[gdal-dev] alpha mask to data value

[William Kyngesburye]

On Apr 2, 2014, at 3:54 AM, Chaitanya kumar CH <chaitanya.ch at gmail.com> wrote:

> William,
> 
> Try gdalwarp after removing the mask band.

Removing the mask band is what I want, but also keeping the nodata values AND changing them to 255.  If I remove the mask nodata is set to 0, but there may be actual black pixels in the real data that will then become nodata when restoring and changing nodata values.  I could take more time in PS and change all black to 1,1,1.  I'm already changing white to 254.254,254 so it doesn't become nodata at some point (though it doesn't matter so much because it will become white in the end anyways).

So far, I have a messy, but working, process:

PS to clear collar - alpha mask
QGIS rectify - copy GDAL commands, alter them to create a tfw, and run manually
PS to flatten rectified tiff so nodata = white (PS destroys georeferencing, but it's safe in tfw)
gdal_translate to assign nodata and projection (needed anyways because QGIS rectifier doesn't assign projection, it's broken)
merge into vrt

> If that didn't work try gdal_merge.py with -init set to 255.
> 
My experience with gdal_merge.py in the past is either it's slower than making a vrt and translating that to geotiff, and it has memory problems with many and/or large rasters because it assembles the raster in memory.

> 
> On Wed, Apr 2, 2014 at 1:44 AM, William Kyngesburye <woklist at kyngchaos.com> wrote:
> I've wrestled with various nodata issues in the past, now it's hitting me again...
> 
> I'm using Photoshop to delete collars on scanned maps, creating an alpha mask.  GDAL has no problem with this.  What I want to do is merge maps together (after rectification), then set any remaining nodata areas to white, RGB 255,255,255.  Just dropping the extra alpha band doesn't work because nodata is set to 0,0,0, which is black.  The a_nodata option in gdal_translate just defines what existing value in the data is nodata.
> 
> I tried using gdalwarp with the -dstnodata option which should set nodata values in the output to a specific value, but it carries along the alpha band and ignores dstnodata.
> 
>   gdalwarp -dstnodata "255 255 255" in.tif out.tif
>   Processing input file in.tif.
>   Using band 4 of source image as alpha.
>   Using band 4 of destination image as alpha.
> 
> 

-----
William Kyngesburye <kyngchaos*at*kyngchaos*dot*com>
http://www.kyngchaos.com/

All generalizations are dangerous, even this one.