James,
It all depends on how software will deal with overviews. Some might
explicitly have logic where it inspects which overviews are present and
decide which one to use. If we are talking about a client like QGIS that
uses the RasterIO() method on the full resolution band with a window of
interest (nXOff, nYOff, nXSize, nYSize) and a target buffer size
(nBufXSize, nBufYSize), GDAL computes a target downscaling factor for
that which is the min(nXSize/nBufXSize, nYSize/nBufYSize). It then
walks through overviews and will select the frist one whose downscaling
factor is greater than target_dowscanling_factor / 1.2
If you have overviews at [2,4,8,16]
target_downscaling_factor selected_overview
------------------------- --------------------------
]0, 2. /1.2[ full resolution band
[2. / 1.2, 4. / 1.2[ 2x downsampled
[4. / 1.2, 8. / 1.2[ 4x downsampled
[8. / 1.2, 16. / 1.2[ 8x downsampled
[16. / 1.2, inf[ 16x downsampled
If you just have overview [16]
target_downscaling_factor selected_overview
------------------------- --------------------------
]0, 16. /1.2[ full resolution band
[16. / 1.2, inf[ 16x downsampled
I've submitted in https://github.com/OSGeo/gdal/pull/8526 a doc
improvement capturing above explanations
Even
Le 08/10/2023 à 00:01, James Sammone via gdal-dev a écrit :
I'm not sure if this is the best channel to ask this question as it
might be beyond the scope, but I've asked it in a few others and have
had no responses aside from others also being curious.
I am trying to understand the relationship between overviews and zoom
levels so I know how to make more efficient Cloud Optimized GeoTIFFs.
Using gdaladdo or gdal.BuildOverviews(), I can create overviews at
[2,4,8,16] or at just [16]. From my understanding, this means the size
is being divided by those values to provide downsample arrays of the
original source. In the first example [2,4,8,16], I've created 4
separate overview arrays into the GeoTIFF that are 2x, 4x, 8x, and 16x
downsampled. And in the second example using only [16], I've built one
overview array into the GeoTIFF that is 16x downsampled.
How can I understand how these overviews are applied when it comes to
zoom levels? Does the 16x downsample appear sooner in the second
example when zooming out than for the first example due to being first
in order? Or do the 16x downsamples appear at the same zoom level for
both cases but the second example has additional 2x, 4x, and 8x
downsamples that also appear before getting there?
Thanks for any insight into this anyone can provide. Despite using
overviews all the time, I've struggled with this for a while and had
largely consigned to not understanding it.
Best,
James
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