[GRASS-dev] Raster data compression confusion: identical CELL file size

[Markus Metz]

On Tue, Sep 6, 2016 at 1:44 PM, Markus Neteler <neteler at osgeo.org> wrote:
> Hi,
>
> since my local drive was filled up again :) I checked raster how data are
> currently compressed in GRASS GIS 7.2.svn.
> According to
>
> https://grass.osgeo.org/grass72/manuals/r.compress.html#used-compression-algorithms
>
> all maps are DEFLATE compressed by default:
>
> "Raster maps are by default ZLIB compressed.
> ...
>
> Floating point (FCELL, DCELL) raster maps never use RLE compression; they
> are either compressed with ZLIB, LZ4, BZIP2 or are uncompressed.
> "
>
> Ehm, now how *are* FCELL, DCELL compressed by default? Not quite clear to
> me! This document needs improvements.

The manual says, as you cited:
"Raster maps are by default ZLIB compressed."
What exactly is unclear about this? Should it say "All raster maps ..." ?

>
> Reality check with Sentinel-2 data (3 different bands, same regional
> extent):
>
> GRASS 7.2.svn (utm37n):~ > r.info -r s2_20151225_B02_10m
> min=0
> max=22937
> GRASS 7.2.svn (utm37n):~ > r.info -r s2_20151225_B04_10m
> min=0
> max=18849
> GRASS 7.2.svn (utm37n):~ > r.info -r s2_20151225_B8A_20m
> min=0
> max=17210
>
> GRASS 7.2.svn (utm37n):~ >:~ > r.compress s2_20151225_B02_10m -p
> <s2_20151225_B02_10m> is compressed (method 2: ZLIB). Data type:
> CELL
>
> GRASS 7.2.svn (utm37n):~ >:~ > r.compress s2_20151225_B04_10m -p
> <s2_20151225_B04_10m> is compressed (method 2: ZLIB). Data type:
> CELL
>
> GRASS 7.2.svn (utm37n):~ >:~ > r.compress s2_20151225_B8A_20m -p
> <s2_20151225_B8A_20m> is compressed (method 2: ZLIB). Data type:
> CELL
>
> So far so nice. Now the suprising part, while the channels are not identical
> (obviously, since covering different spectral parts), the map sizes are
> identical!
>
> GRASS 7.2.svn (utm37n):~ > ls -la
> ...
> -rw-r--r--  1 neteler neteler 2539235691 Jun 14 17:14 s2_20151225_B02_10m
> -rw-r--r--  1 neteler neteler 2539235691 Jun 14 17:19 s2_20151225_B03_10m
> -rw-r--r--  1 neteler neteler 2539235691 Jun 14 17:25 s2_20151225_B04_10m
> -rw-r--r--  1 neteler neteler  634878630 Jun 14 20:36 s2_20151225_B05_20m
> -rw-r--r--  1 neteler neteler  634878630 Jun 14 20:37 s2_20151225_B06_20m
> -rw-r--r--  1 neteler neteler  634878630 Jun 14 20:39 s2_20151225_B07_20m
> -rw-r--r--  1 neteler neteler  634878630 Jun 14 20:40 s2_20151225_B11_20m
> -rw-r--r--  1 neteler neteler  634878630 Jun 14 20:42 s2_20151225_B12_20m
> -rw-r--r--  1 neteler neteler  634878630 Jun 14 20:43 s2_20151225_B8A_20m
>
> I would expect different sizes, compression can hardly lead to identical
> file sizes.

The default ZLIB compression level was invalid, causing ZLIB to not
compress at all. Fixed in r69387,8.

Markus M

>
> Next test: gzip the file
>
> GRASS 7.2.svn (utm37n):~/grassdata/utm37n/PERMANENT/cell > ls -la
> s2_20151225_B03_10m
> -rw-r--r-- 1 mneteler mneteler 2539235691 Jun 14 17:19 s2_20151225_B03_10m
>
> GRASS 7.2.svn (utm37n):~/grassdata/utm37n/PERMANENT/cell > gzip
> s2_20151225_B03_10m
>
> GRASS 7.2.svn (utm37n):~/grassdata/utm37n/PERMANENT/cell > ls -la
> s2_20151225_B03_10m.gz
> -rw-r--r-- 1 mneteler mneteler 1456248453 Jun 14 17:19
> s2_20151225_B03_10m.gz
>
> R
>> 1456248453/2539235691
> [1] 0.5734987
>
> Quite smaller! So I am not at all convinced that these CELL files are
> currently ZLIB compressed.

Compressing a whole file instead of compressing each row separately
(GRASS reads and writes raster data row by row) can lead to higher
compression ratios.

>
> From this ticket I would expect something else:
> https://trac.osgeo.org/grass/ticket/2349
>
> Ah, and no specific environment variables are set:
>
> GRASS 7.2.svn (utm37n):~ > echo $GRASS_<tab>
> $GRASS_ADDON_BASE   $GRASS_GNUPLOT    $GRASS_HTML_BROWSER  $GRASS_PAGER
> $GRASS_PROJSHARE    $GRASS_PYTHON     $GRASS_VERSION
>
>
> A bug?
>
> Markus
>
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