Is there a way to polyfill edge to edge or as much as possible (Res 8) without increasing the resolution.
Black boundary is the polygon boundary I am trying to polyfill completely

>Solution :

Why this happens

This happens because polyfill only returns cells whose centroid falls into the polygon.

What to do instead

In your case, you could use polyfill on a finer resolution and then use h3_to_parent to convert back to your desired resolution.

Here is an example using Geopandas and H3-Pandas, but of course, the logic works with any implementation of the H3 API.

import geopandas as gpd
import h3pandas

# Choose a random country boundary (Tanzania)
gdf = gpd.read_file(gpd.datasets.get_path('naturalearth_lowres')).iloc[1:2]

Simple polyfill

gdf.h3.polyfill_resample(2)

"Finer" polyfill

gdf.h3.polyfill_resample(4).h3.h3_to_parent_aggregate(2)