[postgis-devel] Fwd: [postgis-users] PostGIS Rasters Overview Factor

[Osahon Oduware]

Could anyone help with the questions below?

---------- Forwarded message ----------
From: Osahon Oduware <osahon.gis at gmail.com>
Date: Thu, May 11, 2017 at 11:00 AM
Subject: Re: [postgis-users] PostGIS Rasters Overview Factor
To: PostGIS Users Discussion <postgis-users at lists.osgeo.org>
Cc: David Haynes <haynesd2 at gmail.com>, Regina Obe <lr at pcorp.us>, Pierre
Racine <pierre.racine at sbf.ulaval.ca>


Hi David,

Thanks for your response. Sorry, I forgot to mention that I also performed
the loading using the tile flag *-t 250x250* using the syntax below:

raster2pgsql -s <SRID> -d -Y -e -I -C -F -M -l
2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536
/path/to/raster/file
-t 250x250 <table_name> | psql -h <host_address> -U postgres -p 5432 -d
<database>

I obtained the quoted information below from this link:
http://postgis.net/docs/manual-2.2/using_raster_dataman.html
#RT_Raster_Overviews

"overview_factor - this is the pyramid level of the overview table. The
higher the number the lower the resolution of the table. raster2pgsql if
given a folder of images, will compute overview of each image file and load
separately. Level 1 is assumed and always the original file. Level 2 is
will have each tile represent 4 of the original. So for example if you have
a folder of 5000x5000 pixel image files that you chose to chunk 125x125,
for each image file your base table will have (5000*5000)/(125*125) records
= 1600, your (l=2) o_2 table will have ceiling(1600/Power(2,2)) = 400 rows,*
your (l=3) o_3 will have ceiling(1600/Power(2,3) ) = 200 rows*. If your
pixels aren't divisible by the size of your tiles, you'll get some scrap
tiles (tiles not completely filled). Note that each overview tile generated
by raster2pgsql has the same number of pixels as its parent, but is of a
lower resolution where each pixel of it represents
(Power(2,overview_factor) pixels of the original)."

*Question 1*
The example given in the document above states that "*... *your (l=2) o_2
table will have ceiling(1600/Power(2,2)) = 400 rows,* your (l=3) o_3 will
have ceiling(1600/Power(2,3) ) = 200 rows ...." *It appears the *Power
*function
is being used internally to compute the number of rows for the overview
table based on the overview_factor i.e. Power(2,<overview_factor>). If this
is the case, *does it imply that the factors could be numbers other than
power of 2 (e.g. 3 in this case), but cannot be greater than 1,000 ?*

*Question 2*
After doing a bulk loading of *650 raster source files* with a tiling
option flag, *-t 250x250*, and overview option flag, *-l
2,4,8,16,32,64,128,256*  my raster table had *1,040,000 rows* and the
highest factor overview table, o_256_<table_name>, had *650 rows, *while
the lowest factor table, o_2_<table_name>, had *260,000 rows.*
Going by the example given in the document above, I would expect the
opposite of what I am getting i.e. the highest factor overview table with
more records (rows) than the lowest factor overview table. *What could be
responsible for this?*

*Question 3*
The document above states that "... If your pixels aren't divisible by the
size of your tiles, you'll get some scrap tiles (tiles not completely
filled) ...."
When I viewed the raster on QGIS (*see attached images*), I discovered that
I get black portions on a large portion of the image when completely zoomed
out. I only get to see the raster image when I zoom-in sufficiently (even
for black portions).
*What could be responsible for this black portions (see attached images) as
my pixels are obviously divisible by the tile size (250x250)?*

I would be glad if someone could help out with the above questions.




On Wed, May 10, 2017 at 5:12 PM, David Haynes <haynesd2 at gmail.com> wrote:

> You are correct overviews are powers based. Also it does not look like you
> specified tile size in your raster2pgsql statement
>
>
> raster2pgsql -s <SRID> -d -Y -e -I -C -F -M -t <pixels> x <pixels> -l
> 2,4,8,16,32
>
> On Wed, May 10, 2017 at 11:04 AM, Osahon Oduware <osahon.gis at gmail.com>
> wrote:
>
>> Hi All,
>>
>> I tried loading a raster with overviews using the raster2pgsql tool using
>> the syntax below:
>>
>> raster2pgsql -s <SRID> -d -Y -e -I -C -F -M -l
>> 2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536
>> /path/to/raster/file <table_name> | psql -h <host_address> -U postgres -p
>> 5432 -d <database>
>>
>> but it gave an error message stating that the overview factor cannot be
>> more than 1,000.
>>
>> I would like to know how the Overview-factor works. Must the value for
>> the Overview-factor be in Powers of 2 (i.e. 2,4,8,16,...)?
>>
>> I would be glad if someone could help my understanding.
>>
>> _______________________________________________
>> postgis-users mailing list
>> postgis-users at lists.osgeo.org
>> https://lists.osgeo.org/mailman/listinfo/postgis-users
>>
>
>
> _______________________________________________
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