[Gdal-dev] viewing gdaladdo overlays

[Benjamin Wragg]

Frank,

Thanks for the quick response. Yes I want to check that the overviews
we've built look OK. I'm currently building overviews using GDAL to
increase raster serving performance in Mapserver. The source raster we
are serving is a high resolution digital copy of a map which is
published on paper, so it is filled with place names, etc. When
mapserver serves out the image all the text looks terrible. I know this
problem will never be completely solved as mapserver has to resample the
image for a web browser client and it doesn't know not too drop pixels
which make up the text, but I'm trying to make sure that the overview
being built are as good as they can be.

Cheers,

Benjamin
-----Original Message-----
From: gdal-dev-admin at remotesensing.org
[mailto:gdal-dev-admin at remotesensing.org] On Behalf Of Frank Warmerdam
Sent: Wednesday, 16 July 2003 12:54 PM
To: gdal-dev at remotesensing.org
Subject: Re: [Gdal-dev] viewing gdaladdo overlays


Benjamin Wragg wrote:
> Hi list,
>  
> If I built 4 overlays in a geotiff using gdaladdo, is it possible to
> view each overlay in some type of GIS/Graphical program? If so what 
> programs support this?

Benjamin,

Some programs that support TIFF will let you look at specific sub-images
but this if fairly rare.  An alternative is to use the "tiffsplit"
utility distributed with libtiff to split each TIFF directory into a
seperate file.  That would mean the base image, and each overview would
be a seperate TIFF file. Some information (like the geotiff tags) might
be lost in this process.

Of course, applications that support overviews in TIFF will presumably
let you see the overview levels implicitly.  For instance, if you load
an image in OpenEV that has overviews, go to 1:1 resolution, and then
page out each step should show a different power of 2 overview (assuming
you built your overviews on powers of two).  But it can be hard to be
sure you are getting what you think you should be getting.  I assume you
are wanting to do some quality assurance on the overviews?

Best regards,

-- 
---------------------------------------+--------------------------------
---------------------------------------+------
I set the clouds in motion - turn up   | Frank Warmerdam,
warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent


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