I should have mentioned, this is using GDAL 2.4.1 on macOS. On Tue, 27 Aug 2019 at 03:16, John Baumgartner <[email protected]> wrote:
> Hi all > > I'm a bit confused by the difference in resampling behaviour seen when > using gdal_translate vs gdalwarp, when faced with ncol or nrow that is not > divisible by the aggregation factor. > > For example, consider this 9 cell raster (attached, hopefully) with values > 1 through 9, with resolution 1x1: > > 1 2 3 > 4 5 6 > 7 8 9 > > "gdalwarp in.tif out.tif -r average -tr 2 2" gives cell values 3.0, 4.5, > 7.5, 9.0, while "gdal_translate in.tif out.tif -r average -tr 2 2" gives 3, > 4, 6, 7. > > It seems that gdal_translate reuses cells in the calculation (i.e. reuses > the middle row and middle column, calculating averages of 1,2,4,5 then > 2,3,5,6 then 4,5,7,8 then 5,6,8,9). In contrast, gdalwarp calculates > averages of 1,2,4,5 then 3,6 then 7,8 then 9 (i.e. dividing the raster into > tr x tr blocks where possible, and using fewer data where necessary along > right & bottom edges. > > Things get weirder in some cases. For this 2x5 raster: > > 01 02 03 04 05 > 06 07 08 09 10 > > gdal_translate gives 4, 5.5, 7, seemingly calculating averages of 1,2,6,7 > then 3,8 then 4,5,9,10. > > And for this 2x7 raster: > > 01 02 03 04 05 06 07 > 08 09 10 11 12 13 14 > > gdal_translate gives 5.0, 6.5, 8.5, 10.0. Maybe I'm misreading things, but > is it calculating averages of 1,2,8,9 then 3,10 then 5,12 and then > 6,7,13,14... and ignoring 4,11? > > Is this behaviour documented anywhere? Can it be easily controlled? I > guess an option is to pad the raster with nodata first - can this be done > within the same gdal_translate call? > > Cheers > John > > > > >
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