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

[Even Rouault]

Le lundi 09 mai 2011 21:32:39, William Kang a écrit :
> Hi Even,
> After I set it to 0.125, the speed is comparable to the gdalwarp.exe.
> Thanks a lot for your suggestions.
> 
> Aside from this topic, I have some questions about the reprojection
> process in gdal. My understanding is that the getGeoTransform returns
> the affine coefficients needed to perform the projection change. If we
> can calculate the four corner points by using the geoTransformer and
> get their coordinates in the new projection coordinate system, we can
> calculate the corresponding bounding box and affine transformation
> coefficients.
> 
> But I have read documents and your first reply email suggested that
> some sampling processes along the image edge are necessary to
> calculate the proper bounding box. Why is that? Isn't the four corner
> points enough to determine the new bounding box? Thanks a lot for your
> help.
> 

The 4 corners are not generally sufficient. It is only a rough approximation if 
you don't want to spend more than 20 lines of coding... Sampling along the 
image edges is required for accuracy, especially in case of important warping 
(for example if you reproject between geographic coordinates <--> polar 
projections)