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use geometry_core::{Point, Rect, Transform};
use crate::DrawCommand;
/// Maps clip rect `r` (in the currently-active transform's local space) to window space — the axis-aligned bounds of its four mapped corners. Widgets emit clip rects in their own local space, but the renderer clips in window space, so a clip must be mapped through the active cumulative matrix (scroll/layout translations) to compose correctly.
pub fn transform_clip_rect(m: [f32; 6], r: Rect) -> Rect {
let [a, b, c, d, e, f] = m;
let map = |x: f32, y: f32| (a * x + c * y + e, b * x + d * y + f);
let corners = [
map(r.x, r.y),
map(r.x + r.width, r.y),
map(r.x, r.y + r.height),
map(r.x + r.width, r.y + r.height),
];
let mut min_x = f32::INFINITY;
let mut min_y = f32::INFINITY;
let mut max_x = f32::NEG_INFINITY;
let mut max_y = f32::NEG_INFINITY;
for (x, y) in corners {
min_x = min_x.min(x);
min_y = min_y.min(y);
max_x = max_x.max(x);
max_y = max_y.max(y);
}
Rect {
x: min_x,
y: min_y,
width: max_x - min_x,
height: max_y - min_y,
}
}
/// Flat draw state tracking clips and transforms. Note: `PushLayer` and `PopLayer` commands are intentionally not tracked here; layers are managed outside this struct by the caller.
pub struct DrawState {
clip_stack: Vec<Rect>,
transform_stack: Vec<[f32; 6]>,
pub cumulative_matrix: [f32; 6],
}
impl DrawState {
pub fn new() -> Self {
Self {
clip_stack: Vec::with_capacity(16),
transform_stack: Vec::with_capacity(16),
cumulative_matrix: Transform::IDENTITY.to_array(),
}
}
/// Pushes `rect` as a clip, intersected with whatever is already clipping, and returns the effective
/// scissor.
///
/// A child clip that does *not* meet its parent clips away to nothing, and that case is worth spelling
/// out because getting it wrong is invisible in every test that does not scroll: `intersect` answers
/// `None` for two rects that do not overlap, and falling back to `rect` there hands the child its **own**
/// box as the scissor — outside everything that contains it. A widget that emits a clip of its own (an
/// image does, for a `Cover` overflow or a corner radius) therefore escaped the scroll area it lived in
/// the moment it scrolled out of view: it went on being drawn at its true position, outside the panel,
/// sliding as the content scrolled, and only looked right once its own box fell back inside the viewport.
/// Nothing *without* a clip of its own could show the bug, which is why the text and boxes beside it in
/// the same list were always clipped correctly.
#[inline]
pub fn push_clip(&mut self, rect: Rect) -> Rect {
// The empty rect is placed at the *parent's* origin rather than the child's. An empty clip has no
// position worth keeping, and a backend that cannot express one — wgpu rejects an empty scissor, so
// `physical_scissor` rounds it up to 1×1 — then draws that one pixel somewhere it was already allowed
// to draw, instead of leaving a stray dot outside the panel.
let effective = match self.clip_stack.last() {
Some(¤t) => current
.intersect(rect)
.unwrap_or_else(|| Rect::new(current.x, current.y, 0.0, 0.0)),
None => rect,
};
self.clip_stack.push(effective);
effective
}
#[inline]
pub fn pop_clip(&mut self) -> Option<Rect> {
self.clip_stack.pop();
self.clip_stack.last().copied()
}
#[inline]
pub fn current_clip(&self) -> Option<Rect> {
self.clip_stack.last().copied()
}
#[inline]
pub fn push_matrix(&mut self, matrix: [f32; 6]) {
self.transform_stack.push(self.cumulative_matrix);
// Compose cumulative ∘ matrix: `a.then(b)` yields `b ∘ a`, so `matrix.then(cumulative)` maps a local point through `matrix` first, then the accumulated parent chain.
self.cumulative_matrix = Transform::from_array(matrix)
.then(Transform::from_array(self.cumulative_matrix))
.to_array();
}
#[inline]
pub fn pop_matrix(&mut self) {
if let Some(prev) = self.transform_stack.pop() {
self.cumulative_matrix = prev;
}
}
#[inline]
pub fn apply_point(&self, x: f32, y: f32) -> (f32, f32) {
let p = Transform::from_array(self.cumulative_matrix).apply(Point::new(x, y));
(p.x, p.y)
}
pub fn reset(&mut self) {
self.clip_stack.clear();
self.transform_stack.clear();
self.cumulative_matrix = Transform::IDENTITY.to_array();
}
}
impl Default for DrawState {
fn default() -> Self {
Self::new()
}
}
/// Iterates `cmds` calling `f(cmd, cumulative_matrix)` for every command. PushMatrix/PopMatrix update the matrix before the callback; all other commands see the matrix that was active when they were emitted.
pub fn for_each_with_matrix<F>(cmds: &[DrawCommand], mut f: F)
where
F: FnMut(&DrawCommand, [f32; 6]),
{
let mut state = DrawState::new();
for cmd in cmds {
match cmd {
DrawCommand::PushMatrix { matrix } => state.push_matrix(*matrix),
DrawCommand::PopMatrix => state.pop_matrix(),
_ => {}
}
f(cmd, state.cumulative_matrix);
}
}
#[cfg(test)]
mod tests {
use super::*;
// Guards the push_matrix compose argument order (cumulative ∘ matrix) against the pre-refactor
// hand-rolled `compose_matrix(parent, child)`; a swapped order silently corrupts nested transforms.
#[test]
fn push_matrix_matches_legacy_compose_order() {
let cumulative = [2.0, 0.1, -0.2, 2.0, 10.0, 20.0];
let matrix = [1.0, 0.5, -0.5, 1.0, 5.0, 7.0];
// Old `compose_matrix(parent = cumulative, child = matrix)` computing parent(child(p)).
let [a1, b1, c1, d1, e1, f1] = matrix;
let [a2, b2, c2, d2, e2, f2] = cumulative;
let expected = [
a2 * a1 + c2 * b1,
b2 * a1 + d2 * b1,
a2 * c1 + c2 * d1,
b2 * c1 + d2 * d1,
a2 * e1 + c2 * f1 + e2,
b2 * e1 + d2 * f1 + f2,
];
let mut state = DrawState::new();
state.push_matrix(cumulative);
state.push_matrix(matrix);
assert_eq!(state.cumulative_matrix, expected);
}
/// A clip nested inside one it does not touch must clip away to nothing. The failing case is a widget
/// that emits its own clip — an image with a `Cover` overflow or a corner radius — scrolled out of the
/// viewport containing it: falling back to the child's own rect scissors to a box outside the parent, and
/// the widget goes on being drawn there, over whatever the panel happens to be sitting on.
#[test]
fn a_clip_outside_its_parent_clips_away_to_nothing() {
let viewport = Rect::new(0.0, 0.0, 340.0, 300.0);
let mut state = DrawState::new();
assert_eq!(
state.push_clip(viewport),
viewport,
"the outermost clip is itself"
);
// An avatar 560px down a scrolling column: its own 56×56 clip is nowhere near the viewport.
let escaped = state.push_clip(Rect::new(0.0, 560.0, 56.0, 56.0));
assert_eq!(
(escaped.width, escaped.height),
(0.0, 0.0),
"an image scrolled out of its panel must not be drawn at all, got {escaped:?}"
);
assert!(
viewport
.intersect(Rect::new(escaped.x, escaped.y, 1.0, 1.0))
.is_some(),
"and the empty clip sits inside the parent, so a backend that rounds it up to 1×1 draws that \
pixel where it was already allowed to: {escaped:?}"
);
state.pop_clip();
// The same image scrolled back in is clipped to the part of it the viewport actually shows.
let visible = state.push_clip(Rect::new(0.0, 280.0, 56.0, 56.0));
assert_eq!(visible, Rect::new(0.0, 280.0, 56.0, 20.0));
// And a clip with nothing above it is still itself, which is what the scroll area's own one relies on.
let mut fresh = DrawState::new();
let alone = Rect::new(12.0, 34.0, 56.0, 78.0);
assert_eq!(fresh.push_clip(alone), alone);
}
}