How Photoshop Smooth Selection Rounds Sharp Corners

This article explains how the Smooth selection command in Adobe Photoshop rounds sharp corners on an active selection. You will learn the underlying mechanism Photoshop uses to calculate pixel boundaries, how the radius setting alters corner geometry, and how this process converts hard angles into clean, curved edges without losing the general shape of your subject.

The Underlying Mechanism of Smooth Selection

In Adobe Photoshop, every active selection is fundamentally handled as a 2D grayscale bitmap (an alpha mask), where pixels are either fully selected (white), fully unselected (black), or partially selected (gray values representing anti-aliasing). When you navigate to Select > Modify > Smooth, Photoshop performs an algorithmic modification on this underlying mask to eliminate high-frequency directional changes, such as 90-degree corners.

The process relies on a combination of morphological filtering and neighbor-pixel averaging:

  1. Pixel Neighborhood Analysis: When you specify a Sample Radius (between 1 and 100 pixels), Photoshop sets up a circular search window around each boundary pixel of your selection.
  2. Filtering Outliers: For sharp exterior corners (convex angles), the corner tip contains very few selected neighbors within the circular search window. The algorithm identifies these isolated pixels and unselects them.
  3. Filling Recesses: For sharp interior corners (concave angles), the empty space is surrounded predominantly by selected pixels. The algorithm fills this recess by turning those unselected pixels into selected ones.
  4. Edge Reconstruction: After evaluating the neighboring pixels, Photoshop redraws the boundary along the circumference of the circular radius, transforming sharp points into tangential arcs.

The Role of the Sample Radius

The Sample Radius determines the exact degree of rounding applied to the corners:

  • Low Radius (1–3 pixels): Acts primarily as a clean-up filter. It removes minor pixelation, "stair-stepping" (aliasing), and micro-jaggedness without noticeably altering the structural shape of prominent corners.
  • Medium to High Radius (4–30+ pixels): Bridges larger pixel distances. At a 90-degree corner, a higher radius creates a visibly rounded curve because the algorithm cuts deeper into the exterior apex and builds further out into interior notches.

Because the command calculates distance in absolute pixels, the visual result depends directly on the image resolution. A 10-pixel radius on a low-resolution 72 PPI web graphic will create a substantial, dramatic curve, whereas the same 10-pixel radius on a 300 PPI print-ready image will produce a subtle, tight fillet.