[Gdal-dev] Re: gdalwarp nodata

[Richard Greenwood]

On Mon, 15 Nov 2004 13:15:47 -0500, Frank Warmerdam <warmerdam at pobox.com> wrote:
> Richard Greenwood wrote:
> > Following up on a couple things:
> >
> > 1. The new gdalwarp alpha channel support works great. I built on
> > windows. It does not automatically detect alpha channels created by
> > Photoshop or AutoCAD CadOverlay, but specifying the  -srcalpha switch
> > does the trick.
> 
> Richard,
> 
> Could you provide a sample file to me directly that has this problem?

I put a zip file named 'alpha_chanel.zip' at
     http://www.greenwoodmap.com/frank/
with samples tiff's having alpha channels created with Windows
versions of AutoCAD and Photoshop. The zip file has an readme.txt with
some notes.

I also ran into another annomally. The Python script that you created
to set pixels in the alpha channel to transparent where rgb channels
are  255 255 255 seems to set transparency for more pixels that it
should. It appears to set transpareny for pixels that are also very
nearly, but not exactly, 255 255 255, for example 255 253 255, as if
there were some rounding error.  I have included a simple example of
the is the zip file also.

Thanks,
Rich

-- 
Richard Greenwood
richard.greenwood at gmail.com
www.greenwoodmap.com