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

[Jason Hughes]

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.

Thanks,
Jason Hughes
"Frank Warmerdam" <warmerdam at pobox.com> wrote in message 
news:41941D7B.5020308 at pobox.com...
> Jason Hughes wrote:
>> Is there anyway to specify a pixel value to replace while warping.  I 
>> want a way to differentiate between pixels in my image and the background 
>> that is created from warping.
>>
>> I'm overlaying NITF's on maps and I would like a way to not paint the 
>> background.  So as a workaround  I'm making an intermediate file to 
>> replace the values that equal my background color myself using 
>> GDALCreateCopy().  I then make a third file to warp the image.  This 
>> seems terribly inefficient. Your input is much appreciated.
>
> Jason,
>
> I'm not clear on exactly what you want to accomplish.  You can use the
> -dstnodata argument to gdalwarp to specify a pixel value that will be used
> to mark areas on the output image that were never set from a pixel in the
> input image.  However, if there were pixels in the input image that had
> that "dstnodata" value already you won't be able to distinguish them.
>
> Another alternative is to add an alpha band which would be marked as
> opaque for pixels copied from the source image, and transparent for pixels
> that were never set during the warp.
>
> Does one of these cases match your needs?
>
> 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