[gdal-dev] gdal_translate -expand rgb incompatible with geotiff COPY_SRC_OVERVIEWS

[Even Rouault]

Le jeudi 30 décembre 2010 11:52:34, Vincent Schut a écrit :
> Hi all,
> 
> Kind of a corner case, but i was hoping that gdal_translate's -expand
> option (to expand colorindexed 1-band files into 3-band rgb files) would
> also copy and expand the overviews when run with the (geotiff specific,
> I know) create option "-co COPY_SRC_OVERVIEWS=YES". It appears to not do
> so, and just discards the overviews. Would this be a bug, or maybe a
> feature to add?

When you use -expand, an intermediate VRT file is created to do the color 
expension, but it doesn't retain the source overviews, so when the GeoTIFF 
driver copies the VRT into the final TIFF, there are no more source overviews 
from its point of view. "-co COPY_SRC_OVERVIEWS=YES" will only work if no 
intermediate VRT is used, so basically when you don't use any gdal_translate 
options, except creation options of the target driver.

> 
> Use case: for a classified, indexed image, I have built overviews using
> the 'mode' operator, which is obviously the most usefull for this kind
> of image, and only applicable on 1-band indexed data. I want to have a
> rgb version of this, but with those overviews, so the overviews should
> be an rgb version of the 'mode' overviews of the original. It is
> impossible to create the same overviews once the image has been expanded
> into rgb; mode will result in wrong, and sometimes even weird results (I
> presume mode works band-wise),

yes, mode works band per band

> and average and partners will result in
> colors that are not defined in the original colorindex (which I want to
> avoid, we are talking maps and legends here).
> 
> I think I can work around this by creating a separate file for each
> overview level (bla.ovr, bla.ovr.ovr, and so on), expanding those, and
> than using copy_src_overviews to create one file with internal overviews
> again, but this feels kind of awkward, and the above seemed so elegant...

yes that's a plausible way of proceeding. Making what you wish work would 
require :
1) add support for using/exposing source overviews in the VRT
2) add support for doing color expansion on overviews of the VRT

which I feel would be rather complicated to implement

> 
> Vincent.
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