFunc with a key and a GDALDerivedPixelFunc:

    GDALAddDerivedBandPixelFunc("MyFirstFunction", TestFunction);

A good time to do this is at the beginning of an application when the GDAL drivers are registered.

GDALDerivedPixelFunc is defined with a signature similar to IRasterIO:

Parameters:
papoSources A pointer to packed rasters; one per source. The datatype of all will be the same, specified in the eSrcType parameter.
nSources The number of source rasters.
pData The buffer into which the data should be read, or from which it should be written. This buffer must contain at least nBufXSize * nBufYSize words of type eBufType. It is organized in left to right, top to bottom pixel order. Spacing is controlled by the nPixelSpace, and nLineSpace parameters.
nBufXSize The width of the buffer image into which the desired region is to be read, or from which it is to be written.
nBufYSize The height of the buffer image into which the desired region is to be read, or from which it is to be written.
eSrcType The type of the pixel values in the papoSources raster array.
eBufType The type of the pixel values that the pixel function must generate in the pData data buffer.
nPixelSpace The byte offset from the start of one pixel value in pData to the start of the next pixel value within a scanline. If defaulted (0) the size of the datatype eBufType is used.
nLineSpace The byte offset from the start of one scanline in pData to the start of the next.
Returns:
CE_Failure on failure, otherwise CE_None.
typedef CPLErr
(*GDALDerivedPixelFunc)(void **papoSources, int nSources, void *pData,
                        int nXSize, int nYSize,
                        GDALDataType eSrcType, GDALDataType eBufType,
                        int nPixelSpace, int nLineSpace);

The following is an implementation of the pixel function:

#include "gdal.h"

CPLErr TestFunction(void **papoSources, int nSources, void *pData,
                    int nXSize, int nYSize,
                    GDALDataType eSrcType, GDALDataType eBufType,
                    int nPixelSpace, int nLineSpace)
{

    int ii, iLine, iCol;
    double pix_val;
    double x0, x3, x4, x8;

    /* ---- Init ---- */
    if (nSources != 4) return CE_Failure;

    /* ---- Set pixels ---- */
    for( iLine = 0; iLine < nYSize; iLine++ ) {
        for( iCol = 0; iCol < nXSize; iCol++ ) {
            ii = iLine * nXSize + iCol;

            /* Source raster pixels may be obtained with SRCVAL macro */
            x0 = SRCVAL(papoSources[0], eSrcType, ii);
            x3 = SRCVAL(papoSources[1], eSrcType, ii);
            x4 = SRCVAL(papoSources[2], eSrcType, ii);
            x8 = SRCVAL(papoSources[3], eSrcType, ii);

            pix_val = sqrt((x3*x3+x4*x4)/(x0*x8));

            GDALCopyWords(&pix_val, GDT_Float64, 0,
                          ((GByte *)pData) + nLineSpace * iLine +
                          iCol * nPixelSpace, eBufType, nPixelSpace, 1);
        }
    }

    /* ---- Return success ---- */
    return CE_None;
}

Multi-threading issues

When using VRT datasets in a multi-threading environment, you should be careful to open the VRT dataset by the thread that will use it afterwards. The reason for that is that the VRT dataset uses GDALOpenShared when opening the underlying datasets. So, if you open twice the same VRT dataset by the same thread, both VRT datasets will share the same handles to the underlying datasets.

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