[gdal-dev] Discrepancy in utilizing overviews with gdalwarp

[Rahkonen Jukka]

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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