Python bindings: allow creating & exporting `Transformer` without always needing a source dataset

[HansBrende]

Feature description

I commonly create transformers such as the following:

transformer: gdal.GDALTransformerInfoShadow = gdal.Transformer(my_dataset, None, ['DST_SRS=EPSG:4326'])

which allows me to easily map pixel coordinates to their WGS84 coordinates.

However, the problem I am facing is that I must open the original image every time I want to do this.

What would be ideal is if I could do this same thing once (as above), and then export the relevant data from the transformer that I could, for example, store in a database. And then reinitialize the transformer later with exactly the same semantics without needing to re-open the original dataset to ensure this!

E.g. (suggested syntax):

stuff_I_can_serialize = transformer.getSourceTransform(), transformer.getSourceTransformArg()

later...

transformer: gdal.GDALTransformerInfoShadow = gdal.TransformerFromXYZ(*stuff_I_can_serialize)

I have not tried pickling the transformer or anything like that because even if it worked (doubtful), I wouldn't want it to stop working if a backwards-incompatible change was made to the internal memory representation... being able to serialize/deserialize in some kind of human-readable way would be ideal.

Additional context

No response

[rouault]

you can workaround that a bit, by converting your source dataset to a VRT file, and storing the VRT in your database. Just opening it for the sake of passing it to gdal.Transformer() doesn't require accessing the original raster file

[HansBrende]

@rouault brilliant! I've just tested this strategy out and it works like a charm.

Only possible downside is that the VRT does include a lot of information that is not required, for example the VRTRasterBand elements.

I'm assuming that I could delete all of this unnecessary information in the actual storage process, keeping only a list of the XML elements which are actually required to reconstruct the transform, and then reconstruct a VRT file putting some junk data in for required elements such as VRTRasterBand.

The only elements I'd need to preserve, I'm assuming, would be:

• SRS
• GeoTransform
• GCPList
• plus any metadata items containing RPC information (which I believe are exactly those with attribute domain="RPC"?)

Is that correct or am I missing anything?

[rouault]

Is that correct

yes

[HansBrende]

Great, thanks! Well that gives me a viable path forward. Since the solution is a bit hacky, I'll leave this issue open in case the use-case is helpful in designing any future enhancements. Or if not, feel free to close!