chrome presets

Colour passes over a pane's frame rather than its contents — where="chrome" — each in its own file, ready to paste into ~/.config/slosh/config.kdl.

cat contrib/chrome/shine.kdl >> ~/.config/slosh/config.kdl   # then save

Hand-written, unlike contrib/shaders, which is generated from contrib/shadertoy.html: the toy previews a pane's contents, and a frame is a different rect with no preview to give. So these are checked the only way that means anything — tests/test_chrome_presets.py loads every file here into a real session and asserts that the frame moves and the contents do not.

Try one without editing anything: run contrib/shader-repl in a pane and type :load sine-comet. It applies to that pane only, :paste gives you the include line to keep, and :both puts the pane back.

the perimeter trick

On a frame, x/y are rarely what you want. An effect that travels round a border wants distance along the ring, and the language can work it out — clockwise from the top-left corner, in cells:

(y == 0        ? x
 : x == cols-1 ? cols - 1 + y
 : y == rows-1 ? 2 * cols + rows - 3 - x
 :               2 * cols + 2 * rows - 4 - y)

The perimeter itself is 2 * cols + 2 * rows - 4. Two things fall out of it:

• wrapping. min(d, P - d) is distance the short way round, so an effect crosses the corner it started from instead of stopping at a seam.
• direction. (p - head + P) % P is "how far ahead of the head", which is what makes an arc rotate rather than a blob slide.

There is no way to name a subexpression, so the formula is written out in full in each file. That is the ugly part of these, and the reason the files are long when the effects are small.

the angle idiom

sin and cos take degrees, and PI is 180 because pi in an angle language is a half turn. So TAU * p / perim is "this cell's position, as an angle round one lap", which is the line under most of the newer presets:

• sin(TAU * p / perim) — one cycle over the whole ring, and it wraps at the corner for free, because so does a sine.
• sin(TAU * p * 3 / perim) — three lobes. Any whole number fits the ring exactly, which is what keeps a standing wave standing.
• * 14 - 3000 after it keeps only the top of the crest: an arc rather than a glow over everything. Overdrive then clip is how you sharpen anything here.

A sine costs a table lookup and no more than the triangle wave abs(t / 6 % 510 - 255) it replaces — but it has no corner in it, which is the difference between a frame that pulses and one that flickers.

two rules worth knowing

channel="fg", always, on a frame. A border's background is the terminal's own default; mixing that towards a colour paints a rectangle behind the glyphs instead of colouring them, and leaves it there. fg also keeps the padding inside the border out of it, since a blank cell has no foreground to colour — so the effect lands on exactly the rules, the title and the buttons.

Anything that reads t asks the session to keep painting (anim_ms, 20fps by default) for as long as the pass is running. On a state that is the point: a pulse hung off bell or dead costs a frame clock while a pane is ringing or finished, and nothing for the rest of the session. Hung off every pane, as these files do for previewing, it costs that clock for the whole session — which is fine for a laptop and worth knowing about on a battery.

gradient.kdl reads no clock at all: it is computed once into a per-cell map and reused, so it costs nothing per frame.

states

Any of these is a state chain away from meaning something rather than just looking like something:

states {
    // the arc turns while a pane has not been started yet
    suspended { tint where="chrome" channel="fg" color="#ffcc00" amount="…spinner…" }
    // and a bell nobody has answered flashes, then settles
    bell { tint where="chrome" channel="fg" color="#ffcc00" amount="(since < 250) * 255" }
}

since is how long the pane has been in its current state, which is what makes a one-shot effect possible: t says what time it is, not how long ago something happened. The full list of states, and what each one means, is in config/config.kdl.