[gdal-dev] Unwanted partial transparency when clipping

[Michael Corey]

OK, I did a little more work on this, and I've narrowed down what's 
going on, but I could still use some help in figuring out how to solve it.

Here's my original image:

http://mikejcorey.com/spatial/diablo-orig-5pct.tif

The original appears to have an RGBA setup (RGB channels and an alpha 
channel).

When I run this:

gdalwarp -crop_to_cutline -cutline cutout.shp sourceimage.tif 
diablo-cutline.tif

Here's what I get:

http://mikejcorey.com/spatial/diablo-cutline-5pct.tif

This comes out as an RGB image with no alpha channel, with each channel 
being semi-transparent.

However, if I do this:

gdalwarp -dstalpha -crop_to_cutline -cutline cutout.shp sourceimage.tif 
diablo-dstalpha-cutline.tif

I get this:

http://mikejcorey.com/spatial/diablo-dstalpha-cutline-5pct.tif

This one is strange, because it appears to be a grayscale image. But 
when I open it in Photoshop, I see that it actually has 4 alpha 
channels. I suspect that those are just getting set incorrectly as alpha 
and are in fact the RGB channels, but can someone explain that behavior 
or how to fix it?

What I want to end up with is a clipped RGB image (or RGBA) image where 
nodata is transparent and the RGB isn't translucent.

Thanks again!

Michael Corey



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