[gdal-dev] Unwanted partial transparency when clipping

[Brian Case]

Michael,

another option is to use gdalwarp to merge images together.

Brian

On Thu, 2011-07-07 at 09:47 -0700, Michael Corey wrote:
> Thanks for your responses, Chaitanya and Eli. The semitransparency isn't 
> really noticeable until you put the image on top of something else (in 
> Photoshop or another program).
> 
> It looks like the problem is actually in my gdal_merge.py command. 
> That's where the semitransparency is getting introduced.
> 
> gdal_merge.py -init "255" -o diablo-combined-utm10.tif file file file
> 
> What I'm trying to get is a merged image where no data = transparent. 
> Would this be correct in that case?
> 
> gdal_merge.py -n "255" -ul_lr 380309.272908 4653610.244134 
> 1317796.950998 3614484.073320 -o test-combined.tif file file file
> 
> Michael Corey
> Digital Innovations Editor
> Center for Investigative Reporting
> 510.809.3178
> 
> 
> On 7/7/11 7:13 AM, Eli Adam wrote:
> > Michael,
> >
> >>>> On 7/6/2011 at 5:35 PM, in message<4E14FF42.50903 at cironline.org>, Michael
> > Corey<mcorey at cironline.org>  wrote:
> >> Sure, I've uploaded samples here.
> >>
> >> http://www.mikejcorey.com/spatial/diablo-box-sample.tif
> >> http://www.mikejcorey.com/spatial/diablo-cutout-sample.tif
> > I don't notice the semi-transparency in these scaled down images.  Perhaps it is the way your viewer reads the mask?
> >
> >> These are the same as the images created by the process I described (but
> >> scaled down).
> >>
> >> To your point about specifying size in the first step -- will that make
> >> the process run faster, or does it do the scaling down after it builds
> >> the full-resolution image?
> >>
> >> Also, I notice that my filesize always gets significantly bigger when I
> >> do the cutout step, which seems counter-intuitive to me since in theory
> >> shouldn't there be less information present once the cutout is done?
> > -cutline does not 'discard' any data.  The extent of the data remains the same unless you reset those extents.  You can do that with -crop_to_cutline.  Here are some details from the gdalwarp page, http://gdal.org/gdalwarp.html :
> >
> > -crop_to_cutline:
> >      (GDAL>= 1.8.0) Crop the extent of the target dataset to the extent of the cutline.
> >
> > Polygon cutlines may be used as a mask to restrict the area of the destination file that may be updated, including blending. If the OGR layer containing the cutline features has no explicit SRS, the cutline features must be in the georeferenced units of the destination file. When outputing to a not yet existing target dataset, its extent will be the one of the original raster unless -te or -crop_to_cutline are specified.
> >
> > Best Regards, Eli
> >
> >> Thanks for your help!
> >>
> >> Michael Corey
> >>
> >>
> >> On 7/6/11 5:01 PM, Chaitanya kumar CH wrote:
> >>> Michael,
> >>>
> >>> Can you provide screenshots of
> >>> diablo-combined-center-utm10-70pct-box.tif and
> >>> diablo-combined-center-utm10-70pct-cutout.tif for comparison?
> >>>
> >>> By the way, you can perform the actions of the two gdal_translate
> >>> commands in the first step with the gdal_merge.py script itself unless
> >>> you want to use a specific resampling algorithm.
> >>>
> >>> On Thu, Jul 7, 2011 at 4:28 AM, Michael Corey<mcorey at cironline.org
> >>> <mailto:mcorey at cironline.org>>  wrote:
> >>>
> >>>      Hi all:
> >>>
> >>>      I'm using a shapefile as a clipping mask to cut out the shoreline
> >>>      from some DOQ files that I have merged together. But when I do the
> >>>      clipping step, I end up with unwanted semitransparency in the
> >>>      non-clipped areas.
> >>>
> >>>      I'm pretty sure the problem is only with my gdalwarp step at the end.
> >>>
> >>>      Here's my process:
> >>>
> >>>      gdal_merge.py -init "255" -o diablo-combined-center-utm10.tif file
> >>>      file file file
> >>>
> >>>      gdal_translate -outsize 70% 70% diablo-combined-center-utm10.tif
> >>>      diablo-combined-center-utm10-70pct.tif
> >>>
> >>>      ogrinfo -al ./diablo_canyon_detail_clipper.shp
> >>>      //Extent: (XXXX, YYYY) - (XXXX, YYYY)
> >>>
> >>>      gdal_translate -projwin XXXX YYYY XXXX YYYY
> >>>      diablo-combined-center-utm10-70pct.tif
> >>>      diablo-combined-center-utm10-70pct-box.tif
> >>>
> >>>      gdalwarp -co COMPRESS=DEFLATE -cutline
> >>>      ./diablo_canyon_detail_clipper.shp
> >>>      diablo-combined-center-utm10-70pct-box.tif
> >>>      diablo-combined-center-utm10-70pct-cutout.tif
> >>>
> >>>      Can anyone help?
> >>>
> >>>      Thanks!
> >>>
> >>>      --
> >>>      Michael Corey
> >>>
> >>>      _______________________________________________
> >>>      gdal-dev mailing list
> >>>      gdal-dev at lists.osgeo.org<mailto:gdal-dev at lists.osgeo.org>
> >>>      http://lists.osgeo.org/mailman/listinfo/gdal-dev
> >>>
> >>>
> >>>
> >>>
> >>> -- 
> >>> Best regards,
> >>> Chaitanya kumar CH.
> >>> /t?a???nj?/ /k?m?r/
> >>> +91-9494447584
> >>> 17.2416N 80.1426E
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