[gdal-dev] Problem with gdal1.8 Java bindings gdal.ReprojectImage, produces no data

[Even Rouault]

William,

Using ReprojectImage() requires some involved preliminary steps. It is 
generally not appropriate to reuse the dimensions of the in_ds for the out_ds, 
because the shape of the reprojected image is generally not the same as the in 
image. But the real error is to reuse the in geotransform as the out 
geotransform. It doesn't make any sense when the in and out projections are 
not the same. Unfortunately there's no easy way of guessing the out 
geotransform.

A simple approximation would be to compute the coordinates of the 4 corners of 
the in image, reproject them to the dstRef and compute from them the out 
geotransform.

What you would need is the GDALSuggestedWarpOutput2() function of 
http://trac.osgeo.org/gdal/browser/trunk/gdal/alg/gdaltransformer.cpp, which 
is used by the gdalwarp utility, but it is not available from Java. It is 
generally called with pfnTransformer = GDALGenImgProjTransform and 
hTransformArg = GDALCreateGenImgProjTransformer2( hSrcDS, NULL, papszOptions ) 
for your use case.

But I'm thinking of an easier way. You could use gdal.AutoCreateWarpedVRT() 
that will create a in-memory VRT dataset using the above methods to guess the 
appropriate dimensions and geotransform. Then, you can CreateCopy() into a 
"real" dataset.

See 
http://gdal.org/java/org/gdal/gdal/gdal.html#AutoCreateWarpedVRT(org.gdal.gdal.Dataset,
%20java.lang.String,%20java.lang.String)

Something like :

Dataset vrt_ds = gdal.AutoCreateWarpedVRT(in_ds, null, dstRef.ExportToWkt());
Dataset out_ds = gdal.GetDriverByName("GTiff").CreateCopy(outPath, vrt_ds);
vrt_ds.delete();
out_ds.delete();

This will be a bit slower than ReprojectImage() but this should work.

Best regards,

Even


> Hi folks,
> I am new to this mailing list. Thanks all for your great job on GDAL/OGR.
> 
> I have a problem with the gdal.ReprojectImage from gdal1.8 Java
> bindings. When I perform the reprojection, the projected image is
> totally black. And the origin of the projected image is wrong too.
> There were no error popping up at all. The code is as following:
> 
> 		Dataset in_ds = gdal.Open(inPath, gdalconst.GA_ReadOnly);
> 		Dataset out_ds = in_ds.GetDriver().Create(outPath,
> in_ds.getRasterXSize(), in_ds.getRasterYSize(),
> 				in_ds.getRasterCount());
> 
> 		try {
> 			SpatialReference dstRef = new SpatialReference("");
> 			dstRef.ImportFromEPSG(26919);
> 
> 			out_ds.SetProjection(dstRef.ExportToWkt());
> 			out_ds.SetGeoTransform(in_ds.GetGeoTransform());
> 
> 			if (gdal.ReprojectImage(in_ds, out_ds) == gdalconst.CE_Failure)
> 				System.out.println("something is wrong");
> 		} finally {
> 			in_ds.delete();
> 			out_ds.delete();
> 		}
> 
> Does anybody know what's going on here? Thanks a lot.
> 
> 
> William
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