[gdal-dev] Does GDAL API for Python use <Scale> and <Offset> in VRT?

[Anton Korosov]

Hello everybody!

I'm trying to read values from a VRT file in Python.  When I modify 
<Scale> or <Offset> it seem not to make any effect.

I have a simple VRT of a Landsat7 file (below). It has neither scale nor 
offset. I launch the given script twice:
1st time to test the value of scale and the value from a band and to 
change the value of scale.
2nd time I test value of scale and value from a band. The output shows 
that the scale has changed, but the value is the same. I would expect 
that the second time the value should be 70.

Does it mean that ReadAsArray() doesn't interpret <Scale> and <Offset> 
tags? Ot do I do (expect) something wrong?

Thank you!
Anton



Script:
l7 = gdal.Open('L71211005_00520100824_B10.vrt')
print l7.GetRasterBand(1).GetScale()
print l7.GetRasterBand(1).ReadAsArray()[3000, 3000]
l7.GetRasterBand(1).SetScale(2)
l7.FlushCache()
l7 = None

Output:
 > python test.py
1.0
35
 > python test.py
2.0
35

VRT-file before test:
<VRTDataset rasterXSize="9211" rasterYSize="9021">
   <SRS>PROJCS["WGS 84 / UTM zone 33N",GEOGCS["WGS 
84",DATUM["WGS_1984",SPHEROID["WGS 
84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0],UNIT["degree",0.0174532925199433],AUTHORITY["EPSG","4326"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",15],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AUTHORITY["EPSG","32633"]]</SRS>
   <GeoTransform>  3.6178500000000000e+05,  3.0000000000000000e+01, 
0.0000000000000000e+00,  8.6874150000000000e+06, 
0.0000000000000000e+00, -3.0000000000000000e+01</GeoTransform>
   <Metadata>
     <MDI key="AREA_OR_POINT">Point</MDI>
   </Metadata>
   <VRTRasterBand dataType="Byte" band="1">
     <Metadata />
     <ColorInterp>Gray</ColorInterp>
     <SimpleSource>
       <SourceFilename 
relativeToVRT="1">L71211005_00520100824_B10.TIF</SourceFilename>
       <SourceBand>1</SourceBand>
       <SourceProperties RasterXSize="9211" RasterYSize="9021" 
DataType="Byte" BlockXSize="9211" BlockYSize="1" />
       <SrcRect xOff="0" yOff="0" xSize="9211" ySize="9021" />
       <DstRect xOff="0" yOff="0" xSize="9211" ySize="9021" />
     </SimpleSource>
   </VRTRasterBand>
</VRTDataset>

VRT-file after test:
<VRTDataset rasterXSize="9211" rasterYSize="9021">
   <SRS>PROJCS["WGS 84 / UTM zone 33N",GEOGCS["WGS 
84",DATUM["WGS_1984",SPHEROID["WGS 
84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0],UNIT["degree",0.0174532925199433],AUTHORITY["EPSG","4326"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",15],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AUTHORITY["EPSG","32633"]]</SRS>
   <GeoTransform>  3.6178500000000000e+05,  3.0000000000000000e+01, 
0.0000000000000000e+00,  8.6874150000000000e+06, 
0.0000000000000000e+00, -3.0000000000000000e+01</GeoTransform>
   <Metadata>
     <MDI key="AREA_OR_POINT">Point</MDI>
   </Metadata>
   <VRTRasterBand dataType="Byte" band="1">
     <Metadata />
     <Scale>2</Scale>
     <ColorInterp>Gray</ColorInterp>
     <SimpleSource>
       <SourceFilename 
relativeToVRT="1">L71211005_00520100824_B10.TIF</SourceFilename>
       <SourceBand>1</SourceBand>
       <SourceProperties RasterXSize="9211" RasterYSize="9021" 
DataType="Byte" BlockXSize="9211" BlockYSize="1" />
       <SrcRect xOff="0" yOff="0" xSize="9211" ySize="9021" />
       <DstRect xOff="0" yOff="0" xSize="9211" ySize="9021" />
     </SimpleSource>
   </VRTRasterBand>
</VRTDataset>