<div dir="ltr"><div>Thank Even.</div><div><br></div><div></div><div>I think I found the reason of the problem. It is reproduced with more bands. I created a second band just swapping the values in the diagonal.</div><div>Then, adding the second band to the tmp.vrt (2 normal + 1 alpha), and following your steps, it is noticeable in the second band of the output:<br></div><div><br></div><div>$ cat test2.asc</div><div>ncols 2<br>nrows 2<br>xllcorner 440720.000000000000<br>yllcorner 3750120.000000000000<br>cellsize 60.000000000000<br>NODATA_value -10000<br> -10000 1.0<br> 2.5 0.5</div><div><br></div><div>$ cat tmp.vrt <br><VRTDataset rasterXSize="2" rasterYSize="2"><br> <GeoTransform> 4.4072000000000000e+05, 6.0000000000000000e+01, 0.0000000000000000e+00, 3.7513200000000000e+06, 0.0000000000000000e+00, -6.0000000000000000e+01</GeoTransform><br> <VRTRasterBand dataType="Float32" band="1"><br> <NoDataValue>-10000</NoDataValue><br> <ComplexSource><br> <SourceFilename relativeToVRT="1">test.asc</SourceFilename><br> <SourceBand>1</SourceBand><br> <NODATA>-10000</NODATA><br> </ComplexSource><br> </VRTRasterBand><br> <VRTRasterBand dataType="Float32" band="2"><br> <NoDataValue>-10000</NoDataValue><br> <ComplexSource><br> <SourceFilename relativeToVRT="1">test.2.asc</SourceFilename><br> <SourceBand>1</SourceBand><br> <NODATA>-10000</NODATA><br> </ComplexSource><br> </VRTRasterBand><br> <VRTRasterBand dataType="Float32" band="3"><br> <NoDataValue>-10000</NoDataValue><br> <ColorInterp>Alpha</ColorInterp><br> <ComplexSource><br> <SourceFilename relativeToVRT="1">test_alpha.asc</SourceFilename><br> <SourceBand>1</SourceBand><br> <NODATA>-10000</NODATA><br> </ComplexSource><br> </VRTRasterBand><br></VRTDataset></div><div><br></div><div>$ gdal_translate test.tif /vsistdout/ -of aaigrid -b 2 -outsize 1 1<br>ncols 1<br>nrows 1<br>xllcorner 440720.000000000000<br>yllcorner 3751200.000000000000<br>cellsize 120.000000000000<br>NODATA_value -10000<br> -3332.166748046875</div><div><br></div><div>I have not looked at the code, but seems that the mask is requested only for the first band. Then the pixels in the other bands that are nodata, but are valid in the first band are computed as valid... using -10000 in this case. Looks like requesting the mask per band would fix it, right?</div><div><br></div><div></div><div>If this is the case, the alpha band is not the problem (confirmed removing it from the tmp.vrt file).<br></div><div><br></div><div>Cheers<br></div><div><div><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">.___ ._ ..._ .. . ._. .___ .. __ . _. . __.. ... .... ._ .__<br>Entre dos pensamientos racionales <br>hay infinitos pensamientos irracionales.<br><br></div></div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, 31 May 2021 at 17:28, Even Rouault <<a href="mailto:even.rouault@spatialys.com">even.rouault@spatialys.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<p>Javier,</p>
<p>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.</p>
<p>Confirmed by the following little experiment, so I'm not sure why
you get what you get:<br>
</p>
<p>$ cat test.asc<br>
ncols 2<br>
nrows 2<br>
xllcorner 440720.000000000000<br>
yllcorner 3750120.000000000000<br>
cellsize 60.000000000000<br>
NODATA_value -10000<br>
0.5 1.0<br>
2.5 -10000</p>
<p>$ cat test_alpha.asc <br>
ncols 2<br>
nrows 2<br>
xllcorner 440720.000000000000<br>
yllcorner 3750120.000000000000<br>
cellsize 60.000000000000<br>
NODATA_value -10000<br>
255 255<br>
255 255</p>
<p>$ cat tmp.vrt <br>
<VRTDataset rasterXSize="2" rasterYSize="2"><br>
<GeoTransform> 4.4072000000000000e+05,
6.0000000000000000e+01, 0.0000000000000000e+00,
3.7513200000000000e+06, 0.0000000000000000e+00,
-6.0000000000000000e+01</GeoTransform><br>
<VRTRasterBand dataType="Float32" band="1"><br>
<NoDataValue>-10000</NoDataValue><br>
<ComplexSource><br>
<SourceFilename
relativeToVRT="1">test.asc</SourceFilename><br>
<SourceBand>1</SourceBand><br>
<NODATA>-10000</NODATA><br>
</ComplexSource><br>
</VRTRasterBand><br>
<VRTRasterBand dataType="Float32" band="2"><br>
<NoDataValue>-10000</NoDataValue><br>
<ColorInterp>Alpha</ColorInterp><br>
<ComplexSource><br>
<SourceFilename
relativeToVRT="1">test_alpha.asc</SourceFilename><br>
<SourceBand>1</SourceBand><br>
<NODATA>-10000</NODATA><br>
</ComplexSource><br>
</VRTRasterBand><br>
</VRTDataset><br>
<br>
</p>
<div>$ gdal_translate tmp.vrt test.tif<br>
</div>
<div><br>
</div>
<div>$ gdaladdo -r average test.tif 2</div>
<div><br>
</div>
<div>$ gdal_translate test.tif /vsistdout/
-of aaigrid -b 1 -outsize 1 1<br>
ncols 1<br>
nrows 1<br>
xllcorner 440720.000000000000<br>
yllcorner 3751200.000000000000<br>
cellsize 120.000000000000<br>
NODATA_value -10000<br>
1.3333333730697631836<br>
</div>
<div><br>
</div>
<div>Even<br>
</div>
<div><br>
</div>
<div>Le 31/05/2021 à 16:55, Javier Jimenez
Shaw a écrit :<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div>Hi</div>
<div><br>
</div>
<div>I have a GeoTIFF with this characteristics:</div>
<div>- 6 bands</div>
<div>- last band is "Alpha" (with values 0 or 255, nothing else)<br>
</div>
<div>- Float32</div>
<div>- NoData value = -10000</div>
<div><br>
</div>
<div>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</div>
<div><br>
</div>
<div>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".</div>
<div><br>
</div>
<div>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.<br>
</div>
<div><br>
</div>
<div>Can both Alpha and NoData live together properly?</div>
<div><br>
</div>
<div>Thanks.</div>
<div><br>
</div>
<div>PS I have the impression that with external ovr file it
does not happen. I am testing more.<br>
</div>
<div>
<div>
<div>
<div dir="ltr">.___ ._ ..._ .. . ._.
.___ .. __ . _. . __.. ... .... ._ .__<br>
Entre dos pensamientos racionales <br>
hay infinitos pensamientos irracionales.<br>
<br>
</div>
</div>
</div>
</div>
</div>
<br>
<fieldset></fieldset>
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