[gdal-dev] GDAL_FORCE_CACHING - alters results?

[Daniel Evans]

Hi Evan,

That change in 2.4.0 looks like it's the answer. Looking at the returned data in detail, there is indeed a column of pixels which is duplicated (sampled twice) with the option set, and is replaced by an extra column on the right hand side without it set.

Thanks!

Daniel

-----Original Message-----
From: Even Rouault <even.rouault at spatialys.com>
Sent: 28 January 2020 12:03
To: gdal-dev at lists.osgeo.org
Cc: Daniel Evans <Daniel.Evans at jbarisk.com>
Subject: Re: [gdal-dev] GDAL_FORCE_CACHING - alters results?

On mardi 28 janvier 2020 11:49:01 CET Daniel Evans wrote:
> Hi all,
>
> I've recently tried enabling the GDAL_FORCE_CACHING option, and found
> it gives some significant improvements to TIFF/VRT read speeds.
>
> However, using GDAL 2.3.0, I've now found that when reading large
> VRTs, I can sometimes get subtly different values - up to about 1%
> summed difference across a 1000x1000 or so area. Is there a known
> issue in GDAL_FORCE_CACHING in this older version of GDAL, or
> alternatively have I misunderstood the use of this option and is this expected behaviour?

GDAL_FORCE_CACHING will force I/O requests to happen exactly on VRT block boundaries.
So what you observe is slight inconsistencies in results depending on the window passed to the VRT driver.

I see that 2.4.0 had a potentially related fix
"""VRT: fix source window computation that caused sub-pixel shift with non- nearest resampling"""

But if you use a warped VRT, then what you observe is somewhat expected.
gdalwarp due to approximations in some computations doesn't lead to "consistent" results if you change the output window.
See
https://lists.osgeo.org/pipermail/gdal-dev/2019-November/051177.html

Even

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