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

[Peng Gao]

Any comments? 

> -----Original Message-----
> From: Peng Gao 
> Sent: Thursday, June 30, 2005 8:36 AM
> To: Michael P. Gerlek; warmerdam at pobox.com
> Cc: gdal-dev at xserve.flids.com
> Subject: RE: [Gdal-dev] RE: Strange NoData value from a MrSID 
> image (Peng Gao)
> 
> > -----Original Message-----
> > From: Michael P. Gerlek [mailto:mpg at lizardtech.com]
> > Sent: Friday, June 24, 2005 11:43 AM
> > To: warmerdam at pobox.com; Peng Gao
> > Cc: gdal-dev at xserve.flids.com
> > Subject: RE: [Gdal-dev] RE: Strange NoData value from a MrSID image 
> > (Peng Gao)
> >
> > Frank, Peng:
> >
> > In the MrSID SDK, we are not band-specific -- that is, a pixel is 
> > considered nodata iff all bands match the given values.
> >
> > -mpg
> >
> >
> > -----Original Message-----
> > From: Frank Warmerdam [mailto:fwarmerdam at gmail.com]
> > Sent: Friday, June 24, 2005 10:57 AM
> > To: Peng Gao
> > Cc: Michael P. Gerlek; gdal-dev at xserve.flids.com
> > Subject: Re: [Gdal-dev] RE: Strange NoData value from a MrSID image 
> > (Peng Gao)
> >
> >
> > 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.
> >
> 
> I have a case where MrSID SDK reports NoData values (0, 0, 0) for
> 3 bands, and I have a MrSID image containing black text with 
> value (0, 13, 14).  Since GDAL uses band specific NoData 
> interpretation, the band 1 is nodata. My app considers a 
> multiband pixel NoData if any of the band is nodata. As the 
> result, the black text is displayed transparent.
> I could ignore
> MrSID NoData color at application level. But I can't handle 
> the case where the NoData values come from PAM.
> 
> One solution could be not using native MrSID NoData color as 
> GDAL band specific NoData value (because it's not), and 
> adding a new call at Dataset level to get NoData color for 
> the entire dataset for apps that want to make use of the NoData color?
> 
> What do you think?
> 
> Gao, Peng
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