Parallax

Source: src/gpu/render-graph.ts (initialColumns), src/gpu/pipelines/render-glyphs.ts

Parallax is the depth illusion: some columns read as near (fast, bright, crisp) and others as far (slow, dim, soft). It’s driven entirely by one per-column value — depth — derived from the column’s fall speed.

Depth from speed

When columns are created, each gets a random speed in speedRange = [min, max], and a depth is the column’s normalized position within that range:

$$\text{speed} = \operatorname{lerp}(\text{min},\,\text{max},\,\xi), \qquad \text{depth} = \frac{\text{speed} - \text{min}}{\text{max} - \text{min}} \in [0, 1]$$

(with $\xi$ a uniform random, and a guard so that when min = max the span is zero and depth = 1). So fast = near (depth → 1), slow = far (depth → 0) — exactly how real parallax reads: distant things drift slowly.

The speed spread is the whole effect. Collapse speedRange to a single value and every column shares one depth — a flat field. That’s literally what parallax={false} does: it sets speedRange = [1, 1] and depthDim = 0.

Three depth-correlated cues

depth then feeds three cues in the glyph shader, all keyed on the same value so they reinforce each other:

1. Speed (already above) — far columns fall slower.
2. Brightness dimming — far columns are dimmed toward 1 - depthDim: $\text{depthDimming} = \operatorname{mix}(1 - \text{depthDim},\; 1,\; \text{depth})$ depthDim (default 0.3) is how dark the farthest columns get; near columns (depth → 1) stay full brightness.
3. Edge softness — far columns get a wider anti-aliasing band, faking depth-of-field blur: $\text{softness} = \operatorname{mix}(2.5,\; 1,\; \text{depth})$ Near columns get a crisp ~1px edge; far columns get a 2.5×-wider, blurrier edge. (See Glyph rendering for how softness widens the smoothstep band.)

A note on depthDim vs enabled

There’s no enabled flag on parallax (unlike bloom/crt). It doesn’t need one: “disabled” is fully expressed by the values (speedRange = [1,1], depthDim = 0), so nothing downstream has to branch. depthDim is patched into the uniform every frame (live), while speedRange only takes effect on the next column (re)spawn, since it’s baked into depth at creation.

tailLength is independent

Trail length is rolled separately per column (tailRange) and is not correlated with depth — a deliberate choice for variety. A near, fast column can still have a short trail and vice-versa.