[gdal-dev] preserving alpha channel data with gdal_transform.py

[Jay L.]

What format do you want the alphas channel in after running
gdal_polygonize.py?  I ask because after processing you could open the input
and read just the alpha band (band 4 I assume).  Here is a short script
framework that might get you started.

Something like:
from osgeo import gdal

gdal.open(inpudataset, gdal.GA_ReadOnly)

#Create an output
driverFormat = "GTiff" #Any GDAL supported write format
driver = gdal.GetDriverByName(driverFormat)
outdataset = driver.Create(options.output, dataset.RasterXSize,
dataset.RasterYSize, dataset.RasterCount)
outdataset.SetProjection(dataset.GetProjection())
outdataset.SetGeoTransform(dataset.GetGeoTransform())

inband = gdal.GetRasterBand(4) #Band counts are 1 based.  I am assuming that
your band 4 is alpha.

#Read in the gdal array
input_band = inband.ReadAsArray()
outdataset.GetRasterBand(1).WriteArray(input_band, 0, 0)

Someone with more experience might be able to tell you a method that uses
CreateCopy(), but I think that it isn't possible to selectively copy bands.

Hope that that helps,
Jay

On Mon, Jul 25, 2011 at 8:19 PM, jt2000 <thakkar at nexgenwireless.com> wrote:

> I have a very large raster file with data in the alpha band.  I need to
> vectorize it and associate band 4 data with resulting polygons.
>
> Is there anyway to preserve alpha channel data or get that data associated
> with vectorized file after using gdal_polygonize.py?
>
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