[Gdal-dev] gdal_translate image grow

[Norman Barker]

-----Original Message-----
From: Frank Warmerdam [mailto:fwarmerdam at gmail.com] On Behalf Of Frank
Warmerdam
Sent: Thursday, March 30, 2006 3:11 PM
To: Norman Barker
Cc: gdal-dev at lists.maptools.org
Subject: Re: [Gdal-dev] gdal_translate image grow

Norman Barker wrote:
> Hi,
>
> I am using gdal_translate as the 'engine' behind mod_python as a very 
> simple WMS (so that I can prototype a few ideas before putting them
into 
> MapServer), everything works fine until I specify a BBOX such that the

> extents are outside that of the requested image.  This corresponds to 
> the -projwin option of gdal_translate, executing this I then get an 
> error "-srcwin falls outside of raster size".
> 
> Is there a way to grow the output image and fill in the missing data?
> 
> So for example  
> 
> gdal_translate -not_strict -OF PNG -a_nodata 0 -projwin -180 90 180
-90 
> -outsize 836 654 inputimage outputimage
> 
> would give an output image with world extents, and somewhere in this 
> image would be the inputimage
> 
> Alternatively is there a way to do this with the python bindings or
any 
> other utility.  I like gdal_translate since it does the conversion and

> sampling in one go!

Norman,

If you were willing to limit yourself to directly creatable formats,
you could use gdalwarp to do this.  It supports an output file with a
larger region than the input.  Alternatively a few modifications could
be made to gdal_translate to support a projwin (or srcwin) that is
larger
than the source.

Another approach would be for you to prepare a VRT file as input to
gdal_translate in your VRT script that has the characteristics you want
relative to the original file.  When you subwindow or do other such
transformations in gdal_translate it actually creates a "VRT" image of
the desired output file and uses that for input to translation.  To get
an idea what these VRTs look like you can even do a gdal_translate to
VRT format and you will get the VRT skeleton it created.

Best regards,
-- 
<snip>

Hi,

I have managed to get this working with VRT raster files, they are
seriously cool and I can program them from python forming a basic WMS.

However when I sample from a smaller image to a larger image with
greater spatial extents I use a kernel filter.  It seems that the kernel
filter doesn't stop in the y-axis and continues for the length of the
image (see attached).

Is this something that anyone else has come across? Also how do I make
the output png have transparent values, I have tried -a_nodata 0?

Many thanks,

Norman

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