[gdal-dev] Any trick to run SELECT gdal_get_pixel_value for lots of points?

[Even Rouault]

Hi Jukka,

Recursive Common Table Expressions (CTE) (cf "3.1. Recursive Query 
Examples" of https://www.sqlite.org/lang_with.html) can help if you want 
to sample along one or two axis

Below an example for x in [1,3] and y in [1,4]

$ ogr2ogr -of CSV /vsistdout/ :memory: -sql "WITH RECURSIVE list_x(x) AS 
(VALUES(1) UNION ALL SELECT x+1 FROM list_x WHERE x<3), list_y(y) AS 
(VALUES(1) UNION ALL SELECT y+1 FROM list_y WHERE y<4) SELECT x, y, 
gdal_get_pixel_value('byte.tif', 1, 'pixel', x, y) as val FROM list_y, 
list_x" --config OGR_SQLITE_ALLOW_EXTERNAL_ACCESS YES
x,y,val
"1","1","132"
"2","1","107"
"3","1","123"
"1","2","132"
"2","2","140"
"3","2","132"
"1","3","132"
"2","3","123"
"3","3","123"
"1","4","156"
"2","4","132"
"3","4","140"

Otherwise if you have a list of (x,y) values in a OGR readable format 
(CSV, etc.), you could select the x, y values from it:

$ cat list_points.csv

x,y
1,2
3,4
5,6

$ ogr2ogr -of csv /vsistdout/ list_points.csv -sql "SELECT x, y, 
gdal_get_pixel_value('byte.tif', 1, 'pixel', cast(l.x as real), cast(l.y 
as real)) AS val FROM list_points l" -dialect SQLite --config 
OGR_SQLITE_ALLOW_EXTERNAL_ACCESS YES
x,y,"val"
"1","2","132"
"3","4","140"
"5","6","132"

Even

-- 
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