[Gdal-dev] Re: GDALWarp - NO_DATA Replace?

[Frank Warmerdam]

Jason Hughes wrote:
> Frank - Basicially I'm redrawing the image pixel by pixel onto a mapsurface. 
> For example when warping I set the dstnodata = 0.  Then I paint this onto 
> the map skipping pixels of that value.  As to not get my image pixels 
> confused with the background before I warp it I create a copy in which I set 
> every pixel with that value to +1.  I suppose I could also do this with a 
> 4th raster band but sometimes we work with high resolution data over a large 
> area which would make for an even bigger file and slow my processing time 
> when rescaling for the different zoom levels.
> 
> I was wondering could there be a way to replace my pixel values that match 
> the background during the warping operation as to not create a third file or 
> 4th raster band.  I create the intermediate file so I do not modfiy the 
> original image.

Jason,

Ah yes, I see.  You would like to set a -dstnodata, but you also want to
slightly alter any input pixels that would match it so they won't be
seen as nodata at the end.

On the one hand this seems like a very specialized case, but on the other
I can imagine it coming up fairly often.

There isn't any way of doing this now with gdalwarp.  Normally I would advise
you to do the remapping as a preprocessing step, and then run gdalwarp.

However, you could also construct your own version of "gdalwarp" that did
do this for you if performance is of the essence.

There is a hook in the GDALWarpOptions for a pfnPreWarpChunkProcessor function.
You could install a function that would make one pass over the incoming image
data and convert all zeros to ones for instance.

Other than that the program could be the same as gdalwarp.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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