Bloom

Source: src/gpu/pipelines/bloom.ts

Bloom is the glow around bright heads. It’s the classic three-step image-space effect: extract bright pixels, blur them, add them back. It runs at half resolution (cheap) into HDR targets, only when bloom is enabled.

1. Bright-pass extract

Keep only what’s brighter than the threshold, per channel, with a hard knee:

$\text{bright} = \max(\text{src} - \text{threshold},\; 0)$

Dim pixels go to black; bright pixels survive minus the threshold. Subtracting (rather than just masking) makes the transition into the glow smooth. threshold is the bloom.threshold knob (default 0.8). This is drawn into a half-resolution target — bloom is low-frequency, so half-res is invisible in the result and a 4× pixel saving.

2. Separable Gaussian blur

A true 2D Gaussian blur of radius $r$ costs $(2r+1)^2$ texture samples per pixel. But a Gaussian is separable — a 2D blur equals a horizontal 1D blur followed by a vertical one:

$G_{2D}(x,y) = G_{1D}(x)\,G_{1D}(y)$

so the cost drops to $2(2r+1)$ samples. This effect does exactly that: a horizontal blur pass, then a vertical one (direction = [1,0] then [0,1]), each a 9-tap kernel (radius 4).

The weights are a sampled Gaussian, normalized so the symmetric kernel sums to 1:

$w_i = \frac{e^{-i^2 / 2\sigma^2}}{w_0 + 2\sum_{j=1}^{r} w_j}, \qquad \sigma = 1.8$

and the blurred value is the weighted sum of the center and the four taps each side:

$\text{out} = w_0 c + \sum_{i=1}^{4} w_i\,(c_{+i} + c_{-i})$

Taps step one texel at a time (2 / resolution in the half-res target’s uv space), along the pass direction.

3. Combine

The blurred bloom is added back over the full-resolution scene, scaled by intensity:

$\text{out} = \text{scene} + \text{intensity}\cdot\text{bloom}$

intensity is bloom.intensity (default 1.5). The result goes into an HDR combine target — values can exceed 1.0 here, which is what lets the CRT tone-map step blow highlights out to white.

Head emission — giving bloom something to extract

There’s a catch the threshold and intensity can’t fix on their own: if nothing in the scene exceeds 1.0, there’s barely anything above the threshold to extract, and on the bright pixels themselves scene + intensity·bloom just clamps back to 1.0 (no visible change). Cranking intensity multiplies a near-empty signal; lowering the threshold just blooms the whole scene uniformly. Either way the glow stays weak.

The fix is head emission: the glyph pass multiplies the head by bloom.emission (default 2) so it writes above 1.0 into the HDR target — burning the head hotter than the display can show. Now the extract has real headroom (emission − threshold instead of 1 − threshold), so blooming the heads produces a strong halo, and threshold/intensity actually bite.

A side effect: pushing all channels past 1.0 means the head core clamps toward white while the green survives in the surrounding glow (the green channel dominates after the blur) — the classic white-hot head + green halo. Lower emission toward 1 to keep greener cores; emission = 1 disables it (heads stay in [0,1]). Emission only applies while bloom is enabled.

Why HDR + half-res

• HDR (rgba16float) throughout so the extracted highlights and the scene + intensity·bloom sum aren’t clipped before the final tone-map.
• Half-res for extract + blur because the glow is smooth; you can’t see the resolution drop, and it quarters the work.

When bloom={false}, the whole extract → blur → blur → combine chain is skipped and the scene goes straight to the final pass — a real GPU saving, which is why it’s a boolean gate rather than intensity: 0.