bloom() makes the bright parts of the picture glow into what is around them: the soft light of the last consoles of the era. Only what is brighter than threshold glows, intensity is how strong the glow is and radius about how far it reaches.
It takes several reads of the frame: the bright parts are picked out at half the game's size, blurred across and down at a quarter, and added back over the sharp picture. A wide glow costs about what a narrow one does. The glow is added the way light adds on a screen, so it brightens without ever passing white.
↑ ↓ for the threshold, ← → for the intensity and Space to switch it off. Watch the highlights on the balls as the light swings.