Definition
The bloom effect is a post-processing technique used in 3D games and film graphics that makes light glow and softly bleed out around very bright light sources. It gets past a screen's physical brightness limit and gives you the vivid feeling that an object is shining so intensely it almost hurts to look at.
A Trick to Fool the Limits of Your Screen
The sun or a laser beam on your screen doesn't give off enough real energy to make you squint like real light does. That's because a monitor has a hard limit on its maximum brightness: pure white.
So graphics developers came up with a clever trick to fool your eyes. When you look at a very strong light in the real world, the light scatters inside a camera lens or the lens of your eye. It spills past the edge of the source and spreads out in all directions as a white haze.
3D graphics copy this optical effect. They paint a soft halo of light around glowing objects. Then your brain gets fooled into thinking the object is shining brighter than the screen could ever really show.
How Does a Computer Make Light Spread?
If you just blur the whole screen, you get a fuzzy, out-of-focus, low-quality image. So the bloom effect works in three careful steps.
First, from the finished original frame, it picks out only the pixels brighter than a threshold the developer sets. Dark night sky and ordinary brick stay behind. Only the high-brightness areas, like streetlights or magic spells, get pulled out.
Next, it applies several layers of a soft blur filter to that bright-light image to create a gentle glow. Finally, it layers the blurred light image on top of the sharp original frame. The result is a natural halo of light around each light source.
To Be a Bit More Precise
To be a bit more precise, bloom doesn't physically calculate how particles of light travel. It's a post-processing graphics technique painted on after the frame is finished.
It turns an optical limit to its advantage: the way light partly scatters inside a camera lens or an eye. It boosts realism that way. But if you overdo it, the whole screen looks foggy, object outlines get mushy, and it can cause eye strain.
That's why modern game engines pair bloom with HDR (high dynamic range) rendering. The engine calculates a much wider range of brightness than a monitor can show. This is an internal calculation method in the engine, and it's different from display features like an HDR monitor. Thanks to it, bloom can be applied delicately to only the light sources that were actually calculated as very bright, giving a clear, three-dimensional look.
Common misconceptions
The bloom effect is a technique that blurs the entire screen.
It doesn't blur the whole screen. It's a precise compositing technique that picks out only areas brighter than a set threshold, spreads that light out, and layers it over the original.
Where you meet it
In a sci-fi game, when you swing a lightsaber, a bright, dazzling halo of light radiates around the blade.
When you look out from inside a dark cave, light spreads from the bright edge of the cave entrance onto the dark rocks around it.
In one sentence
The bloom effect is a graphics post-processing technique that softly spreads light to overcome a monitor's brightness limit and capture the glare and glow of bright light.