Matrix digital rain · deepseek-v4.1-flash

Let me build the Matrix rain app. I'll write index.html, then test in browser.

Let me create a solid implementation.

Key requirements:
- multiple independently moving columns of green katakana glyphs
- brighter leading character, fading trail
- vary column speed and start position
- recycle columns after bottom
- Pause/Resume and Restart buttons
- Resize canvas to viewport without stretching
- local fonts only, procedural

I'll implement with requestAnimationFrame, time-based movement. Use canvas 2d.

Let me write it.

Design: column width ~16px, fontSize 18px. Each column has y position in pixels, speed px/s, trail length, glyphs array. Draw with fade: fill canvas with semi-transparent black each frame gives trail naturally. But restart must clear trails — that's fine, fill solid black.

Actually with full-screen fade approach, recycling and independent movement is simple. But "clears previous trails" on restart: clearing canvas plus resetting works.

I'll use trail array approach for precise bright leading char. Each column draws `len` glyphs; leading is bright white/green, others alpha decreasing. Each frame fill black with alpha ~0.08 to create fade — but then old glyphs fade. Combined with per-glyph alpha that's fine.

Better: don't fade whole canvas; clear each frame and draw trail explicitly. That gives cleaner control and restart clears nicely. But explicit trail means glyphs stay same per cell while column moves; typical matrix changes glyphs as it moves. I'll update glyph char randomly occasionally.

I'll do explicit clearing approach: clearRect, then draw each column's glyphs from head backwards with alpha based on distance.

Movement in continuous pixels; head position. Columns have cellH = fontSize. The trail is a list of positions (head - i*cellH). Draw each.

For "fading trail", alpha = 1 - i/len. Leading char bright (near white).

Speed: vary, and some columns start off-screen at negative y or at random positions. On recycle, reset head above top and new random params.

Pause: stop rAF updates, keep last frame. Resume continues with proper dt (avoid jump by resetting lastTimestamp).

Resize: set canvas width/height to devicePixelRatio scaled, recompute columns count, preserve or reset. Requirement: "fit viewport without ... leaving an uncovered strip". On resize, rebuild columns to fill width. Reset columns is fine and simplest. I'll rebuild preserving progress optionally; just reset.

Use dpr for crispness. Set canvas.style width/height 100%.

Let me write the file.

Font: use "monospace" and a stack of locally available: "MS Gothic", "Hiragino Kaku Gothic", monospace. Fallback monospace. Katakana glyphs drawn procedurally.

Half-width vs full-width katakana: use full-width katakana range U+30A1-U+30F6.

Let me write.

I'll include a simple overlay of controls with buttons at top-left, styled.

Let me code.

const KATA = "アァカサタナハマヤャラワガザダバパイィキシチニヒミリヰギジヂビピウゥクスツヌフムユュルグズブヅプエェケセテネヘメレヱゲゼデベペオォコソトノホモヨョロヲゴゾドボポヴッン";

Column class:
- x
- headY (pixels)
- speed (px/s)
- glyphs: array length trailLen of chars, index 0 = head
- trailLen
- fontSize? global cellH.

Draw:
for i in 0..trailLen-1:
  y = headY - i*cellH
  if y < -cellH or y > H+cellH continue
  alpha = i===0 ? 1 : (1 - i/trailLen) * 0.9
  head color: rgb(220,255,220) whiteish; else green rgb(0, 255, 70) with alpha.
  Also maybe first few brighter.

Random char change: for head each frame maybe change char occasionally to simulate flicker. I'll update each glyph with small probability.

Recycle: when headY - trailLen*cellH > H (fully left bottom) reset. Actually when headY > H and trail gone. Use: if (headY - trailLen*cellH) > H reset.

Reset: headY = -Math.random()*... start above; speed random; trailLen random; new glyphs.

Initial start position varied: headY = random between -H and H. "Vary start position" yes.

Let me write it now.

I'll cap dt to avoid huge jumps.

Let's write file.

Now test in browser.

The screen is blank/black. The initial render seems empty. Possibly the font glyphs aren't rendering? Or the animation columns all start off-screen with negative headY, so nothing visible initially? Let me think: resetColumn freshStart sets headY = -Math.random()*(cssH + trail*CELL_H), so between 0 and -(H+...), i.e. above screen. Many would be above. After 80ms not much. But wait the screenshot is at +0ms after reload, could just be frame 1 before much movement.

Also I notice a stray glyph at top-left "T"? Let me sample again after some time using motion frames.

Actually wait, there may be an issue: initial render happens before start, and all columns start above the viewport, so blank. Over time they fall in. Let me check with motion.

The animation works. Columns are moving, brighter leading glyphs visible (white-ish heads), fading trails. Great. But I notice the rain only appears in the top ~40%? Actually in frame 4 the trails extend down to ~610px out of 800. The bottom portion appears mostly empty. Wait is that because trails are short and columns are high up? Actually there seem to be full trails visible. It's fine—the effect occupies the visible area as columns fall.

Hmm, but the trails look like they're not spanning the full screen; there are large black regions. That's acceptable for matrix rain. But maybe column density is too sparse at bottom because trail lengths are 8-28 and cellH=20, max ~560px trail. Fine.

Now I want to verify brighter leading glyphs. In frame 4, I see the bright heads at the bottom of each column (e.g., "バ", "ハ", "ボ"). Good.

Now test Pause. Click pause button at ~(55,29).

Paused. Button now says RESUME. Let me take two screenshots at intervals to confirm frozen. Actually motion while paused should show identical frames. Let me use inspect to check isPaused and sample column headY twice. I'll use browser motion which captures frames. But if paused, all frames identical. Let me capture motion.

Actually I can't run arbitrary JS with the browser tool except DOM inspect? There's no eval action. Let me just use motion (multiple frames) while paused, should be identical.

All four frames identical while paused — confirmed frozen. Now resume and check movement. Then Restart to check trails cleared.

The glyphs changed (heads moved down, glyphs mutated). Good — resumed. Now test Restart: click Restart at ~(160,29). After restart, it clears and rebuilds; initial frame mostly black since columns start above. Let me click then screenshot immediately and after some time.

Actually note restart render() draws columns which start above screen -> mostly blank. That demonstrates trails cleared. Let me click restart and immediately motion capture.

Restart cleared all trails — canvas is black. Good. Now let it run to confirm recovery.

Also test resize behavior. Let me check the canvas covers viewport. There's a slight concern: `canvas` resets transform, fine. Let me verify resize by... browser tool has no resize. I can check via inspecting computed style. But I can't run JS. The resize handler is standard. I'll trust it but let me verify the uncovered strip concern: canvas is `position:fixed; inset:0; width:100vw; height:100vh` plus set explicitly in resize. Good coverage.

One issue: on resize, buildColumns resets all columns, which is acceptable.

Let me capture motion after restart to confirm rain returns. Then done. Also check no console errors (only sandbox warning). Good.

Let me do a final motion capture.

Rain has returned and continues. Everything works. The head glyphs are bright white/green, trails fade. Independent columns moving.

One thing I notice: the trails are somewhat sparse and there are big empty vertical gaps. That's a bit unusual for Matrix rain but acceptable — variation is required. However, an aesthetic concern: the effect is confined to the top ~50%? Looking at frames, the glyphs extend down to ~600+ in some frames, but much of lower screen is black. Because trails max 28*20=560px and columns recycle once tail passes bottom, but heads move continuously so at any moment rain should be spread. Actually many columns start above and fall. It's fine.

Wait, actually there's a possible issue: density. Columns are 18px apart, so ~66 columns across 1200px. In the screenshots I only see maybe 12-15 columns with glyphs. That means many columns are above the viewport at any time. That's because resetColumn freshStart distributes headY over the full height so initially many visible, but over time as they fall at different speeds they bunch? No—they should spread evenly over time. Hmm, but the recycle resets to only above the top with small random offset, so once settled, columns are distributed across [top - trail, ...]. Actually the distribution should be roughly uniform across the time it takes to traverse. It seems fine—the screenshots show about 15 columns, but there might be more faint ones hard to see against black.

Actually look at frame 4 of last motion: I count maybe 15 columns. With 66 columns and roughly 560px trail in 800px screen, expected visible fraction should be high. Something's off: perhaps the recycling makes columns cluster? No.

Hmm, wait. Let me reconsider. The screenshot only captures a portion? No, full viewport 1200x800.

Let me count columns in the frame: positions roughly x=0, 45, 90, 150, 230, 300, 365, 420, 490, 570, 640, 720, 800, 880, 960, 1050, 1130, 1190. That's ~18 columns, i.e. spacing ~65px, not 18px. That suggests only every ~3-4th column is visible. Interesting.

Why? CELL_W=18, count = ceil(1200/18)+1 = 68 columns. Each column occupies x = i*18. But text is drawn at col.x. With font 17px, adjacent columns 18px apart would be very dense. In screenshots spacing is much wider. Unless... the browser viewport isn't 1200 CSS px? The devicePixelRatio maybe 1. The screenshot is 1200 wide.

Wait, maybe font rendering makes wide katakana (17px monospace -> each full-width katakana renders ~17px wide?) Actually monospace font with full-width katakana may render wider, but x spacing is still 18px.

Hmm, but the observed spacing ~65px suggests CELL_W effectively larger. Let me check: canvas width set to cssW*dpr where dpr = devicePixelRatio. If dpr is e.g. 1, fine. If dpr were 3.5, canvas.width=4200 but style width 1200, and setTransform scale 3.5, so drawing at col.x=18 -> 18 css px. Fine.

So why sparse? Let me look again: maybe I'm miscounting because trails are short and many columns are off-screen above at that moment. The columns that ARE visible each have long trails ~400px. Between them there are columns with heads above screen (trail not yet visible) or below (recycled). Actually a column's trail spans 160-560px vertically. If heads are uniformly distributed over an extended range, at any instant roughly fraction of columns visible = trail_height / cycle_height. But cycle just wraps from bottom to top with random small offset, so distribution of head positions is uniform over [-(trail*20), H]. Then visible fraction ≈ trail*20/(H+trail*20) ≈ 400/1200 ≈ 33%. With 68 columns, ~22 visible. That matches ~18!

So the visual is fine and intended. But for a denser Matrix look, I could increase density by reducing CELL_W or increasing trail lengths. The requirement only says multiple columns, vary. It's acceptable. But denser looks better. I have time budget. Let me improve aesthetics modestly: increase trail length range and reduce column spacing? Reducing CELL_W to 16 and increasing trail gives denser rain. But too dense could look cluttered. Current look is actually quite nice and clearly Matrix-like.

I think it's good. However, I want to double-check resize doesn't leave a strip. Can't easily trigger resize with the tool. The code sets canvas width/height and style explicitly, plus buildColumns. Reasonably safe.

Actually, one potential bug: on initial load `resize()` is called, which calls buildColumns then render. Then start(). Good.

One more: during resize while paused, render() draws, but not step. Fine.

I'm fairly confident. Let me do a final quick check of the source file for correctness and report. Also verify there are no external resources: none. Fonts are local stack. Good.

Let me also make the effect a bit denser for a better Matrix feel since I have budget. I'll bump trail range and add a second layer? No, keep simple. I'll reduce CELL_W to 16 and trail 10-30. Actually, changing risks introducing bugs. Current works and looks good. I'll leave it.

Actually, let me reconsider the requirement "brighter leading character and a fading trail." The head is #d9ffe4 with glow; trail green fading. Confirmed in screenshots (white/cyan heads at bottom of columns). Good.

Report done. Final file already saved. Let me just quickly read the file to confirm it's intact, not necessary. I'll finish.

Let me verify final file exists.

grep found 0 "http" matches (exit 1 due to no match) — confirms no external URLs. Good.

Done. Report.