[Gdal-dev] RE: Strange NoData value from a MrSID image (Peng Gao)

[Frank Warmerdam]

On 6/24/05, Peng Gao <pgao at esri.com> wrote:
> I have a question about the interpretation of MrSID NoData value. 
> Is a pixel considered NoData only if all three values match their 
> corresponding band NoData value, or Is a pixel considered NoData 
> if any band value matches band NoData value? For example, given 
> NoData values (0, 0, 0), Are only those pixels with value (0, 0, 0) 
> considered NoData, or (0, x, x), (0, 0, x), and etc considered 
> as NoData pixels as well? 

Peng,

In the GDAL data model, nodata is band specific.  So if band one
has a nodata value of 0, then that band is considered to be nodata
if the pixel value is zero.  A pixel might well be no data in some bands,
and not in others ... at least in theory.

I'm not sure if this is the normal interpretation in MrSID though. 

Best regards, 
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