Cloudshadow - Drifting Cloud Shadows for GameMaker

The shadows of passing clouds, drifting across your rooms. Cloudshadow reads the room you already built (which cells are water, roof, cliff, wall or open ground) and draws the weather's shadow over it every frame, as one quad. Nothing to author, no sprite to paint, and nothing your game draws is replaced: what is under a cloud is darkened and cooled, the way ground looks when a cloud takes the sun.

// put obj_cls_shadow on a layer above your ground: that is the installation. Then, if you like:
cls_surface_layer("Tiles_Harbour", "water");   // say what a tile layer is (names like Water, Roof, Cliff are read for you)
cls_set_sky("squall", 6);                      // crossfade to a squall line over six seconds
var my_light_lost = cls_shade_at(x, y);        // 0 in full sun .. 0.58 under the thickest cloud

What your players see

• Thirteen skies, each built differently, not recoloured: clear, fair, popcorn cumulus, scattered, broken, a mackerel sky in rows across the wind, cirrus streaks, a thin high veil, overcast, fog banks, a squall line, towering cumulonimbus with soft anvils sheared downwind, and storm. Changing sky crossfades over the seconds you choose.
• Three altitudes drift at three speeds (0.56, 1.28 and 2.0 cells a second at the designed wind) along a wind that slowly veers, so the weather never moves as one rigid sheet.
• The shadow lands by surface. Water loses the most light and goes coolest. A roof stands two cells up, so a cloud reaches it before the ground beside it. A wall face takes a grazing change. A covered cell takes none. Measured on the demo valley under a storm: the lake loses 52.4% of its light, the roof 44.1%, the meadow 40.9%.
• Never a black hole. The deepest shadow is capped at 58% of the light, whatever the intensity.

Built to plug in

• It reads your room: tile layers by their names (Water, Roof, Cliff, Wall, Interior...), or your rules: a layer, a list of tile indices, an object's box, a rectangle. The sky is derived from the room's make-up unless you set one; a room with almost nothing open to the sky opts itself out. cls_draw_surface_map() shows you what it read.
• Your game logic can ask: cls_shade_at(x, y) returns the light the clouds take at a point right now, from the same arithmetic the shader runs: plants that want sun, a stealth bonus in shade.
• Day and night: cls_set_daylight(0..1) from your own cycle, and the shadow eases off as the sun goes.
• Saving: cls_save() / cls_load() as a short JSON string; weather also carries from room to room on its own.
• Cheap: one quad and one shader per frame, nothing read back from the GPU while you play. A sky's first use costs 34 to 64 ms on the CPU and about a millisecond on the GPU (measured, shared machine).

What is in the download

Cloudshadow.yymps (Tools > Import Local Package) with raw, readable GML and two short GLSL ES shaders, obj_cls_shadow (the drop-in), and a demo room: a painted valley with a lake, a cottage, a cliff and a road, where LEFT and RIGHT change the sky, W turns the wind, N switches day and night and M shows the surface map. Plus the Readme and the licence.

Verification

Ported from our RPG Maker MZ plugin Cloudshadow and held to it: the in-engine suite checks every one of the 315 cloud masses and the shade at 810 points of the MZ edition's five skies against the MZ code, exactly. 156 in-engine checks in a real GameMaker build, including every sky's baked sheet and the drawn shadow read back from the GPU; the package installed into a blank project and the Readme's code run as written; 156 hostile calls on the documented functions with no crash. Built and tested on GameMaker 2024.14 (runtime 2024.14.4.268, the VM, Windows); HTML5, mobile and YYC were not built.

From the same studio

• Welkin draws the SKY above a side-view scene; Cloudshadow draws the clouds' shadow on a top-down ground. $14.99.
• Verdure, a cultivation system that draws its own plants (they would like to know whether they are in the sun).
• Holloway, one of our games. $7.99.

Use it in unlimited games, commercial or free. Made by Core Systems Asset Factory. Code and graphics were made with AI assistance, disclosed below.