Particles and weather

1. Animate one particle at a time, then the next. Each keeps its shape until it dies; lone pixels are allowed only on a particle's last frame — except 1 px rain and snow, which are the unit.
2. Motion law: start fast, slow down, shrink, darken or fade. Gravity acts before the fade. Add hue variation between particles of one burst (rules://20-color-for-pixel-art).
3. Depth by three layers: far, mid, near. Size ×2, speed ×2, value contrast +2 steps from far to near (derived). Far leaves are the near frame sampled at every other row and column (nearest neighbour); far snow moves at half the near amplitude and twice its period.
4. Loops are arithmetic. On a tile H px tall with F frames, a layer shifts k·H/F px per frame, k an integer, with wrap. Then every layer returns to its start on frame F. Pick F that divides H. Rain: H=16, F=4 → far 4 px, near 8 px per frame. Snow: H=16, F=8 → far 1 px, near 2 px per frame (derived).
5. No grids. Two particles of one layer never share a column within 4 px, and offsets never repeat in a pattern. Place by hand; check with look op ascii.
6. Rain = a short vertical streak, head brighter than tail. Near 3 px, far 2 px at 16 px canvases. Copy the streak down the lane over 4–5 frames; when it lands, play a 3–4 frame splash at the impact point. Wind: slant by 1 px of x per 2–3 px of y and shift the loop along that diagonal. Pale blue-grey, not white, on light backgrounds.
7. Snow = 1 px far, 1 px bright mid, 2×2 near, falling on a gentle S-curve. Near flake x-offset per 2 px fallen: 0 +1 +1 0 −1 −1 0 +1 (16 px period). Far: 0 0 +1 +1 0 0 −1 −1 per 4 px (derived). Let ~1 flake in 10 drift upward. Snow sits only on upward-facing surfaces. Breath puffs on a character are optional.
8. Leaves tumble in 4 frames at 125 ms: long diagonal → broad oval → edge-on → opposite diagonal. Fall 1 px/frame on a sine path; a 2×2 leaf flutters by dropping its top half every other frame. Near leaf ≈ 6×5, far ≈ 3×3. More wind = more leaves, faster travel and tumble; calm = none; all drift one direction per scene and flip together.
9. Wind is invisible: draw what it moves. Dust, leaves, cloth, hair, grass. Flag or hair waves: flow points 12 px apart moving 2 px/frame = a 6-frame loop. Tornado: stacked spinning discs, always the same rotation, 2 colours for depth, dust fading from the base.
10. Dust is born on the ground and takes the ground's colour. It accelerates fast, slows, becomes a swirl, then breaks into smaller bits. Landing dust only when fall speed is above ~50 % of terminal speed (Celeste).
11. Sparks, embers, ash: 1–3 px, three sizes, a sine drift, shrink then drop to a darker background tone, never pure white on dark. At most ~8 live particles per 32×32 area (derived) — "too many" is noise.
12. Weather must not eat the sprites. Overlay particles stay within 3 value steps of the background; keep the nearest layer lightest. If the sprites sink after assembly, drop the background layers by S −20, C −15, B +15 rather than brightening the rain.
13. Twinkle, don't flash. Stars: 1 px, plus, or 2×2 in 2–3 colours; a few change at a time, out of sync; a few bright points add rhythm. Pickup sparkle: 3–4 frames, a plus growing into a 4-point star and back; several variants, randomised.
14. Palette cycling does not exist here. The classic rain/snow-by-cycling trick needs engine support and a custom palette per background; draw frames instead.

Timing: rain 50–60 ms per frame, snow 100–120 ms, leaves 125 ms, dust 60–80 ms (derived), sparkle 60–80 ms.

Rain tile, frame 0 (16×16, seamless on both axes). Five near drops (3 px) and seven far (2 px). For F=4: frame n is this tile with far rows shifted 4n, near rows 8n, wrapped.

D = rain-near-tail  #8b9bb4
M = rain-near-mid   #a9b8d0
L = rain-near-head  #dbe4f2
d = rain-far-tail   #4f5f80
m = rain-far-head   #6f82a6
---
............d...
.D..........m...
.M..............
.L........D.....
..........M.....
........d.L....d
...d....m......m
...m............
................
d.....D.........
m.....M.........
......L.......D.
..............M.
....D...d..d..L.
....M...m..m....
....L...........

Splash, 3 frames (5×3). Put on the ground line at the end of the streak; frame 4 is empty.

L = rain-light  #c0cbdc
---
.....
..L..
.LLL.

L = rain-light  #c0cbdc
---
.L.L.
L...L
.....

M = rain-mid  #8b9bb4
---
.....
.M.M.
.....

Snow tile, frame 0 (16×16, seamless). Three near 2×2 flakes, six mid, eight far. Apply the S-curve offsets per layer on top of the vertical shift.

N = snow-near   #ffffff
n = snow-shade  #c0cbdc
P = snow-mid    #e4ecf7
F = snow-far    #8b9bb4
---
....F...........
..NN........F...
..Nn....P.......
..............P.
................
.....P..........
...........NN...
........F..Nn...
.P.............F
.........P......
...F............
.............P..
......NN........
......Nn........
F.........F.....
..............F.

Leaf tumble, 4 frames (6×5). Play 1 → 2 → 3 → 4 at 125 ms, drifting along the wind. Recolour for autumn (rules://20-color-for-pixel-art): same shapes, orange/brown ramp.

L = leaf-light  #99e550
M = leaf-mid    #6abe30
D = leaf-dark   #37946e
S = leaf-stem   #8f563b
---
....LL
..LLMM
.LMMMD
SMMDD.
......

L = leaf-light  #99e550
M = leaf-mid    #6abe30
D = leaf-dark   #37946e
S = leaf-stem   #8f563b
---
......
.LLLL.
SMMMMM
.DDDD.
......

L = leaf-light  #99e550
M = leaf-mid    #6abe30
D = leaf-dark   #37946e
---
......
..LM..
...D..
......
......

L = leaf-light  #99e550
M = leaf-mid    #6abe30
D = leaf-dark   #37946e
S = leaf-stem   #8f563b
---
LL....
MMLL..
DMMML.
.DDMMS
......

Landing dust, 3 frames (10×5). Bottom row is the ground line. Recolour from the ground tile.

L = dust-light  #d9a066
M = dust-mid    #b07a4e
---
..........
..........
...LL.LL..
.LLMMLMML.
.MMMMMMMM.

L = dust-light  #d9a066
M = dust-mid    #b07a4e
---
..........
.LL....LL.
LMML..LMML
.MM....MM.
..........

L = dust-light  #d9a066
M = dust-mid    #b07a4e
---
.L......L.
..........
.M..M..M..
..........
..........

Twinkle (5×5). Sequence: single centre pixel → plus → star → plus → pixel, 60–80 ms each.

W = spark-core  #ffffff
L = spark-arm   #c0cbdc
---
.....
..L..
.LWL.
..L..
.....

W = spark-core  #ffffff
L = spark-arm   #c0cbdc
---
..W..
..L..
WLWLW
..L..
..W..

1. Decide the layers (far/mid/near) and the tile H × F; write the shift per layer from rule 4 before drawing a single drop.
2. New sprite or layer at tile size (sprite_manage op new); draw frame 0 with draw op grid from a template, hand-moving offsets so no lanes align.
3. frame op duplicate to F frames. For each frame and layer, move the layer's pixels with transform op translate (dy = shift). Pixels that leave the bottom must re-enter at the top: draw op blit the bottom strip to the top before translating.
4. Add splashes: draw op grid at each streak's landing point, 3 frames.
5. look op filmstrip, then check the loop: frame F must lead into frame 1, and frame 0 shifted by H must equal frame 0.
6. frame op set_duration (rain 50–60 ms), tag op create the loop.
7. Composite over the background for the readability check (rule 12), then run validate; expect orphan-pixel flags on rain and snow — they are the material.

• Three layers differ in size, speed and value; no two lanes align.
• Filmstrip loops with no jump at the seam.
• Rain has head brighter than tail and a splash where it lands.
• Snow and leaves follow a curved path, not a straight line.
• Particles shrink or darken before vanishing; lone pixels only on last frames.
• Overlay does not reduce the contrast of sprites against the background.

• Saint11 tutorials: SmokeSheet, Blood, Wind, Ice, Vegetation, Stars, Fabric.
• SqdPxl "Easy Pixel Rain" and "Simple Pixel Snow"; Slynyrd Pixelblog 33 (wind effects) and 63 (readability tweak).
• Sprite Fusion "Bowch!" particle spec (leaf sizes and tumble); Celeste landing-dust threshold.
• Ferrari, palette-cycling talk (why not to cycle here).
• Gurney, Color and Light (snow colour); loop arithmetic and layer ratios are ours (derived).