[GRASS-dev] [bug #4452] (grass) r.patch makes mosaics slowly

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this bug's URL: http://intevation.de/rt/webrt?serial_num=4452
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Subject: r.patch makes mosaics slowly

r.patch currently takes current_region_rows * current_region_columns *
number_of_input_maps time to compile mosaics. It has some reasonable
optimizations for patching an existing large map with many small patches, but
is quite unsuited to mosaics.

For mosaics the fast thing to do would be to iterate over input maps (bounds
of input, resolution of current region), copying values to nulls in the
output. (and non-zero values to zeros or nulls in the output with the -z switch).

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