How to Use the GEGL Spiral Filter in GIMP

This article provides an overview of the GEGL Spiral filter in GIMP, detailing how it generates mathematical spiral patterns including Archimedean, logarithmic, and linear types. You will learn the specific computational behavior of each spiral type, how the filter maps these curves onto an image or canvas, and how to configure key parameters such as radius, thickness, and balance to achieve precise graphic results.

The Core Function of the GEGL Spiral Filter

The GEGL Spiral filter in GIMP is a procedural pattern generator built on GIMP’s GEGL (Generic Graphical Library) engine. Its primary function is to compute and render mathematical spirals directly onto a layer or selection using parametric equations. Rather than requiring manual vector drawing, the filter generates continuous, anti-aliased spiral bands with customizable dimensions, rotation angles, and color balances.

Within the filter's settings, the Type dropdown determines the mathematical formula used to map the spiral's arms outward from the origin.

Spiral Types Generated by the Filter

1. Archimedean Spiral

In an Archimedean spiral, the distance between successive turnings remains constant. The radius \(r\) is directly proportional to the angle of rotation \(\theta\) (\(r = a \cdot \theta\)).

  • Behavior in GIMP: As the spiral extends outward from the center, each loop maintains an identical width and spacing relative to the adjacent loop.
  • Visual Characteristics: Uniform, tightly controlled, and mechanical.
  • Common Uses: Coils, watch springs, vinyl record groove simulations, and maze-like radial designs.

2. Logarithmic Spiral

In a logarithmic (or equiangular) spiral, the distance between turnings increases geometrically with each revolution (\(r = a \cdot e^{b\theta}\)).

  • Behavior in GIMP: The bands start extremely compact near the center and widen exponentially as they move toward the outer edge of the canvas.
  • Visual Characteristics: Dynamic, organic, and expansive. The angle between the tangent and radial line at any point is constant, preserving the overall shape regardless of scale (self-similarity).
  • Common Uses: Emulating natural structures such as nautilus shells, whirlpools, tropical cyclones, and spiral galaxies.

3. Linear Spiral

The linear spiral option in the GEGL filter modifies the mapping function so that the progression and edge transitions follow a strict linear interpolation rather than standard radial curve smoothing.

  • Behavior in GIMP: It produces a direct, uniform rate of outward displacement along the radial axis, creating high-contrast, evenly distributed bands with hard or direct transitions depending on the balance settings.
  • Visual Characteristics: Sharper, graphical geometric patterns where equal distribution across the plane is required.
  • Common Uses: Graphic backdrops, optical illusion art, and stylized sunburst or vortex effects.

Key Parameters for Controlling the Spiral

To modify the structure of linear, Archimedean, or logarithmic spirals, the filter provides several core controls:

  • Base Radius: Sets the initial offset radius from the center point before the spiral pattern begins expanding outward.
  • Thickness: Controls the stroke or band thickness of the spiral arms throughout the curve.
  • Balance: Determines the ratio between the foreground and background segments of the spiral stripes (e.g., thinning the primary color arm while widening the secondary space).
  • Rotation: Rotates the entire coordinate system around the center anchor, allowing precise alignment with existing image components.
  • Direction: Toggles between clockwise and counter-clockwise winding.