Add an ArLayer
==============

The [ArLayer](../../argaze.md/#argaze.ArFeatures.ArLayer) class defines a space where to make matching of gaze movements and AOIs and inside which those matchings need to be analyzed.

![Empty layer area](../../img/ar_layer_empty.png)

## Add ArLayer to ArFrame JSON configuration file

An [ArFrame](../../argaze.md/#argaze.ArFeatures.ArFrame) instance can contains multiples [ArLayers](../../argaze.md/#argaze.ArFeatures.ArLayer).

Here is an extract from the JSON ArFrame configuration file with a sample where one layer is added:

```json
{
	"name": "My FullHD screen",
	"size": [1920, 1080],
	...
	"layers": {
		"MyLayer": {
			"aoi_scene" : {
        		"upper_left_area": [[0, 0], [960, 0], [960, 540], [0, 540]],
        		"upper_right_area": [[960, 0], [1920, 0], [1920, 540], [960, 540]],
        		"lower_left_area": [[0, 540], [960, 540], [960, 1080], [0, 1080]],
        		"lower_right_area": [[960, 540], [1920, 540], [1920, 1080], [960, 1080]]
        	},
        	"aoi_matcher": {
                "DeviationCircleCoverage": {
                    "coverage_threshold": 0.5
                }
            },
        	"aoi_scan_path": {
        		"duration_max": 30000
        	},
			"aoi_scan_path_analyzers": {
				"Basic": {},
				"TransitionMatrix": {},
				"NGram": {
					"n_min": 3,
					"n_max": 5
				}
			}
		}
	}
}
```

!!! note

	Timestamped gaze movements identified by parent [ArFrame](../../argaze.md/#argaze.ArFeatures.ArFrame) are passed one by one to each [ArLayer](../../argaze.md/#argaze.ArFeatures.ArLayer). So, the execution of all [ArLayers](../../argaze.md/#argaze.ArFeatures.ArLayer) is done during parent [ArFrame.look](../../argaze.md/#argaze.ArFeatures.ArFrame.look) method call as explained in [previous chapter](ar_frame_configuration_and_execution.md).

Now, let's understand the meaning of each JSON entry.

### "MyLayer"

The name of the [ArLayer](../../argaze.md/#argaze.ArFeatures.ArLayer). Basically useful for visualisation purpose.

### AOI Scene

The [AOIScene](../../argaze.md/#argaze.AreaOfInterest.AOIFeatures.AOIScene) defines a set of 2D [AreaOfInterest](../../argaze.md/#argaze.AreaOfInterest.AOIFeatures.AreaOfInterest) registered by name.

![AOI Scene](../../img/ar_layer_aoi_scene.png)

### AOI Matcher

The first [ArLayer](../../argaze.md/#argaze.ArFeatures.ArLayer) pipeline step aims to make match identified gaze movement with an AOI of the scene.

![AOI Matcher](../../img/ar_layer_aoi_matcher.png)

The matching algorithm can be selected by instantiating a particular [AOIMatcher](../../argaze.md/#argaze.GazeFeatures.AOIMatcher) from the [argaze.GazeAnalysis](../../argaze.md/#argaze.GazeAnalysis) submodule or [from another python package](advanced_topics/module_loading.md).

In the example file, the choosen matching algorithm is the [Deviation Circle Coverage](../../argaze.md/#argaze.GazeAnalysis.DeviationCircleCoverage) which has one specific *coverage_threshold* attribute.

!!! warning
	JSON *aoi_matcher* entry is mandatory. Otherwise, the AOIScanPath and AOIScanPathAnalyzers steps are disabled.

### AOI Scan Path

The second [ArLayer](../../argaze.md/#argaze.ArFeatures.ArLayer) pipeline step aims to build a [AOIScanPath](../../argaze.md/#argaze.GazeFeatures.AOIScanPath) defined as a list of [AOIScanSteps](../../argaze.md/#argaze.GazeFeatures.AOIScanStep) made by a set of successive fixations/saccades onto a same AOI.

![AOI Scan Path](../../img/ar_layer_aoi_scan_path.png)

Once identified gaze movements are matched to AOI, they are automatically appended to the AOIScanPath if required.

The [AOIScanPath.duration_max](../../argaze.md/#argaze.GazeFeatures.AOIScanPath.duration_max) attribute is the duration from which older AOI scan steps are removed each time new AOI scan steps are added.

!!! note
	JSON *aoi_scan_path* entry is not mandatory. If aoi_scan_path_analyzers entry is not empty, the AOIScanPath step is automatically enabled.

### AOI Scan Path Analyzers

Finally, the last [ArLayer](../../argaze.md/#argaze.ArFeatures.ArLayer) pipeline step consists in passing the previously built [AOIScanPath](../../argaze.md/#argaze.GazeFeatures.AOIScanPath) to each loaded [AOIScanPathAnalyzer](../../argaze.md/#argaze.GazeFeatures.AOIScanPathAnalyzer).

Each analysis algorithm can be selected by instantiating a particular [AOIScanPathAnalyzer](../../argaze.md/#argaze.GazeFeatures.AOIScanPathAnalyzer) from the [argaze.GazeAnalysis](../../argaze.md/#argaze.GazeAnalysis) submodule or [from another python package](advanced_topics/module_loading.md).