[mapserver-users] preparing orthos for Mapserver?

[Frank Warmerdam]

On Sun, Oct 6, 2013 at 12:41 AM, Andrea Aime
<andrea.aime at geo-solutions.it> wrote:
> On Sat, Oct 5, 2013 at 4:21 PM, Frank Warmerdam <warmerdam at pobox.com> wrote:
>>
>> Great advice from Jukka, and I'd just note that GDAL
>> *will* efficiently pull out reduced resolution images on
>> the fly without overviews if you have a file organized
>> with a one scanline block even in the year 2013.  For
>> map serving you are still much better off for several
>> reasons with the tiles+overviews approach, but I do
>> love the properly that a one-block-scanline file can be
>> quickly subsamled by viewer applications like OpenEV
>> and QGIS.
>
>
> Hi Frank,
> this is quite interesting. Can you elaborate a bit of what "one block
> scanline" means?

Andrea,

It means using a strip organized file with the strip one row long.
This can be accomplished with the BLOCKYSIZE creation option as
mentioned by Jukka.

> E.g., what is the actual tiling structure of this file?
> Does presence of compression affect the ability to do the subsampling?

Note that the sampling advantage is still only in the vertical.  A
high resolution horizontal scanline is still read and subsampled
conventionally.  It is not clear that compression will make any more
difference than normal.

> How about quality of the output? (e.g., is GDAL reading one pixel every N to
> generate
> the reduced resolution version?)

Well, the normal GDAL subsampling rules apply which is nearest
neighbour for simple windowed subsamples.  So building overviews gives
the advantage that you can apply a better rule, like averaging.

Best regards,
-- 
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