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|
//! Small, backend-neutral fixed-step animations.
//!
//! A transition always interpolates from its saved endpoints. It never folds
//! the rounded value from one frame into the next, so channels are monotonic,
//! completion is exact, and a different backend cadence cannot accumulate a
//! different rounding error. Values opt in by providing
//! `interpolate(from, to, step, steps)`.
const std = @import("std");
/// Frontends aim for one animation step per display frame. Ten 16 ms steps is
/// deliberately short: enough to make a palette change legible without
/// turning theme browsing into something the user has to wait through.
pub const frame_ms: u32 = 16;
pub const frame_ns: u64 = frame_ms * std.time.ns_per_ms;
pub const transition_steps: u16 = 10;
pub fn Transition(comptime Value: type) type {
return struct {
const Self = @This();
from: Value,
to: Value,
displayed: Value,
step: u16 = transition_steps,
pub fn init(value: Value) Self {
return .{ .from = value, .to = value, .displayed = value };
}
pub fn isActive(a: *const Self) bool {
return a.step < transition_steps;
}
/// Begin again from the value on screen, not the old target. This is
/// what makes a mid-flight retarget continuous.
pub fn retarget(a: *Self, target: Value) void {
a.from = a.displayed;
a.to = target;
a.step = if (std.meta.eql(a.from, target)) transition_steps else 0;
if (a.step == transition_steps) a.displayed = target;
}
pub fn advance(a: *Self) void {
if (!a.isActive()) return;
a.step += 1;
// Assign the endpoint directly. Besides documenting the contract,
// this keeps exact completion independent of an interpolator's
// internal rounding choices.
a.displayed = if (a.step == transition_steps)
a.to
else
Value.interpolate(a.from, a.to, a.step, transition_steps);
}
/// Initialization and dump restore use snap: their first frame is the
/// selected theme, never an animation from a compiled-in default.
pub fn snap(a: *Self, value: Value) void {
a.* = init(value);
}
};
}
/// `Transition`'s interface with the animation taken OUT: a value that is only ever the one it was
/// last set to.
///
/// This exists so that a build which never fades does not carry the machinery for fading. A runtime
/// flag around the same `Transition` cannot achieve that - the endpoints stay in the struct and
/// `Value.interpolate` stays in the binary, reachable and therefore emitted. Selecting a different
/// type at comptime is what makes the interpolator genuinely unreachable, and on a target whose whole
/// display is a 115200-baud serial line, absent code and unspent frames are the same saving twice.
///
/// Every method here is the trivial one, and `retarget` is deliberately `snap` rather than an error:
/// callers ask for a new palette and get it, on the next frame, in one step. Nothing about the
/// interface says how many frames the arrival takes.
pub fn Immediate(comptime Value: type) type {
return struct {
const Self = @This();
displayed: Value,
pub fn init(value: Value) Self {
return .{ .displayed = value };
}
pub fn isActive(_: *const Self) bool {
return false;
}
pub fn retarget(a: *Self, target: Value) void {
a.displayed = target;
}
pub fn advance(_: *Self) void {}
pub fn snap(a: *Self, value: Value) void {
a.displayed = value;
}
};
}
/// Linear RGB interpolation with nearest-integer rounding. The weighted-sum
/// form stays unsigned for both rising and falling channels.
pub fn interpolateRgb(from: [3]u8, to: [3]u8, step: u16, steps: u16) [3]u8 {
if (step == 0) return from;
if (step >= steps) return to;
var out: [3]u8 = undefined;
for (&out, from, to) |*dst, a, b| {
const numerator = @as(u32, a) * (steps - step) + @as(u32, b) * step;
dst.* = @intCast((numerator + steps / 2) / steps);
}
return out;
}
const TestColor = struct {
rgb: [3]u8,
pub fn interpolate(from: TestColor, to: TestColor, step: u16, steps: u16) TestColor {
return .{ .rgb = interpolateRgb(from.rgb, to.rgb, step, steps) };
}
};
// The substitute has to be interchangeable, and the property that matters is the one a caller could
// otherwise get wrong: it must arrive at the SAME palette a completed fade arrives at. A fade whose
// endpoint differed by a rounding step would make the build option a visible change of colors rather
// than a change of how long they take.
test "Immediate lands where a completed Transition lands" {
const from: TestColor = .{ .rgb = .{ 240, 3, 90 } };
const to: TestColor = .{ .rgb = .{ 5, 222, 90 } };
var faded = Transition(TestColor).init(from);
faded.retarget(to);
for (0..transition_steps) |_| faded.advance();
var instant = Immediate(TestColor).init(from);
try std.testing.expect(!instant.isActive());
instant.retarget(to);
try std.testing.expectEqual(faded.displayed, instant.displayed);
// Never active, so a frontend that renders only while something is animating stops immediately
// rather than spending ten frames discovering there is nothing to draw.
try std.testing.expect(!instant.isActive());
instant.advance();
try std.testing.expectEqual(to, instant.displayed);
instant.snap(from);
try std.testing.expectEqual(from, instant.displayed);
}
test "fixed-step interpolation has exact monotonic endpoints" {
const Tween = Transition(TestColor);
const from: TestColor = .{ .rgb = .{ 240, 3, 90 } };
const to: TestColor = .{ .rgb = .{ 5, 222, 90 } };
var tween = Tween.init(from);
tween.retarget(to);
try std.testing.expectEqual(from, tween.displayed);
var previous = tween.displayed;
for (0..transition_steps) |_| {
tween.advance();
try std.testing.expect(tween.displayed.rgb[0] <= previous.rgb[0]);
try std.testing.expect(tween.displayed.rgb[1] >= previous.rgb[1]);
try std.testing.expectEqual(@as(u8, 90), tween.displayed.rgb[2]);
previous = tween.displayed;
}
try std.testing.expect(!tween.isActive());
try std.testing.expectEqual(to, tween.displayed);
tween.advance();
try std.testing.expectEqual(to, tween.displayed);
}
test "retarget starts at the currently displayed value" {
const Tween = Transition(TestColor);
const first: TestColor = .{ .rgb = .{ 0, 40, 200 } };
const second: TestColor = .{ .rgb = .{ 200, 140, 0 } };
const third: TestColor = .{ .rgb = .{ 20, 10, 250 } };
var tween = Tween.init(first);
tween.retarget(second);
tween.advance();
tween.advance();
tween.advance();
const on_screen = tween.displayed;
tween.retarget(third);
try std.testing.expectEqual(on_screen, tween.from);
try std.testing.expectEqual(on_screen, tween.displayed);
try std.testing.expect(tween.isActive());
for (0..transition_steps) |_| tween.advance();
try std.testing.expectEqual(third, tween.displayed);
}
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