[Gdal-dev] VRT format Schema

[Even Rouault]

Benoît,

yes this XSD idea apparently did not go further than this dicussion. The best 
description of the VRT format is to be found on 
http://www.gdal.org/gdal_vrttut.html

You can try gdal_translate -of VRT or gdalbuildvrt to have examples of valid 
VRT files.

For your use case, "gdalbuildvrt -separate out.vrt in1.tif in2.tif in3.tif" 
should generate what you expect.

Best regards,

Even

> Hi list !
>
> I need to merge three tif files, each one having a single Float32 band.
> I want each file to use a separated band of the final tif.
>
> I think the best way to do this is to use the VRT driver.
> I would have liked to not use the C# binding or the native C++ library.
> I would prefer to create a file on the disk using C# serialization.
>
> I am so searching for the schema of the VRT format and I've found this
> quite old thread. I saw the XSD in one of the messages of the topic. Is it
> correct ? It is mentionned too that the XSD should be in the CVS/SVN in
> gdal/frmts/vrt but I've not found it, I assume the idea has been
> abandonned.
>
> I am also interested in any other suggestion ! ^^
>
> Thanks in advance.
>
> Regards,
> Benoit Andrieu
>
> Antoine Hue wrote:
> >>>/  There are still a few dark zones and area to improves:
>
> />>>/  - ComplexSource and AverageSource need a complete definition
> />/  Any information about these sources? They do not appear in the VRT tut
> />/  and I did not get there purpose/functioning.
> />>>/  - Better definition of the subClass property of the VRTRasterBand
> />>>/  needed.
> />>/
> />>/  What did you need to know?  It seems that values include
> />>/  "VRTWarpedDataset",
> />>/  "VRTDerivedRasterBand" and "VRTRawRasterBand".
> />/  OK, I will add this. What is the impact this parameter on the VRT
> />/  processing?
> /
> Antoine,
>
> It determines what elements are legal on the VRTRasterBand and
> internally it tells GDAL which particular class to instantiate.
> This "subClass" attribute approach does not really lend itself to
> describing the grammar with XML schema as far as I know.
>
> >>>/  - An explicit definition of the raw raster source (to go with
>
> />>>/  SimpleSource, KernelFilteredSource,..) would improve homogeneity.
> />>/
> />>/  I'm not sure what you mean. Isn't the ".vrt Descriptions for Raw
> Files" />>/  on the gdal_vrttut.html page appropriate?
> /  >
>
> >/  For example, Simple source are described in the<SimpleSource>  tag.
>
> />/  However, there is no<RawRasterSource>   tag but a set of tags specific
> />/  to the raw raster band (<VRTRasterBand subClass="VRTRawRasterBand">)
> /
> That is correct, there is no RawRasterSource or equivelent.  Raw, warped
> and "derived" data can only be implemented as complete bands, not sources
> on a sourced band.
>
> >/  This actually appears in the schema:
>
> />/
> />/  <!-- Raster Band -->
> />/  <xsd:complexType name="vrt_raster_band_t">
> />/      ....
> />/          <xsd:choice maxOccurs="unbounded">
> />/              <xsd:element name="SimpleSource" type="simple_source_t" />
> />/              <xsd:element name="KernelFilteredSource"
> />/  type="kernel_source_t" />
> />/              <xsd:element name="ComplexSource" />  <!-- Need refinement
> --> />/              <xsd:element name="AveragedSource" />  <!-- Need
> refinement --> />/              <xsd:sequence>
> />/                  <!-- Raw source -->
> />/                  <xsd:element name="SourceFilename"
> />/  type="source_file_name_t"/>
> />/                  <xsd:element name="ImageOffset"
> />/  type="xsd:nonNegativeInteger" />
> />/                  <xsd:element name="PixelOffset"
> />/  type="xsd:nonNegativeInteger" />
> />/                  <xsd:element name="LineOffset"
> />/  type="xsd:nonNegativeInteger" />
> />/                  <xsd:element name="ByteOrder" type="byte_order_t" />
> />/              </xsd:sequence>
> />/          </xsd:choice>...
> />/  </xsd:complexType>
> />/
> />/  One more question about it, the extract above allows mixed type
> sources />/  within a raster band. Is this correct?
> /
> A "sourced" raster band (the default kind of VRTRasterBand) can have
> a mixture of different sources.
>
> >>/  Were you thinking of the .xsd file as being primarily useful for
>
> />>/  automated validation of .vrt files?  As educational?
> />/  Why not both?
> />/  Schema is more extensive than the tutorial and more readable than C++.
> />/  It may also simplify C++ by checking validity of the VRT in place of
> the />/  C++.
> /
> As I mention, I think I have approached the .vrt files in a way that is
> not conveniently expressed in XML Schema and I would contend that the
> schema files are not all that readable as a description.
>
> >/  Schema may also make it possible to version VRT grammar and enable
>
> />/  evolution of the format.
> /
> Well, keep in mind that the code is using a schema supporting parser
> of any kind, so the .xsd files aren't systematically useful to GDAL
> itself.  As for versioning, I anticipate handling this in an adhoc
> fashion, primarily by just extending the format rather than changing
> existing items.
>
> Best regards,