[gdal-dev] Bad gdalwarp interpolation

[Calogero Mauceri]

Thanks Javier for your reply.

I'd like to avoid changing the source file work around.
We are using gdalwarp C++ API as part of a general application that can
open any kind of file.



On Fri, Jul 26, 2024 at 12:26 PM Javier Jimenez Shaw <j1 at jimenezshaw.com>
wrote:

> Hi
>
> I do not know if there is anything wrong in GDAL. It looks like
>
> As a workaround, you can first convert the projection to be centred at
> lon=0 with something like this (I kept the same radius as the original to
> minimize distortion). You can add bilinear interpolation or not, as you
> wish.
> gdalwarp -t_srs '+proj=eqc +a=2439400 +b=2439400 +lon_0=0.000000
> +lat_0=0.000000 +x_0=0 +y_0=0 +units=m +no_defs'-overwrite
> msgr_dem_low_res.tif centred.tif
> And then cut with your command.
>
> I know it is not optimal because it is converting the full image. But the
> output is not blur.
>
> Cheers,
> Javier.
>
>
>
> On Fri, 26 Jul 2024 at 09:49, Calogero Mauceri via gdal-dev <
> gdal-dev at lists.osgeo.org> wrote:
>
>> Hi,
>>
>> I'm rendering a 256x256 tile from a mercury dem. I noticed the gdalwarp
>> is producing a bad output when using interpolation with the following
>> command
>>
>> gdalwarp -of PNG -r bilinear -t_srs '+proj=eqc +a=2439700 +b=2439700
>> +lon_0=0.000000 +lat_0=0.000000 +x_0=0 +y_0=0 +units=m +no_defs' -ts 256
>> 256 -te -958067.949620 -958067.950079 0.000460 0.000000 -overwrite
>> msgr_dem_low_res.tif tile.png
>>
>> The output image is kind of blurred/low resolution
>> [image: tile.png]
>>
>> On the other side, it looks good if nearest interpolation is used
>>
>> gdalwarp -of PNG -t_srs '+proj=eqc +a=2439700 +b=2439700 +lon_0=0.000000
>> +lat_0=0.000000 +x_0=0 +y_0=0 +units=m +no_defs' -ts 256 256 -te
>> -958067.949620 -958067.950079 0.000460 0.000000 -overwrite
>> msgr_dem_low_res.tif tile.png
>>
>> [image: tile.png]
>>
>>
>> The problem seems related to the fact that the right edge of the tile is
>> around longitude 0, while the source image is a full globe/360 degrees
>> image centered at 180 longitude
>>
>> If I reduce the tile size to 255x255 for avoiding the 0 longitude at the
>> edge, then the interpolated output file looks much better
>>
>> gdalwarp -of PNG -r bilinear -t_srs '+proj=eqc +a=2439700 +b=2439700
>> +lon_0=0.000000 +lat_0=0.000000 +x_0=0 +y_0=0 +units=m +no_defs' -ts 255
>> 255 -te -958067.949620 -950583.044220 -7484.905400 0.000000 -overwrite
>> msgr_dem_low_res.tif tile_255.png
>>
>> [image: tile_255.png]
>>
>> Am I doing something wrong or is this a bug in GDAL?
>>
>> I'm using the mercury dem from here
>> https://pdsimage2.wr.usgs.gov/Messenger/MESSDEM_1001/DEM/GLOBAL/IMG/
>> Here is a lower resolution of the same dem for testing purpose
>> https://
>> <https://pdsimage2.wr.usgs.gov/Messenger/MESSDEM_1001/DEM/GLOBAL/IMG/>
>> files.actgate.com/terrain/mercury/msgr_dem_low_res.tif
>>
>> Thanks in advance for your help,
>> Calogero Mauceri
>>
>> --
>> Calogero Mauceri
>> Software Engineer
>>
>> Applied Coherent Technology Corporation (ACT)
>> www.actgate.com
>> _______________________________________________
>> gdal-dev mailing list
>> gdal-dev at lists.osgeo.org
>> https://lists.osgeo.org/mailman/listinfo/gdal-dev
>>
>

-- 
Calogero Mauceri
Software Engineer

Applied Coherent Technology Corporation (ACT)
www.actgate.com
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