Polygon

The Polygon object lets you place a custom polygon-shaped stimulus on a frame by clicking to define its vertices in the task editor. Once drawn, the polygon is fully editable: you can drag existing vertices, add midpoints, and remove points. Polygon fill and stroke appearance is configurable, and you can link the shape to a stimulus information variable.

A common application is eye tracking: by tracing the outline of an irregular stimulus with many vertices, you can use the Polygon as a precise AOI mask in the Analyzer. This is especially useful when the stimulus shape cannot be captured with a rectangle and an SVG is not available.

Adding the Polygon Drawing Object

In the task editor toolbar, open the Images & Drawing group and select Polygon to add a Polygon object to the current frame.

The element is added to the frame as an empty polygon. Drawing mode starts automatically so you can begin placing vertices immediately.

Drawing a Polygon

With the Polygon Drawing Object selected and drawing mode active, click anywhere on the frame canvas to place a vertex. Each click adds a new point to the polygon outline.

To close the polygon (connect the last vertex back to the first), click on the circular handle at the first vertex. Alternatively, check the Closed property in the properties panel to force the shape closed without clicking the handle.

To stop adding vertices without closing the shape, click Stop drawing (ESC) in the properties panel, or press ESC. The polygon remains as an open polyline until Closed is enabled.

Modifying an Existing Polygon

After stopping drawing, the polygon enters edit mode. All existing vertex handles remain visible on the canvas.

• Move a vertex: Click and drag any vertex handle to a new position. The polygon edges update in real time.
• Add a midpoint: In the Points Table, click the + icon on any row. A new vertex is inserted at the midpoint between that point and the next one in the sequence.
• Remove a vertex: In the Points Table, click the × icon on the row for the vertex you want to delete.

To resume adding new vertices at the end of the sequence, click Continue drawing in the properties panel.

Object Properties

All properties below are found in the properties panel when the Polygon Drawing Object is selected.

Points Table

The Points section in the properties panel shows a table of all current vertices. Each row displays the vertex index (nr), its horizontal position (x), and its vertical position (y) in frame coordinates.

Use the row controls to:

• Click + to insert a new vertex at the midpoint between that row and the next.
• Click the remove icon to delete the vertex at that row.

You can also set the polygon's points programmatically using a data frame variable and the Set Object Property action in the event system. Each row in the data frame represents one vertex, with columns for x and y coordinates.

Clearing the Drawing

To remove all vertices and start the polygon from scratch, click Clear all in the properties panel. This resets the shape to an empty polygon and re-enters drawing mode automatically.

Using the Polygon as an Eye Tracking AOI

The Polygon object can serve as an invisible AOI mask for eye tracking. The typical setup is to draw the polygon tracing the outline of an irregular stimulus, then leave it completely unstyled: no fill, no stroke. The polygon is invisible to participants but its vertex boundary is used for gaze and fixation hit-testing.

Use this approach when a rectangular AOI is too coarse for the stimulus shape and an SVG element is not available. Because you can place as many vertices as needed, you can closely follow any irregular contour.

Gaze and fixation samples are tested against the polygon's exact vertex set rather than a bounding rectangle. Metrics such as dwell time and percentage of time in AOI are computed using the full polygon boundary.

If your polygon shape differs per condition, the correct vertex set is resolved per trial automatically using the modifier system.

Give the polygon a clear Stimulus Info label so it is easy to identify when configuring AOI metrics.

Modifying the Polygon at Runtime

Use the Set Object Property action in the event system to update the polygon during an experiment. The following properties can be set at runtime:

Example: replace the polygon's shape when a trial starts

Trigger: On Frame Start
Action: Set Object Property -> Set Target: your Polygon object -> Property: points -> Value: your variable containing the new coordinate array

Further Reading