[mapserver-users] Average/Bilinear Resampling and Alpha Bands

[Robert Sanson]

Hi Jason
 
Well, ECW use wavelet compression, which is lossy, and data values can get changed slightly.
 
regards,

Robert

>>> Jason Beverage <jasonbeverage at gmail.com> 16/03/2010 10:27 a.m. >>>
Hey again,

Some more information, the file I was using was an ECW I converted from a tif. The black splotches appear if I use the ECW, but if I use the original tif file I don't see them.

However, when using AVERAGE or BILINEAR, it appears as if the alpha values are either completely transparent or completely opaque. The image I'm using is an image I used the gdalwarp -cblend option on to do some polygon cutting so I'm losing my blending line if I do average or bilinear resampling. I'll see if I can create a small example that shows the issue.

Thanks,

Jason

On Mon, Mar 15, 2010 at 3:55 PM, Robert Sanson <SansonR at asurequality.com> wrote:


Hi Jason
That's not unexpected. RESAMPLE=NEAREST preserves the "nodata" vales used in the mask. BILINEAR or AVERAGE combine values from adjoining pixels, so the "nodata" pixels get some other value.
regards,

Robert

>>> Jason Beverage <jasonbeverage at gmail.com> 16/03/2010 4:53 a.m. >>>

Hi all,

I have some imagery that is in UTM that contains an alpha band that
masks out areas with no data. When I use MapServer to reproject it to
epsg:4326, the masked out area looks good if I set RESAMPLE=NEAREST on
the layer. If I change the RESAMPLE option to BILINEAR or AVERAGE I
get black splotches in the area that is transparent.

Has anyone else seen this behavior?

I'm using MapServer 5.6.1

Thanks,

Jason
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