[gdal-dev] nodata value and alpha in Float32

[Even Rouault]

Javier,

mixing Alpha and NoData is generally not a good idea. Behavior will be 
different according to which part of the code is triggered. As far as I 
can see, the warping code should be able to handle both, but overview 
building will use the return of GDALRasterBand::GetMaskBand(), and when 
you have both nodata and alpha, this should be from the nodata value.

Confirmed by the following little experiment, so I'm not sure why you 
get what you get:

$ cat test.asc
ncols        2
nrows        2
xllcorner    440720.000000000000
yllcorner    3750120.000000000000
cellsize     60.000000000000
NODATA_value  -10000
  0.5 1.0
  2.5 -10000

$ cat test_alpha.asc
ncols        2
nrows        2
xllcorner    440720.000000000000
yllcorner    3750120.000000000000
cellsize     60.000000000000
NODATA_value  -10000
  255 255
  255 255

$ cat tmp.vrt
<VRTDataset rasterXSize="2" rasterYSize="2">
   <GeoTransform>  4.4072000000000000e+05, 6.0000000000000000e+01,  
0.0000000000000000e+00, 3.7513200000000000e+06,  0.0000000000000000e+00, 
-6.0000000000000000e+01</GeoTransform>
   <VRTRasterBand dataType="Float32" band="1">
     <NoDataValue>-10000</NoDataValue>
     <ComplexSource>
       <SourceFilename relativeToVRT="1">test.asc</SourceFilename>
       <SourceBand>1</SourceBand>
       <NODATA>-10000</NODATA>
     </ComplexSource>
   </VRTRasterBand>
   <VRTRasterBand dataType="Float32" band="2">
     <NoDataValue>-10000</NoDataValue>
     <ColorInterp>Alpha</ColorInterp>
     <ComplexSource>
       <SourceFilename relativeToVRT="1">test_alpha.asc</SourceFilename>
       <SourceBand>1</SourceBand>
       <NODATA>-10000</NODATA>
     </ComplexSource>
   </VRTRasterBand>
</VRTDataset>

$ gdal_translate  tmp.vrt test.tif

$ gdaladdo  -r average test.tif 2

$ gdal_translate test.tif /vsistdout/ -of aaigrid -b 1 -outsize 1 1
ncols        1
nrows        1
xllcorner    440720.000000000000
yllcorner    3751200.000000000000
cellsize     120.000000000000
NODATA_value  -10000
  1.3333333730697631836

Even

Le 31/05/2021 à 16:55, Javier Jimenez Shaw a écrit :
> Hi
>
> I have a GeoTIFF with this characteristics:
> - 6 bands
> - last band is "Alpha" (with values 0 or 255, nothing else)
> - Float32
> - NoData value = -10000
>
> The 5 first bands may have "nodata" pixels, not necessarily on all 
> bands simultaneously. (nodata pixels are usually broken or saturated 
> pixels from the sensor, and each band comes from a different sensor). 
> The valid pixels have values between 0 and 1
>
> When I try to generate embedded overviews (with average interpolation) 
> with C++ or CreateCopy as COG (with default values), looks like it is 
> using the -10000 values as valid values, and producing results in the 
> overviews like -9990.6... that obviously are not anymore considered 
> "nodata".
>
> The first noticeable effect opening the file in QGIS, is that the min 
> and max value for each band are not anymore 0 and 1, but nonsense 
> negative numbers.
>
> Can both Alpha and NoData live together properly?
>
> Thanks.
>
> PS I have the impression that with external ovr file it does not 
> happen. I am testing more.
> .___ ._ ..._ .. . ._. .___ .. __ . _. . __..  ... .... ._ .__
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> hay infinitos pensamientos irracionales.
>
>
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