[gdal-dev] gdal_translate vs gdalwarp aggregation behaviour

[John Baumgartner]

I should have mentioned, this is using GDAL 2.4.1 on macOS.

On Tue, 27 Aug 2019 at 03:16, John Baumgartner <johnbaums at gmail.com> 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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