Reverse part of the tone scale
Solarization leaves darker tones near their source values and reverses brighter tones toward a negative. The threshold selects the boundary, while Tone reversal controls how far pixels move toward the inverted result.
Threshold softness prevents an abrupt contour by blending the effect across a luminance interval. Set it to 0 for a hard graphic transition or raise it for photographic gradients.
A darkroom accident, formalized
The look comes from the Sabattier effect: flashing a half-developed print with light reverses the tones that were still developing. Darkroom prints also show a thin bright line where reversed and normal tones meet, called a Mackie line, and a soft threshold at low values reproduces a similar glowing boundary.
Digital control removes the luck. The threshold slider does what exposure timing did by chance, repeatably and per pixel.
Example: metallic architecture shot
Buildings with bright sky behind them solarize well. Set the threshold near 140 so the sky and reflective glass reverse while the structure holds, keep Tone reversal at 100, and leave Keep color on: the channel crossovers turn a blue sky bronze and glass facades chrome-like. If the result is too loud, pull Tone reversal back to 60 rather than moving the threshold, so the geometry of the reversal stays where you placed it.
For the classic 1930s portrait look, switch Keep color off and raise softness above 40.
Placing the reversal line
The threshold is the whole composition tool here. Sweep it slowly and watch which surfaces flip: skies, skin, chrome, and water each cross at different luminance levels, so a small threshold move can hand a different object to the negative side. The Show before toggle under the preview helps confirm which regions actually changed.