The scene is rendered to a texture, then a chain of full-screen passes — the EffectComposer — reworks the image before it hits the screen. Toggle UnrealBloomPass and watch emissive surfaces glow. Crank the threshold to pick what blooms.
Bloom only lifts pixels brighter than threshold. The glowing core is an emissive material; the dim cubes stay sharp because they fall below the cutoff. Resize re-syncs composer.setSize() just like the renderer.