[Gdal-dev] VRT format Schema

[Antoine Hue]

Le 22 mai 06, à 21:10, Frank Warmerdam a écrit :

> Antoine Hue wrote:
>> Since I got no answer to my question about an available DTD or Schema  
>> for the VRT format; and since I wanted to understand a bit more about  
>> the VRT, I have created a schema for it based on the HTML doc and  
>> based on code browsing.
>> Please find it attached as well as a picture generating from it by  
>> oXygen.
>
Figure is available from
http://ip666.org/~antoine/pti/v/etudes/gdal_vrt.png.html? 
g2_imageViewsIndex=1
(initially dropped by the Mailman)

>
>> 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?
>
>> - 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">)
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?

>
> 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++.
Schema may also make it possible to version VRT grammar and enable  
evolution of the format.

Antoine