A blurred copy of the image added back via screen or add blend is a cheap bloom/glow effect

Bloom (the glowing halo around bright areas) is normally produced by expensive multi-pass blurring pipelines in real-time engines. A cheap approximation is to take a blurred copy of the current image and composite it back over the original using an additive or screen blend mode. Screen blend lightens without oversaturation; add blend is brighter and can oversaturate. Because blur-soften averages nearby pixels, bright areas spread their brightness outward in the blurred copy, which when composited back creates the perception of glow. This is a fast trick requiring only one blur pass and one blend operation, and is effective for creating ethereal or radiant looks without additional rendering complexity.

Examples

Hydra: src(o0).blur(2).add(src(o0),0.5).out() — blurred copy added back at 50% mix creates a glow around bright elements. Increasing blur radius extends the glow.

Assessment

Describe the two-step procedure for creating a cheap bloom effect from blur-soften. State which blend modes are appropriate (and the difference between them), and explain what aspect of human visual perception makes this approximation convincing.