Photoshop Similar Command: Pixel Selection Criteria

Adobe Photoshop’s Similar command expands an existing selection across the entire canvas to encompass non-adjacent pixels that share matching tonal and chromatic values. Rather than relying on physical proximity, this command operates globally by evaluating specific color thresholds, active selection parameters, and document layer settings. Understanding these underlying criteria allows for precise, predictable selections of dispersed elements, such as skies, backgrounds, or specific recurring colors.

The Source Selection Color Range

The Similar command takes its baseline data directly from the pixels currently selected in the document. It does not select colors based on an average; instead, it analyzes every individual color value contained within the initial selection boundaries and uses each of those values as an independent reference point for the search.

Magic Wand Tolerance Settings

Although accessed through the main application menu (Select > Similar), the command's primary filtering mechanism is governed by the Magic Wand tool's options bar. The Tolerance value directly dictates how strictly or loosely the command evaluates color matches:

  • Low Tolerance (e.g., 0–10): Restricts the selection to pixels that are nearly identical in brightness and hue to the reference pixels. A tolerance of 0 selects only identical color values.
  • High Tolerance (e.g., 32–100+): Broadens the criteria, selecting pixels that fall within a wider tonal and color variation relative to the sampled area.
  • Range: The tolerance value is measured on a scale from 0 to 255, reflecting the 8-bit channel color spectrum.

Color Channel Values

Photoshop evaluates pixel data based on the numeric values across the active color mode's channels (RGB, CMYK, or Lab). For a non-adjacent pixel to be included:

  • Each corresponding channel must fall within the range defined by the reference pixel value plus or minus the set tolerance.
  • If specific channels are hidden or targeted in the Channels panel, the Similar command limits its evaluation strictly to the active channels.

Anti-Aliasing

The Anti-alias option, also controlled via the Magic Wand tool settings, dictates the edge transition of the resulting selection. When active, Photoshop softens the boundaries between selected and unselected pixels by introducing semi-transparent edges, preventing harsh, pixelated fringes across the canvas.

Layer Sampling Scope

The canvas-wide search respects the Sample All Layers setting located in the Magic Wand options:

  • Disabled: The command analyzes and selects non-adjacent pixels located only on the currently active layer.
  • Enabled: The command samples the composite color values of all visible layers across the canvas, though the resulting selection marquee is placed onto the active layer.

Bypassing Contiguity

The defining operational trait of the Similar command is that it systematically ignores the Contiguous setting of the Magic Wand tool. While the standard Magic Wand tool can be constrained to neighboring pixels, the Similar command operates under a mandatory non-contiguous rule, applying the tolerance and color parameters universally to every pixel visible within the canvas boundaries.