[FWTools] gdaladdo & mapserver OFFSITE

Frank Warmerdam warmerdam at pobox.com
Tue Mar 20 11:59:05 EST 2007

Emmanuel BELO wrote:
> Dear List,
> by preprocessing black and white tif images (in grayscale mode) with
> gdaladdo we produce optimized images to use with mapserver.
> In our mapfile, we set white to transparent with OFFSITE command.
> When viewing the data at the original scale with an orthophoto behind,
> it looks perfect, but when zooming out, a small white strip appears
> between the black lines and the transparent area.
> Is it due to the use of gdaladdo or is it due to Mapserver resampling
> process?
> What are the best practices for processing this kind of data?


Do you use "-r average" with gdaladdo when generating the overviews?  If
you want a discrete background value you should avoid this.

I can't think of any other reason this would be happening.

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    | President OSGeo, http://osgeo.org

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