[gdal-dev] gdalwarp performance with many GCPs

[Knut-Frode Dagestad]

Hi Jan,

That sounds interesting and promising.

For the mentioned file it takes about 2 minutes with "-et 5" (low 
accuracy), and 12 minutes without this switch. Without -tps it takes 
less than 0.5 seconds.
The machine is quite decent with 16 GB memory and SSD disk.

Best regards from Knut-Frode



On 12. des. 2012 13:24, Jan Hartmann wrote:
> Hi Knut,
>
> What do you mean by "very slow"? I regularly use gdalwarp -tps on much
> larger rasters with a few thousand gcp-s, and never noticed unacceptable
> delays. Do you have very little physical memory, or many parallel
> processes running?
>
> Jan
>
> On 12/12/2012 01:12 PM, Knut-Frode Dagestad wrote:
>> Hi list,
>>
>> When warping images with many GCPs, the -tps switch (Thin Plate
>> Spline) is found to be necessary to get decent accuracy. This makes
>> however warping very slow. The only method I found to increase speed
>> is the -et switch, but at cost of spatial accuracy.
>>
>>
>> Below some comments about the other tries which did not help:
>>
>> - compiling GDAL with armadillo support had no effect on speed.
>> Btw, to compile with armadillo I had to manually insert -llapack in
>> the following line in configure:
>> if test -z "`${CXX} testarmadillo.cpp -o testarmadillo -larmadillo
>> 2>&1`" ; then .....
>> Perhaps ${LIBS} should be added permanently to this line in trunk?
>>
>> - compiling GDAL with OpenCL had also no effect. At first surprising,
>> but looking at the opencl warp kernel it seems that it only makes a
>> difference for other resampling algorithms than nearest neighbour?
>>
>> - Increasing  memory with -wm had no effect
>>
>> - Using several threads (-multi -wo NUM_THREADS=ALL_CPUS) actually
>> increased computing time significantly (my proj version is 4.7.1).
>>
>> From the debug output, a lot of the time is apparently spent on:
>> "WARP: Copying metadata from first source to destination dataset"
>> Is this an indication that much time is simply spent on reading and
>> writing the GCPs to/from file? If so, we could perhaps expect improved
>> performance if Geolocation Arrays could be used instead (not possible
>> due to http://trac.osgeo.org/gdal/ticket/4907).
>>
>>
>> The tests are made with the following file and command on Ubuntu:
>> http://dl.dropbox.com/u/15885758/testgcp.tif  (3212 GCPs and 2048x2511
>> pixels covering Southern Europe)
>>
>> time gdalwarp --debug on -et 5 -tps -t_srs '+proj=merc' testgcp.tif
>> out.tif  (+ other swithces mentioned above)
>>
>>
>> Best regards from Knut-Frode
>>
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>
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