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Re: WarpedVRT and resampling ?

Sean Gillies
 

Hi Vincent,

Can you explain what processes produced those images? What program produced the blurry one and what program produced the non-blurry one? Which resampling algorithms were used?

Yes, you can introduce resampling in two places. I think it is not a good idea to do so: if you use the WarpedVRT, you should define its extents so that you don't need to use any resampling at all when you read from it. Also, I don't fully understand the edge effects when you resample in read().


On Fri, Apr 19, 2019 at 9:37 PM <vincent.sarago@...> wrote:
This was previously discussed on multiple Issues and PR: 
- https://github.com/mapbox/rasterio/issues/1206
- https://github.com/mapbox/rasterio/pull/1238
- https://github.com/mapbox/rasterio/pull/1239

While working with https://github.com/cogeotiff/rio-tiler on different data format (Int16, Float ...) I've encountered some strange behavior when using non-nearest resampling algorithm. 

## Contexte 

When working with `WarpedVRT` you can choose the resampling algorithm twice: 
```
with WarpedVRT(src, resampling=enums.Resampling.nearest) as vrt:
    arr = vrt.read(out_shape=(1, 512, 512, resampling=enums.Resampling.nearest))
```
Sean try to explain this in https://github.com/mapbox/rasterio/pull/1238#issuecomment-353123776

rio-tiler code: https://github.com/cogeotiff/rio-tiler/blob/master/rio_tiler/utils.py#L296-L321

## Behavior 

On the images attached we can see that there are `blurry` lines on the bottom. Those lines appear on the bottom of the tile returned by `rio-tiler` (bottom of the WarpedVRT returned value). 

Observation: 
- Only visible when working on Linux (https://github.com/RemotePixel/amazonlinux-gdal/tree/gdal2.4.0, or with rasterio wheels) 
- Only with Int16 data with negative NoData 
- Not visible when using gdalwarp directly
- Visible only when not using `resampling=enums.Resampling.nearest` in WarpedVRT params

I'm not 100% to understand what is going on and if I should or should not use `enums.Resampling.nearest`  as default resampling in WarpedVRT params so I'm looking for guidance here. 

Thanks,

Vincent 



--
Sean Gillies
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