ImageGear for C and C++ on Windows v19.10 - Updated
Solarize Effect
User Guide > How to Work with... > Image Processing and Cleanup > Special Effects > Solarize Effect

IG_FX_solarize mimics the accidental exposure of photographic film to light.

Solarize inverts the colors of pixels whose lightness exceeds a given threshold. If lightness is considered on a 0 to 1.0 scale, the default threshold used by various paint programs is 0.5.

In the following set of images, Solarize was performed with a threshold that caused the color of the sky to be inverted while the rest of the image remained unaffected.

Original

Solarize Effect