sl0ppty: six chrome presets that need the new operators

    sine-comet      an arc gliding round, with a sine's shoulders
    marching-ants   two on, two off, stepping -- the selection marquee
    standing-wave   three lobes that do not travel; the pattern breathes
    bit-drift       position XOR clock, three bits: a machine thinking
    heartbeat       lub-dub, then a rest. For a state
    xor-barcode     the top rule as a barcode, sliding a character at a time

Each is a thing the old language could not say. `sin` and `TAU` give
`sin(TAU * p / perim)` -- this cell's position as an angle round one lap -- which
wraps at the corner for free, because so does a sine, and which fits any whole
number of lobes to the ring exactly. That last property is what makes a standing
wave stand: three lobes and three nodes, in the same places, breathing.

`>> 1 & 1` is a two-cell dash pattern in two operators, with no multiply and no
table; adding a slice of the clock marches it. `^` gives a motif that reshuffles
rather than slides, which is what a machine looks like when it is thinking and
not what any ramp looks like. And `abs(sin())` over a full turn is two humps, so
a heartbeat is one expression and a window mask.

Two techniques worth stealing from these, and now in the README: **overdrive then
clip** (`* 14 - 3000` after a sine keeps only the top of the crest, turning a glow
over everything into an arc), and the **angle idiom** itself.

Every one of them was measured before it was written down -- the perimeter
unrolled into an intensity strip, frame by frame, so an effect that "should"
travel is seen travelling. Two candidates did not survive that: a Sierpinski
`(x & y) == 0`, which on a ring lights the top and left solid and reads as a bug,
and a two-sine interference, which read as noise. `tests/test_chrome_presets.py`
picks up all six with no change, and asserts of each what it asserts of the other
seven: the frame moves, and the pane's own output does not.

1 files changed, 23 insertions, 0 deletions

diff --git a/contrib/chrome/README.md b/contrib/chrome/README.md
index 12f275f..645d009 100644
--- a/contrib/chrome/README.md
+++ b/contrib/chrome/README.md
@@ -22,6 +22,12 @@ real session and asserts that the frame moves and the contents do not.
 | `gradient.kdl` | not animated: lit from the top-left, so the frame reads as a surface |
 | `corner-glints.kdl` | the four corners breathing together |
 | `title-sheen.kdl` | the top rule only, where the title sits |
+| `sine-comet.kdl` | an arc gliding round with a sine's shoulders, not a ramp's corner |
+| `marching-ants.kdl` | two on, two off, stepping round — the selection marquee |
+| `standing-wave.kdl` | three lobes that do not travel; the pattern breathes in place |
+| `bit-drift.kdl` | position XOR clock, three bits — a machine thinking |
+| `heartbeat.kdl` | lub-dub, then a rest. For a state |
+| `xor-barcode.kdl` | the top rule as a barcode, sliding a character at a time |
 
 ## the perimeter trick
 
@@ -47,6 +53,23 @@ 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