[gdal-dev] Discrepancy in utilizing overviews with gdalwarp

[Even Rouault]

Filed as https://github.com/OSGeo/gdal/issues/10174

Le 10/06/2024 à 10:57, Denis Rykov via gdal-dev a écrit :
> Thank you, Jukka, for reproducing the behavior and explaining the 
> reason behind it!
>
> On Mon, Jun 10, 2024 at 9:19 AM Rahkonen Jukka 
> <jukka.rahkonen at maanmittauslaitos.fi> wrote:
>
>     Hi,
>
>     Those two commands behave in quite different ways. With “-oo
>     OVERVIEW_LEVEL=1” the output size is the size of the second
>     overview. In my test case the original has 12000 x 12000 pixels
>     and the command creates a 3000 x 3000 sized output
>
>     GDAL: GDALOverviewDataset(vrttest.vrt, this=000001CB3D728890)
>     creation.
>
>     GDAL: Using GTiff driver
>
>     GDAL: Computing area of interest: 26.8805, 63.2011, 27, 63.255
>
>     Creating output file that is 3000P x 3000L.
>
>     …
>
>     GDAL: GDALWarpKernel()::GWKNearestNoMasksOrDstDensityOnlyByte()
>     Src=0,0,3000x3000 Dst=0,0,3000x3000
>
>     ...10...20...30...40...50...60...70...80...90...100 - done.
>
>     With -ovr 1 the output has the size of the full resolution image,
>     12000 x 12000 pixels, even the data is read from the 3000 x 3000
>     overview. The pixels will be upsampled.
>
>     GDAL: Using GTiff driver
>
>     GDAL: Computing area of interest: 26.8805, 63.2011, 27, 63.255
>
>     Creating output file that is 12000P x 12000L.
>
>     WARP: Selecting overview level 1 for vrttest.vrt
>
>     …
>
>     GDAL: GDALWarpKernel()::GWKNearestNoMasksOrDstDensityOnlyByte()
>     Src=0,0,1500x750 Dst=0,0,6000x3000
>
>     ...10.GDAL:
>     GDALWarpKernel()::GWKNearestNoMasksOrDstDensityOnlyByte()
>     Src=1500,0,1500x750 Dst=6000,0,6000x3000
>
>     ..20..GDAL:
>     GDALWarpKernel()::GWKNearestNoMasksOrDstDensityOnlyByte()
>     Src=0,750,1500x750 Dst=0,3000,6000x3000
>
>     .30...GDAL:
>     GDALWarpKernel()::GWKNearestNoMasksOrDstDensityOnlyByte()
>     Src=1500,750,1500x750 Dst=6000,3000,6000x3000
>
>     40...50GDAL:
>     GDALWarpKernel()::GWKNearestNoMasksOrDstDensityOnlyByte()
>     Src=0,1500,1500x750 Dst=0,6000,6000x3000
>
>     ...60.GDAL:
>     GDALWarpKernel()::GWKNearestNoMasksOrDstDensityOnlyByte()
>     Src=1500,1500,1500x750 Dst=6000,6000,6000x3000
>
>     ..70..GDAL:
>     GDALWarpKernel()::GWKNearestNoMasksOrDstDensityOnlyByte()
>     Src=0,2250,1500x750 Dst=0,9000,6000x3000
>
>     .80...GDAL:
>     GDALWarpKernel()::GWKNearestNoMasksOrDstDensityOnlyByte()
>     Src=1500,2250,1500x750 Dst=6000,9000,6000x3000
>
>     90...100 - done.
>
>     GDAL: Flushing dirty blocks:
>     0...10...20...30...40...50...60...70...80...90...100 - done.
>
>     The behavior is the same when using a TIFF with internal overviews
>     as input instead of VRT. Obviously the -ovr option has been
>     designed to force the use of some better-quality overview as
>     source data when warping yields a smaller, downsampled image. The
>     forced upsampling does not make much sense to me. I wonder if
>     gdalwarp should even allow that?
>
>     The gdal_translate utility has also -ovr option
>     https://gdal.org/programs/gdal_translate.html#cmdoption-gdal_translate-ovr.
>     It behaves in a different way  when it comes to the size of the
>     output, but with gdal_translate it makes sense.
>
>     “When -ovr is specified to an integer value, and none of -outsize
>     and -tr is specified, the size of the overview will be used as the
>     output size.”
>
>     Maybe the thing to do is to improve the documentation of the
>     gdalwarp option
>     https://gdal.org/programs/gdalwarp.html#cmdoption-gdalwarp-ovr and
>     mention that this option does not have on effect on the size of
>     the output.
>
>     -Jukka Rahkonen-
>
>     *Lähettäjä:* gdal-dev <gdal-dev-bounces at lists.osgeo.org> *Puolesta
>     *Denis Rykov via gdal-dev
>     *Lähetetty:* maanantai 10. kesäkuuta 2024 2.27
>     *Vastaanottaja:* gdal dev <gdal-dev at lists.osgeo.org>
>     *Aihe:* [gdal-dev] Discrepancy in utilizing overviews with gdalwarp
>
>     I'd anticipate that both commands would yield the same outcome:
>
>     $ gdalwarp -oo OVERVIEW_LEVEL=1 -dstalpha input.vrt ovr1.tif
>     $ gdalwarp -ovr 1 -dstalpha input.vrt ovr1.tif
>
>     However, the reality is different. The first command behaves as
>     intended, utilizing overviews from |*.vrt.ovr|, while the second
>     command operates noticeably slower, seemingly disregarding the
>     overviews.
>
>
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