Create Optical Funnels Using GIMP Whirl and Pinch

This guide explains how to use the Whirl and Pinch filter in GIMP to generate convincing optical funnel effects. By manipulating the Pinch parameter alongside radius and whirl settings, you can warp flat patterns, grids, or starfields into deep, perspective-bending spatial vortexes and gravitational wells.

Understanding the Pinch Parameter

The Pinch setting controls whether pixels are drawn toward the center of the selection or pushed outward toward the boundary:

  • Positive Values (0.000 to 1.000): Pull the image inward toward the center point. This compresses pixels radially, creating the illusion of extreme depth, recess, or an inward-sloping funnel.
  • Negative Values (-1.000 to 0.000): Push the image outward from the center, producing a bulbous or convex lens effect.

To simulate an optical funnel, you will primarily work with positive values to produce an inward cone.

Step-by-Step Implementation

1. Prepare the Base Layer

Funnel distortions rely on underlying structure to convey depth. Start with a layer featuring high-contrast geometric detail:

  • A checkerboard pattern (Filters > Render > Pattern > Checkerboard)
  • A radial grid or concentric circular lines
  • A dense starfield or abstract linear texture

Ensure your target layer or selection is square if you want a uniform, symmetrical funnel.

2. Open the Filter

Navigate to Filters > Distorts > Whirl and Pinch... to open the tool dialog and enable the on-canvas preview.

3. Configure the Funnel Geometry

  • Pinch: Set this to a high positive value, typically between 0.600 and 1.000. This draws the texture tightly inward, creating the steep walls of the funnel.
  • Radius: Adjust the radius slider to define how far outward the funnel reaches. A value between 1.000 and 2.000 ensures a smooth transition between the distorted center and the undistorted outer edges of the canvas.
  • Whirl Angle: Set this between 0.0 and 90.0. A setting of 0 creates a straight, symmetrical plunge, while a moderate angle adds a rotational twist typical of accretion disks and fluid vortexes.

4. Compound the Pinch for Greater Depth

A single pass at maximum pinch (1.000) may not create enough depth for extreme perspective. To achieve a deeper funnel:

  1. Apply the filter with Pinch set to 0.700–0.800.
  2. Press Ctrl + F (or Cmd + F) to re-apply the filter, compounding the inward pull without causing severe pixel distortion at the outer boundary.

5. Enhance the Optical Illusion with Shading

An optical funnel requires lighting cues to establish physical volume:

  1. Create a new transparent layer above your distorted texture.
  2. Select the Gradient Tool (G), set the shape to Radial, and choose a gradient that fades from transparent to black.
  3. Drag the gradient from the center of the funnel outward to place deep shadow at the core and highlight the outer rim.
  4. Set the shadow layer’s blend mode to Multiply or Overlay and lower the opacity to suit your scene.