[Fwd: [Gdal-dev] Nodata bitmasks / null processing]

[guillaume huby]

> -------- Message original --------
> Sujet: [Gdal-dev] Nodata bitmasks / null processing
> Date: Sat, 04 Aug 2007 13:06:04 -0400
> De: Frank Warmerdam <warmerdam at pobox.com>
> Pour :: gdal-dev <gdal-dev at lists.maptools.org>
>
> Folks,
>
> Currently GDAL supports two mechanisms for identifying nodata/transparent
> areas in a raster.  One is use of a particular pixel value as a nodata
> value (returned by the GDALRasterBand::GetNodataValue() method) and the
> other
> is by specifying an alpha band with alpha=0 marking transparent areas.
>
> However, a number of file formats support a concept of "nodata masks" -
> essentially bitmasks marking areas of the raster that are nodata
> areas.  But
> this is accomplished without having to reserve a special marker value to
> be
> the nodata value.
>


> I am looking for some feedback from the community.  In particular I'm
> interested in examples of file formats that include bit masking.


Frank,

I'm not sure being able to say you what is the best way to implement this.
But I can say that this is an interesting project. I can provide you a
sample I would like to see implemented. MODIS HDF files are all provided
with a Quality Assurance bit mask. For exemple you can take surface
reflectance files (http://edcdaac.usgs.gov/modis/myd09gqv5.asp, there is
download links in this page). These files have two datasets for surface
reflectance (http://edcdaac.usgs.gov/modis/myd09gqv5_sds.asp) and one for
Quality Assurance (http://edcdaac.usgs.gov/modis/qa/myd09gq_qa_v5.asp). I
think that your project can be very usefull in this case.

I hope this will help you !

Best regards,
Guillaume.
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