That glow is a fabrication. The lily on plain paper sends no extra photons to the eye, yet the visual system insists on treating it as a faint light source rather than just a paler patch of pigment.
At the heart of this effect is contrast, not brightness. Tiny differences along the lily’s outline set up strong luminance gradients, and edge detection circuits in the retina and visual cortex boost those gradients through lateral inhibition, so the interior looks slightly overexposed even when a photometer would record identical reflectance to the background.
More radical is what the brain assumes. Human vision runs a form of predictive coding, constantly inferring where light must be coming from to explain the raw signal, and when a pale region is smoothly shaded and bounded by darker contours, higher visual areas classify it as illuminated surface rather than just lighter paint, assigning a sensation of self luminous softness without any change in physical light.
