# Styling layers

## Understanding layer styles

A layer's style is defined in a JSON-based called [the Felt Style Language](https://developers.felt.com/felt-style-language), or FSL for short. Editors can view the current style of a layer inside a Felt map by clicking on `Actions > Edit styles` in a layer's overflow menu (three dots).

Here is an example of a simple visualization, expressed in FSL:

```json
{
  "config": {"labelAttribute": ["type"]},
  "legend": {},
  "paint": {
    "color": "blue",
    "opacity": 0.9,
    "size": 30,
    "strokeColor": "auto",
    "strokeWidth": 1
  },
  "type": "simple",
  "version": "2.1"
}
```

## Fetching a layer's current style

A layer's FSL can be retrieved by performing a simple `GET` request to a layer's endpoint:

{% tabs %}
{% tab title="curl" %}

```bash
# Your API token should look like this:
# FELT_API_TOKEN="felt_pat_ABCDEFUDQPAGGNBmX40YNhkCRvvLI3f8/BCwD/g8"
FELT_API_TOKEN="<YOUR_API_TOKEN>"
MAP_ID="<YOUR_MAP_ID>"
LAYER_ID="<YOUR_LAYER_ID>"

curl -L \
  -H "Authorization: Bearer ${FELT_API_TOKEN}" \
  "https://felt.com/api/v2/maps/${MAP_ID}/layers/${LAYER_ID}"
```

{% endtab %}

{% tab title="Python" %}

```python
import requests

# Your API token should look like this:
# api_token = "felt_pat_ABCDEFUDQPAGGNBmX40YNhkCRvvLI3f8/BCwD/g8"
api_token = "<YOUR_API_TOKEN>"
map_id = "<YOUR_MAP_ID>"
layer_id = "<YOUR_LAYER_ID>"

r = requests.get(
  f"http://felt.com/api/v2/maps/{map_id}/layers/{layer_id}",
  headers={"Authorization": f"Bearer {api_token}"}
)
assert r.ok
print(r.json())
```

{% endtab %}

{% tab title="felt-python" %}

```python
import os

from felt_python import get_layer_details

# Setting your API token as an env variable can save
# you from repeating it in every function call
os.environ["FELT_API_TOKEN"] = "<YOUR_API_TOKEN>"

map_id = "<YOUR_MAP_ID>"
layer_id = "<YOUR_LAYER_ID>"

layer_details = get_layer_details(map_id, layer_id)
```

{% endtab %}
{% endtabs %}

## Updating an existing layer's style

To update a layer's style, we can send a `POST` request with the new FSL to the same layer's `/update_style` endpoint.

{% tabs %}
{% tab title="curl" %}

```bash
curl -L \
  -X POST \
  "https://felt.com/api/v2/maps/${MAP_ID}/layers/${LAYER_ID}/update_style" \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer ${FELT_API_TOKEN}" \
  --data '{"style": {"paint": {"color": "green", "opacity": 0.9, "size": 30, "strokeColor": "auto", "strokeWidth": 1}, "legend": {}, "type": "simple", "version": "2.1"}}'
```

{% endtab %}

{% tab title="Python" %}

```python
new_fsl = {
  "paint": {
    "color": "green",
    "opacity": 0.9,
    "size": 30,
    "strokeColor": "auto",
    "strokeWidth": 1
  },
  "legend": {},
  "type": "simple",
  "version": "2.1"
}

r = requests.post(
  f"http://felt.com/api/v2/maps/{map_id}/layers/{layer_id}/update_style",
  json={"style": new_fsl},
  headers={"Authorization": f"Bearer {api_token}"}
)
assert r.ok
print(r.json())
```

{% endtab %}

{% tab title="felt-python" %}

```python
from felt_python import update_layer_style

new_fsl = {
  "paint": {
    "color": "green",
    "opacity": 0.9,
    "size": 30,
    "strokeColor": "auto",
    "strokeWidth": 1
  },
  "legend": {},
  "type": "simple",
  "version": "2.1"
}

update_layer_style(
    map_id=map_id,
    layer_id=layer_id,
    style=new_fsl,
)
```

{% endtab %}
{% endtabs %}

## FSL examples

You can find examples of FSL for different visualization types in [the Felt Style Language section](https://developers.felt.com/felt-style-language) of these docs:

* [Simple visualizations](https://developers.felt.com/felt-style-language/types-of-visualizations/simple-visualizations): same color and size for all features (vector) or pixels (raster).
* [Categorical visualizations](https://developers.felt.com/felt-style-language/types-of-visualizations/categorical-visualizations): different color per feature or pixel, based on a categorical attribute
* [Numeric visualizations](https://developers.felt.com/felt-style-language/types-of-visualizations/numeric-visualizations-color-and-size): different color or size per feature or pixel, based on a numeric attribute.
* [Heatmaps](https://developers.felt.com/felt-style-language/types-of-visualizations/heatmaps): a density-based visualization style, for vector point layers.
* [Hillshade](https://developers.felt.com/felt-style-language/types-of-visualizations/hillshade): a special kind of visualization for raster elevation layers.