# Geometry masks

*Geometry masks* are Shapely polygons that the chain rasterizes at tile level before calling the core resampling function. All source-array masks (raster mask + geometry masks) are merged into a single mask that is passed to `array_grid_resampling`. This feature is only available in the chain layer; the core function does not see polygons directly.

**What you'll learn**

- The role of `array_src_geometry_pair` (valid + invalid polygons)
- How to use `array_src_geometry_origin` to shift polygon coordinates
- How geometry masks combine with the source raster mask

## Setting things up

We reuse the source raster, grid file, grid mask and source raster mask from previous notebooks.


## Defining the geometries

`array_src_geometry_pair` is a length-2 sequence of Shapely polygons:
- index 0 -- the **valid** geometry: pixels inside are kept valid,
- index 1 -- the **invalid** geometry: pixels inside are forced invalid.

Either entry may be `None`. Coordinates are expressed in the source raster frame; `array_src_geometry_origin` shifts them if needed.


```python
from shapely.geometry import Polygon

geometry_valid  = Polygon([(40, 10), (300, 30), (270, 200), (40, 200)])
invalid_polygon = create_star_polygon(100, 100, 50)
```


![geometry_input.png](grid_resampling_chain_005_geometry_masks_files/geometry_input.png)


The figure above shows the green valid polygon and the red star-shaped invalid polygon, together with the grid nodes and the source raster mask.

## Running the chain with geometry masks


```python
with rasterio.open(GRID_IN_F64, "r") as grid_in_ds, \
        rasterio.open(RASTER_IN, "r") as array_src_ds, \
        rasterio.open(grid_mask_in_path, "r") as grid_mask_in_ds, \
        rasterio.open(raster_mask_in_path_u8, "r") as raster_mask_in_ds, \
        rasterio.open(output_raster_path, "w", **raster_out_open_args) as array_out_ds, \
        rasterio.open(output_mask_path, "w", **mask_out_open_args) as mask_out_ds:

    basic_grid_resampling_chain(
        grid_ds                        = grid_in_ds,
        grid_row_coords_band           = 1,
        grid_col_coords_band           = 2,
        grid_resolution                = grid_resolution,
        array_src_ds                   = array_src_ds,
        array_src_bands                = 1,
        array_out_ds                   = array_out_ds,
        interp                         = "cubic",
        nodata_out                     = 0,
        mask_out_ds                    = mask_out_ds,
        grid_mask_in_ds                = grid_mask_in_ds,
        grid_mask_in_unmasked_value    = grid_mask_in_valid_value,
        grid_mask_in_band              = 1,
        array_src_mask_ds              = raster_mask_in_ds,
        array_src_mask_band            = 1,
        array_src_mask_validity_pair   = (array_src_mask_validity_valid,
                                          array_src_mask_validity_invalid),
        array_src_geometry_origin      = (0., 0.),
        array_src_geometry_pair        = [geometry_valid, invalid_polygon],
    )
```


![geometry_mask_output.png](grid_resampling_chain_005_geometry_masks_files/geometry_mask_output.png)