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

Thu May 11 06:31:46 PDT 2017

Osahon,

1)      The overview factors do not have to be factors of 2.

2)      You get fewer rows the higher the factor because the factor gives you lower resolution.

So if your original number of rows (tiles) = 1000

An overview factor of 3 = 1000/Power(2,3)  = 1000/(2*2*2) = 125 rows

3)      If you tiled your main raster in 250x250 then all factors have tile sizes 250x250, but the higher rasters are packing more of the original tiles (geometry space)  in each tile because they are lower resolution.

So in the over view factor =2 case – each tile contains 4 tiles of the original raster (but at 1/4 the resolution).

4)      You get scrap tiles because each tile must maintain the same size but includes more than 1 tile from original, but must contain a whole tile (not a half tile for example)

So lets say you had not 1000 tiles but 1001 tiles, that last tile of your over view 3 would have only 1 tile of the original and the remaining 7 slots would be filled in with no data filler

so that the tile size of 250x250

From: Osahon Oduware [mailto:osahon.gis at gmail.com] 
Sent: Thursday, May 11, 2017 6:37 AM
To: PostGIS Development Discussion <postgis-devel at lists.osgeo.org>; Regina Obe <lr at pcorp.us>; Pierre Racine <pierre.racine at sbf.ulaval.ca>
Subject: Fwd: [postgis-users] PostGIS Rasters Overview Factor

Could anyone help with the questions below?

---------- Forwarded message ----------
From: Osahon Oduware <osahon.gis at gmail.com <mailto: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 <mailto:postgis-users at lists.osgeo.org> >
Cc: David Haynes <haynesd2 at gmail.com <mailto:haynesd2 at gmail.com> >, Regina Obe <lr at pcorp.us <mailto:lr at pcorp.us> >, Pierre Racine <pierre.racine at sbf.ulaval.ca <mailto: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 <mailto: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 <mailto: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.

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