[gdal-dev] Expected runtime of polygonize (GDAL 3.9.0) for few very large features.

[Even Rouault]

Hi,

I've got a chance to have a look at your test dataset. In 
https://github.com/OSGeo/gdal/pull/10477, I've reduced the runtime to 8 
minutes (with GeoParquet output, without spatial sorting), by optimizing 
some implementation details. I believe this could be further reduced as 
most of the time is still spent in malloc/free of temporary objects (the 
output is 90 million polygons!) and some objects could be reused, but 
that would be more extensive changes

Even

Le 01/07/2024 à 18:40, Meyer, Jesse R. (GSFC-618.0)[SCIENCE SYSTEMS AND 
APPLICATIONS INC] via gdal-dev a écrit :
>
> Hi,
>
> We’ve encountered a few images with what seems like pathological 
> performance problems with polygonise. The details below are a report 
> from another developer that I haven’t yet independently verified.
>
> We threshold a raster image to a binary mask in a memory dataset, use 
> that as its own mask to mask out the background.
>
> gdal.Polygonize(nn_mem_band, nn_mem_band, ogr_mem_lyr, -1)
>
> We have a number of 32k x 32k raster images that feature number of 
> very large same-valued regions (some as large as 80% of the entire 
> raster).  We’re seeing ~10hrs on a modern workstation to complete the 
> line of code above.  OpenCV can apparently construct a connected 
> components list in mere seconds, on the same workstation and image, so 
> we’re considering constructing the OGR geometries directly from those 
> as a temporary work around.
>
> Is this situation a known pitfall with the current algorithm / data 
> structures behind Polygonize?
>
> I’m able to share the problematic tile(s) if of interest,
>
> Best
>
> Jesse
>
>
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