[gdal-dev] Discussing DTM compression in GeoTIFF

[Andreas Neumann]

Hi, 

I was reading through
https://kokoalberti.com/articles/geotiff-compression-optimization-guide/
(very interesting article) and I am wondering about how to interpret
read-speed vs compression. With this article I learned about the
relatively new zstd compression and LERC, which is esp. interesting for
LIDAR/DTM data in float32. 

In general, when you compress more, the read-speed decreases. So an
uncompressed file would read very fast. But isn't this gain in read
speed at the expense that you have to read a substantially higher amount
of data when you use uncompressed or low-compressed data? 

E.g. in the table
https://kokoalberti.com/articles/geotiff-compression-optimization-guide/read_results.png


when you read a float32 dtm with zstd compression, predictor 3 and
zstd_level 9, it shows a substantially lower read rate compared to
predictor 1 (not optimized for float32). But given that predictor 3
compresses substantially better than predictor 1 for float32, isn't the
read-rate "overall" just as good with predictor 3? 

Formulating my question different: isn't the read-rate at
https://kokoalberti.com/articles/geotiff-compression-optimization-guide/read_results.png
a bit distorted by the fact that higher-compression means that you also
have to read substantially smaller file sizes? Shouldn't this table with
the read-rate be kind of "normalized" with the file sizes it has to read
for exactly the same data? 

Currently, it just lists MB/CPU-sec. 

Thanks for the discussion, 

Andreas
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