rasterize=True flips image

[ahuang11]

[jbednar]

I can reproduce this using holoviews.operation.datashader.rasterize(), so I think it's an issue with rasterize() itself rather than anything in hvplot:

It seems to be specific to QuadMesh objects, because the same data doesn't get flipped for Image objects:

[jbednar]

I've made a possible fix for this problem on holoviews: holoviz/holoviews#2883

@ahuang11, could you try it out and see if it works for you? It definitely fixes it in the example I show above, but I don't know if this is the right fix, i.e., whether it will break some other case.

[ahuang11]

Looks good for this case too. Thanks for the quick support!

(Interestingly, quadmesh without rasterize=True before a fix was also flipped: #23)

[ahuang11]

Oh I explored other cases too to see if it broke anything; when I pass in crs=crs.PlateCarree() it crashes, but to use gv.operation.project_image() I need to specify the crs...

import os
import xarray as xr
import holoviews as hv
import geoviews as gv
import hvplot.xarray
hv.extension('bokeh')

os.system(
    'wget -nc '
    'ftp://ftp.cdc.noaa.gov/'
    'Datasets/ncep.reanalysis.dailyavgs/'
    'surface/'
    'air.sig995.2018.nc'
)

obs_ds = xr.open_dataset('air.sig995.2018.nc')
qm = obs_ds.hvplot.quadmesh(x='lon', y='lat', z='air', groupby='time',
                       rasterize=True, crs=crs.PlateCarree(central_longitude=360)))
gv.operation.project_image(qm, projection=ccrs.NorthPolarStereo())

but works if it's an Image

qm = obs_ds.hvplot.image(x='lon', y='lat', z='air', groupby='time',
                         rasterize=True, crs=crs.PlateCarree(central_longitude=360))
gv.operation.project_image(qm, projection=ccrs.NorthPolarStereo()).options(projection=ccrs.NorthPolarStereo())

and also works if I supply a projection...

obs_ds.hvplot.quadmesh(x='lon', y='lat', z='air', groupby='time',
                       crs=crs.PlateCarree(central_longitude=360),
                       projection=ccrs.NorthPolarStereo())

but if I add rasterize=True to the last bit, it gives an empty globe (until I interact by zooming out)

obs_ds.hvplot.quadmesh(x='lon', y='lat', z='air', groupby='time',
                       crs=crs.PlateCarree(central_longitude=360),
                       rasterize=True,
                       projection=ccrs.NorthPolarStereo())

[jbednar]

Probably best to let @philippjfr review this, as he knows all the various ways this code can get used, whereas I can see only this specific case. Unfortunately, he may not be able to respond any time soon...

[ahuang11]

Not a problem at all; there's no rush! Thanks for all the help and making these amazing tools!

[philippjfr]

Should be fixed in holoviz/holoviews#3017

[ahuang11]

Fixed.