html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/issues/3620#issuecomment-565671981,https://api.github.com/repos/pydata/xarray/issues/3620,565671981,MDEyOklzc3VlQ29tbWVudDU2NTY3MTk4MQ==,2448579,2019-12-14T02:18:15Z,2019-12-14T02:19:17Z,MEMBER,"I should have said ""discrete lazily evaluated form (which we support through dask)"". I think we already have what you want in principle (caveats at the end).
Here's an example:
``` python
import dask
import numpy as np
import xarray as xr
xr.set_options(display_style=""html"")
def arbitrary_function(dataset):
return dataset[""a""] * dataset[""wavelength""] * dataset.attrs[""wcs_param""]
ds = xr.Dataset()
# construct a dask array.
# In practice this could represent an on-disk dataset,
# with data reads only occurring when necessary
ds[""a""] = xr.DataArray(dask.array.ones((10,)), dims=[""wavelength""], coords={""wavelength"": np.arange(10)})
# some coordinate system parameter
ds.attrs[""wcs_param""] = 1.0
# complicated pixel to world function
# no compute happens since we are working with dask arrays
# so this is quite cheap.
ds.coords[""azimuth""] = arbitrary_function(ds)
ds
```
So you can carry around your coordinate system parameters in the `.attrs` dictionary and the non-dimensional coordinate `azimuth` is only evaluated when needed e.g. when plotting
``` python
# Both 'a' and 'azimuth' are computed now, since actual values are required to plot
ds.a.plot(x=""azimuth"")
```
In practice, there are a few limitations. @djhoese and @snowman2 may have useful perspective here.
1. xarray tends to compute ""non-dimensional coordinates"" more than necessary. The more egregious examples have been fixed (#3068, #3311, #3454, #3453) but there may still be some places where fixes are needed (#3588).
2. there's some discussion about carrying around Earth-specific coordinate system parameters here: https://github.com/pydata/xarray/issues/2288; https://github.com/pydata/xarray/issues/2996.
Additional info:
1. https://docs.dask.org/en/latest/array.html
2. https://xarray.pydata.org/en/stable/dask.html
3. https://blog.dask.org/2019/06/20/load-image-data
PS: If it helps, I'd be happy to chat over skype for a half hour getting you oriented with how we do things.","{""total_count"": 1, ""+1"": 1, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,537772490
https://github.com/pydata/xarray/issues/3620#issuecomment-565625792,https://api.github.com/repos/pydata/xarray/issues/3620,565625792,MDEyOklzc3VlQ29tbWVudDU2NTYyNTc5Mg==,2448579,2019-12-13T22:05:07Z,2019-12-13T22:05:07Z,MEMBER,"It would also be good to hear about ""sub-pixel metadata"" → this seems to be the main reason why you want to carry around the analytic rather than the discrete evaluated form (which we basically support through dask). Is that right or am I missing something? ","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,537772490