[postgis-devel] ST_Rescale vs. ST_Resize

Sandro Santilli strk at keybit.net
Tue Jul 1 09:10:37 PDT 2014

I implemented automatic resizing of raster tiles in a mapnik plugin
for postgis raster that's under development. The initial implementation
has been using ST_Resize() passing as scale arguments the factor between
the raster scale and the target scale.

Ie, for target scales 0.6, the call was:

 ST_Resize(r, ST_ScaleX(r)/0.6, ST_ScaleY(r)/0.6)

That works very nicely, but then I found there's an ST_Rescale function
and tried to use it, to save some calls at the SQL level.

Unfortunately the ST_Rescale version:

 ST_Rescale(r, 0.6, -0.6)

Results in visible tile boundaries, subtle gaps between a tile and another,
as shown here: http://strk.keybit.net/tmp/pgrast-cdb-prescale-gaps.png

Is that expected or is it a bug ?
Should the two calls above give the same result or not ?



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