How the Polygonal Lasso Tool Works in Photoshop

The Polygonal Lasso Tool in Adobe Photoshop creates precise, straight-edged selections by connecting user-defined anchor points with linear segments. This article explains the underlying mechanics of how the tool plots vertices, calculates straight-line vector segments, and converts those geometric coordinates into an active raster pixel selection known as "marching ants."

Core Anchoring Mechanics

The Polygonal Lasso Tool operates by mapping discrete geometric points onto the canvas:

  1. Setting the Origin: Clicking anywhere on the canvas establishes the initial coordinate point (vertex).
  2. Generating Vector Segments: As you move the cursor away from the initial point, Photoshop renders a dynamic line—often called a "rubber band"—that follows the cursor in real time.
  3. Placing Subsequent Anchor Points: Each subsequent left-click locks a new vertex at that specific coordinate. Photoshop calculates a direct, linear mathematical path between the previous anchor and the newly established one.

By chaining these points together, you define the perimeter of a polygon tailored to straight-sided subjects such as architecture, geometric shapes, or hard-edged objects.

Real-Time Modification and Correction

While drawing the selection path, the Polygonal Lasso Tool allows active adjustments without canceling the entire operation:

  • Removing Points: Pressing the Backspace (Windows) or Delete (macOS) key deletes the most recently placed anchor point in reverse chronological order, allowing you to backtrack along the path.
  • Navigation: Holding the Spacebar temporarily activates the Hand Tool, enabling you to pan across large images without accidentally dropping a vertex. Releasing the key returns immediately to the Polygonal Lasso mode.
  • Temporary Freehand Switching: Holding the Alt (Windows) or Option (macOS) key and dragging the mouse temporarily switches the tool into the standard freehand Lasso Tool, allowing curves to be drawn. Releasing the modifier key snaps back to straight-line mode.

Closing the Path into a Pixel Selection

An active selection requires a completely closed boundary. The Polygonal Lasso Tool converts its open vector-like path into a closed selection using one of three methods:

  • Snapping to the Origin: Hovering the cursor directly over the initial starting point causes a small circle icon to appear next to the lasso cursor. Clicking this point closes the loop.
  • Double-Clicking: Double-clicking at any stage instructs Photoshop to automatically draw a straight line from the current cursor position back to the original starting point, instantly closing the boundary.
  • Keyboard Shortcut: Pressing Enter or Return accomplishes the same result as double-clicking, auto-connecting the final position to the origin point.

Once the perimeter is closed, Photoshop's rendering engine evaluates the interior of the polygon against the canvas resolution, applies any predefined feathering or anti-aliasing settings, and converts the mathematical path into an active, animated pixel selection.