[Gdal-dev] VRTWarpedDataset w/ -dstalpha

[Frank Warmerdam]

On 5/20/05, Dardo D Kleiner - CONTRACTOR <dkleiner at cmf.nrl.navy.mil> wrote:
> So I have a set of source imagery in a particular non-WGS84 projection that I
> want to convert to a set of raw tiles in WGS84.  I figured I could make use
> of the fantastic VRT format to minimize the number of stages required.  So I
> build a "layered" vrt tree like this (forgive the ASCII art):
> 
> [src1.tif] [src2.tif] [src3.tif] . . . [srcN.tif] (say, 10k x 10k images)
>      |          |          |                |
> [src1.vrt] [src2.vrt] [src3.vrt] . . . [srcN.vrt] (add georeferencing)
>      \          \          /                /
>       \          \        /                /
>        \          \      /                /
>         \---------[src.vrt]--------------/ (i.e. a vrt mosaic)
>                       |
>                       | gdalwarp -of vrt -s_srs EPSG:xxxx -t_srs EPSG:4326 \
>                       |       -co ALPHA=YES -dstalpha src.vrt dest.vrt
>                       |
>         /---------[dest.vrt]-------------\
>        /           /     \                \
>       /           /       \                \
>      /           /         \                \
> [dest1.vrt] [dest2.vrt] [dest3.vrt] . . . [destM.vrt] (512 x 512 tile grid)
>      |           |           |                 |
> [dest1r.vrt] [dest2r.vrt] [dest3r.vrt] . . . [destMr.vrt] (VRTRawRasterBand)
>      |           |           |                 |
> [dest1.8888] [dest2.8888] [dest3.8888] . . . [destM.8888] (raw tiles)
> 
> I'd like to just be able to run a pass over the dest? files doing
> 'gdalwarp -of vrt dest?.vrt dest?r.vrt' and have the dest?.8888 files
> "magically" created.  In the GDAL Virtual Format Tutorial it says "The
> VRTRawRasterBand supports in place update of the raster, whereas the source
> based VRTRasterBand is always read-only.", which leads me to believe that I
> can write to a vrt that has VRTRawRasterBand.  And gdalwarp says "Mosaicing
> into an existing output file is supported if the output file already exists."
> 
> I've encountered two problems.
> 
> 1) When I try to access dest.vrt (say, gdal_translate dest.vrt dest.tif), I
> get alot of these errors:
> 
> ERROR 6: WriteBlock() not supported for this dataset.
> 
> and the resulting image is "wrong", in that the color is very "blue".  If I
> don't use "-co ALPHA=YES -dstalpha" the output is "correct".  But I need the
> alpha channel to avoid displaying the black edges that result from the warp
> operation.

Dardo, 

I'm not sure I follow your whole tree of .vrt's concept properly.  Is dest.vrt
using a bunch of <SimpleSource>'s to refer to individual tiles?  You should
be able to read from such a thing, but you cannot write to it. 
 
> 2) When I try to 'gdalwarp -of vrt dest?.vrt dest?.raw' it always overwrites my
> raw files with a new VRTWarpedDataset definition, rather than writing "into"
> the existing raw VRT and producing the raw tile output file.

Try not including the "-of vrt"  part.  When gdalwarp sees you have 
"-of VRT" it does alot of special stuff to write out a new VRT describing
the warp instead of actually performing the warp.  But you should be able
to warp into a preexisting raw file described with a .vrt file.  

> Is what I'm attempting supported?

I think so, with the -of caveat. 

>  On a more philosophical level, is it insane?

Umm, you should be aware that you are stressing things a bit beyond what
I had in mind.  In particular, be aware that having .vrt files using
<SimpleSource>
to describe mosaics of many tiles may have performance problems.  The 
implementation of the <SimpleSource> mechanism does *not* scale gracefully
up to hundreds of thousands of files.  Though it should work fine for dozens
of files.

Best regards,

-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent