Sprite Lab in Lab2: effects - #75703

[written by Claude]

Two new blocks on the free play level: set weather to [none | rain | snow] fills the whole scene with weather that reacts to the world, and burst N [sparks | confetti] from [sprite] flings a flourish out of a sprite. Both run as a particle layer in the engine, not as sprites: they never count against the sprite budget, answer no events, and appear in no picker.

Both descend from legacy Sprite Lab's effect blocks. create [burst | pop | rain | spiral] effect with N [image] sprites (gamelab_makeBurst) spawns a one-shot batch of real sprites, each given a behaviour and a short lifetime, from the stage centre and against the sprite limit; do [effect] effect (gamelab_setForegroundEffect) paints a canvas overlay with rain, confetti and a starburst among its looks. The burst block keeps the first one's shape and gains an origin sprite and a look instead of an image; the weather block takes the overlay's rain and makes it continuous and world-aware. Neither legacy block is in a Lab2 toolbox.

What students see

• Rain falls at a slight angle, splashes off tiles and characters without reaching into them, and is already falling on the first frame. Snow sways down, rests on a tile for half a second and fades, and melts when a character touches it without being pushed. None turns it off.
• Weather follows gravity. Flip it and the drops slow, turn, and come up to speed the other way, landing on the undersides of tiles. Zero gravity still reads as a side view, so weather falls as under the default.
• Sparks are hot whites and golds, fast and brief, shrinking as they fade. Confetti is coloured paper that launches gentler, floats down slower, tumbles, and lasts two to three times longer. Bursts pass through everything. A second burst adds to the first; an image on several sprites bursts from each of them; no sprite means the middle of the stage.
• Weather lands on things and bounces off them but never moves anything and never fires a touch event.

Recordings

Rain & sparks

Snow & confetti

Rain with a gravity flip

How

• weather.ts: a pool of plain records per kind with a rule table (count, speed, sway, what a contact with a tile or a body does), a felt gravity that eases toward the scene's, and drawing in world space. bursts.ts: one-shot bursts with a table per look (launch, drag, fall, life, size, shape, palette) and a fade through the canvas alpha, since in our p5 a tint's alpha is tint strength, not opacity.
• The engine steps and draws both inside the paint after the sprites, so they sit under the camera, and steps once per frame even when a thumbnail capture paints twice. Weather gets the tiles and every other sprite as bodies through the platform resolver's own collision geometry (wallBox, bodyBox, now exported from platformPhysics.ts). The spark image (static/spritelab_lab2/particles/spark.png, drawn in code) is tinted once per palette colour into a p5.Image; a p5.Graphics would not do, since its canvas is scaled by the pixel density while image() reads it at its logical size.
• Two lab-defined blocks, added to both scene toolboxes of the free play level; the burst's sprite field joins the rename and delete cascades. Bursts are capped at 100 per call and 400 in flight.

Tried and set aside

• Particles as sprites through the resolver: plain records were simpler and cheaper, and the resolver's geometry could be shared without them.
• Weather anchored to a sprite (chimney smoke, a sparkling treasure): deferred for world-wide weather; the pool and rules would carry over.
• A splash ring where rain lands read as a drawn effect; the drop now bounces, fades and vanishes instead.
• Snow pushed aside by the player felt like shoving snow around; a touched flake now keeps its path and melts.
• Hanging in zero gravity: a weightless scene still reads as a side view, so zero falls as the default.
• Bursts of a project image that landed, bounced and rested read as items to pick up, not a flourish; replaced by sparks that touch nothing.
• Sparks and confetti as one motion in two palettes were indistinguishable; each look now has its own motion and shape.

Where this could go

• A student-editable system. Each weather kind and burst look is a handful of settings (count, speed, what happens on contact, colours), which is the shape the Systems idea from the behavior2 prototype needs: blocks like "set rain to N drops" or "make snow bounce off tiles" would change those settings, and the engine would run them as it does now.
• Events from contacts. The body contact in the weather step knows which box it hit, so "when rain hits [sprite]" is a lookup and a dispatched event.
• Anchored emitters and student images are each one more setting (where particles start, what they are drawn with) plus a block to set it.

Testing

• Unit tests for the weather (pool, recycling, contacts for both kinds, seeding outside tiles, a bouncing drop's streak staying above its tile, gravity easing and flip, undersides, zero gravity, drawing), the bursts (spread, colours, drag and gravity, the two looks' motion, the fade, image and paper drawing), the exported boxes, and both blocks' generators and dropdowns.
• In the browser as a test student: rain and snow on the platformer demo level at 61fps; a gravity flip landing drops under tiles; both blocks generating their calls from the free play flyout and running on Play, including sparks after the p5.Image fix.
• Not tested: a touch device.