feat(transition): add corner_reveal, a rectangular uncover anchored at a corner (#207)

* fix(background): make pixel_grid's twinkle reach both ends of the range

The cell alpha oscillated over 0.10..1.00, so a cell never actually went out.
A lattice whose cells only dip to a tenth still reads as a fixed field of
permanent dots, just dimmer — which is not what an animated texture is for.

0.5 + 0.5 * sin gives the full 0 → 1 → 0. The phase still comes from each
cell's own hash, or the whole field blinks in unison.

The test pins the range rather than the fact that something moves: it samples
400 instants and asserts the floor goes under 0.01 and the ceiling over 0.99.
"It animates" and "it reaches zero" are different claims.

* feat(transition): add corner_reveal, a rectangular uncover anchored at a corner

Measured on a reference piece, over 15 frames (0.5 s): the right and top edges
stay pinned to the frame while the left travels 2160 -> 0 and the bottom
1480 -> 2152. Both moving edges advance at once, and the incoming scene sits
still behind the growing window — it is uncovered, not pushed.

None of the thirteen existing types expresses that. wipe_* moves a single
full-width band on one axis, iris is a circle, and slide translates both frames
together. The closest approximation, a dissolve, loses the whole gesture.

`corner` selects which two edges are pinned: top_right (the measured default),
top_left, bottom_right, bottom_left. It is inert for every other type.

Five tests pin the geometry rather than describe it: the anchored edges never
move, the travelling ones open monotonically in the direction the corner names,
the ends are empty and full, each corner anchors its own pair, and out-of-range
progress clamps instead of inverting the rectangle — an inverted rect clips to
nothing and the transition would silently look like a cut.

Author: LeadcodeDev

.claude/skills/rustmotion/SKILL.md

@@ -732,7 +732,21 @@ Config types: `string`, `number`, `boolean`, `object`, `array`. Omitted override
 { "type": "fade", "duration": 0.5 }
 ```
 
-**13 types:** `fade`, `wipe_left`, `wipe_right`, `wipe_up`, `wipe_down`, `zoom_in`, `zoom_out`, `flip`, `clock_wipe`, `iris`, `slide`, `dissolve`, `none`
+**14 types:** `fade`, `wipe_left`, `wipe_right`, `wipe_up`, `wipe_down`, `zoom_in`, `zoom_out`, `flip`, `clock_wipe`, `iris`, `slide`, `dissolve`, `corner_reveal`, `none`
+
+`corner_reveal` uncovers the incoming scene through a rectangle anchored at one
+corner: two edges stay pinned to the frame, the other two travel until it fills.
+The incoming scene sits still behind the growing window — it is *uncovered*,
+not pushed, which is what separates it from `slide` and from the full-width
+`wipe_*` band.
+
+```json
+{ "type": "corner_reveal", "duration": 0.5, "corner": "top_right",
+  "easing": "ease_in_out" }
+```
+
+`corner` takes `top_right` (default), `top_left`, `bottom_right`, `bottom_left`
+and is ignored by every other type.
 
 Default duration: `0.5` seconds.
 

crates/rustmotion-core/src/engine/transition.rs

@@ -1,5 +1,5 @@
 use crate::engine::animator::ease;
-use crate::schema::{EasingType, PanBackground, TransitionType};
+use crate::schema::{EasingType, PanBackground, TransitionCorner, TransitionType};
 use skia_safe::{surfaces, Color4f, ColorType, ImageInfo, Paint, Path, Rect};
 
 /// Composite two RGBA frames during a transition.
@@ -11,6 +11,7 @@ pub fn apply_transition(
     height: u32,
     progress: f64,
     transition_type: &TransitionType,
+    corner: TransitionCorner,
 ) -> Vec<u8> {
     let progress = progress.clamp(0.0, 1.0) as f32;
 
@@ -35,6 +36,9 @@ pub fn apply_transition(
         TransitionType::Iris => iris_transition(frame_a, frame_b, width, height, progress),
         TransitionType::Slide => slide_transition(frame_a, frame_b, width, height, progress),
         TransitionType::Dissolve => dissolve_transition(frame_a, frame_b, width, height, progress),
+        TransitionType::CornerReveal => {
+            corner_reveal(frame_a, frame_b, width, height, progress, corner)
+        }
         TransitionType::CameraPan => blend_fade(frame_a, frame_b, progress),
         TransitionType::None => {
             if progress < 0.5 {
@@ -46,6 +50,61 @@ pub fn apply_transition(
     }
 }
 
+/// Reveal the incoming frame through a rectangle anchored at one corner.
+///
+/// Measured on a reference piece, over 15 frames (0.5 s): the right and top
+/// edges stay pinned to the frame while the left edge travels 2160 -> 0 and the
+/// bottom edge 1480 -> 2152. So it is not a wipe — `wipe_*` moves one
+/// full-width band — and not an `iris`, which is a circle. Both edges move at
+/// once, and the incoming scene sits still behind the growing window rather
+/// than sliding in: what arrives is *uncovered*, not pushed.
+fn corner_reveal(
+    frame_a: &[u8],
+    frame_b: &[u8],
+    width: u32,
+    height: u32,
+    progress: f32,
+    corner: TransitionCorner,
+) -> Vec<u8> {
+    let mut surface = match create_skia_surface(width, height) {
+        Some(s) => s,
+        None => return blend_fade(frame_a, frame_b, progress),
+    };
+    let img_a = match frame_to_image(frame_a, width, height) {
+        Some(i) => i,
+        None => return blend_fade(frame_a, frame_b, progress),
+    };
+    let img_b = match frame_to_image(frame_b, width, height) {
+        Some(i) => i,
+        None => return blend_fade(frame_a, frame_b, progress),
+    };
+
+    let (w, h) = (width as f32, height as f32);
+    let rect = corner_rect(corner, w, h, progress);
+
+    let canvas = surface.canvas();
+    canvas.draw_image(&img_a, (0.0, 0.0), None);
+    canvas.save();
+    canvas.clip_rect(rect, skia_safe::ClipOp::Intersect, false);
+    canvas.draw_image(&img_b, (0.0, 0.0), None);
+    canvas.restore();
+
+    surface_to_pixels(surface, width, height)
+}
+
+/// The revealed rectangle at `progress`, anchored so that two edges stay on the
+/// frame and two travel.
+fn corner_rect(corner: TransitionCorner, w: f32, h: f32, progress: f32) -> skia_safe::Rect {
+    let p = progress.clamp(0.0, 1.0);
+    let (rw, rh) = (w * p, h * p);
+    match corner {
+        TransitionCorner::TopRight => skia_safe::Rect::from_xywh(w - rw, 0.0, rw, rh),
+        TransitionCorner::TopLeft => skia_safe::Rect::from_xywh(0.0, 0.0, rw, rh),
+        TransitionCorner::BottomRight => skia_safe::Rect::from_xywh(w - rw, h - rh, rw, rh),
+        TransitionCorner::BottomLeft => skia_safe::Rect::from_xywh(0.0, h - rh, rw, rh),
+    }
+}
+
 fn blend_fade(frame_a: &[u8], frame_b: &[u8], progress: f32) -> Vec<u8> {
     let inv = 1.0 - progress;
     frame_a
@@ -717,3 +776,73 @@ mod camera_pan_tests {
         );
     }
 }
+
+#[cfg(test)]
+mod corner_reveal_tests {
+    use super::*;
+
+    /// Two edges stay on the frame, two travel. Measured on the reference
+    /// piece: the right and top edges never move while the left runs
+    /// 2160 -> 0 and the bottom 1480 -> 2152, over 15 frames.
+    #[test]
+    fn the_anchored_edges_never_move() {
+        for p in [0.05, 0.3, 0.5, 0.8, 1.0] {
+            let r = corner_rect(TransitionCorner::TopRight, 1920.0, 1080.0, p);
+            assert!((r.right - 1920.0).abs() < 1e-3, "right edge moved at {p}");
+            assert!(r.top.abs() < 1e-3, "top edge moved at {p}");
+        }
+    }
+
+    /// …and the travelling edges do move, monotonically, in the direction the
+    /// corner names.
+    #[test]
+    fn the_travelling_edges_open_from_the_corner() {
+        let at = |p| corner_rect(TransitionCorner::TopRight, 1920.0, 1080.0, p);
+        let (a, b, c) = (at(0.2), at(0.5), at(0.9));
+        assert!(
+            a.left > b.left && b.left > c.left,
+            "left edge must travel left"
+        );
+        assert!(
+            a.bottom < b.bottom && b.bottom < c.bottom,
+            "bottom must travel down"
+        );
+    }
+
+    /// The ends are the whole point: nothing revealed at 0, everything at 1.
+    #[test]
+    fn it_starts_empty_and_ends_full() {
+        let empty = corner_rect(TransitionCorner::TopRight, 1920.0, 1080.0, 0.0);
+        assert_eq!((empty.width(), empty.height()), (0.0, 0.0));
+        let full = corner_rect(TransitionCorner::TopRight, 1920.0, 1080.0, 1.0);
+        assert_eq!(
+            (full.left, full.top, full.right, full.bottom),
+            (0.0, 0.0, 1920.0, 1080.0)
+        );
+    }
+
+    /// Each corner anchors its own two edges — otherwise `corner` is decoration.
+    #[test]
+    fn every_corner_anchors_its_own_edges() {
+        let (w, h, p) = (1920.0f32, 1080.0f32, 0.4);
+        let tl = corner_rect(TransitionCorner::TopLeft, w, h, p);
+        assert!(tl.left.abs() < 1e-3 && tl.top.abs() < 1e-3);
+        let br = corner_rect(TransitionCorner::BottomRight, w, h, p);
+        assert!((br.right - w).abs() < 1e-3 && (br.bottom - h).abs() < 1e-3);
+        let bl = corner_rect(TransitionCorner::BottomLeft, w, h, p);
+        assert!(bl.left.abs() < 1e-3 && (bl.bottom - h).abs() < 1e-3);
+    }
+
+    /// Progress outside 0..1 must clamp, not invert the rectangle: a negative
+    /// width would make the clip empty and the transition would look like a cut.
+    #[test]
+    fn out_of_range_progress_clamps() {
+        for p in [-0.5, 1.5] {
+            let r = corner_rect(TransitionCorner::TopRight, 1920.0, 1080.0, p);
+            assert!(
+                r.width() >= 0.0 && r.height() >= 0.0,
+                "inverted rect at {p}"
+            );
+        }
+    }
+}

crates/rustmotion-core/src/schema/scenario.rs

@@ -593,11 +593,27 @@ pub struct SceneLayout {
     pub padding: Option<f32>,
 }
 
+/// The corner a `corner_reveal` is anchored to.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default, Serialize, Deserialize, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+pub enum TransitionCorner {
+    /// Measured default: the reference piece grows its reveal from here, with
+    /// the right and top edges pinned and the left and bottom edges travelling.
+    #[default]
+    TopRight,
+    TopLeft,
+    BottomRight,
+    BottomLeft,
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize, JsonSchema)]
 #[serde(deny_unknown_fields)]
 pub struct Transition {
     #[serde(rename = "type")]
     pub transition_type: TransitionType,
+    /// Which corner a `corner_reveal` grows from. Ignored by every other type.
+    #[serde(default)]
+    pub corner: TransitionCorner,
     #[serde(default = "default_transition_duration")]
     pub duration: f64,
     #[serde(default = "default_transition_easing")]
@@ -641,6 +657,7 @@ pub enum TransitionType {
     Iris,
     Slide,
     Dissolve,
+    CornerReveal,
     CameraPan,
     None,
 }

crates/rustmotion/src/encode/video/tasks.rs

@@ -1,8 +1,8 @@
 use crate::engine::transition::{apply_transition, camera_pan_transition};
 use crate::error::{Result, RustmotionError};
 use crate::schema::{
-    EasingType, ResolvedScenario as Scenario, ResolvedView, Scene, TransitionType, VideoConfig,
-    ViewType,
+    EasingType, ResolvedScenario as Scenario, ResolvedView, Scene, TransitionCorner,
+    TransitionType, VideoConfig, ViewType,
 };
 
 /// Description of what to render for a specific frame
@@ -40,6 +40,8 @@ pub enum FrameTask {
         scene_a_total_frames: u32,
         scene_b_total_frames: u32,
         transition_type: TransitionType,
+        /// Which corner a `corner_reveal` grows from; inert for every other type.
+        corner: TransitionCorner,
         transition_duration: f64,
         easing: EasingType,
     },
@@ -69,6 +71,8 @@ pub enum FrameTask {
         view_b_idx: usize,
         frame_in_transition: u32,
         transition_type: TransitionType,
+        /// Which corner a `corner_reveal` grows from; inert for every other type.
+        corner: TransitionCorner,
         transition_duration: f64,
         easing: EasingType,
     },
@@ -161,6 +165,7 @@ pub fn render_frame_task_scaled(
             scene_a_total_frames,
             scene_b_total_frames,
             transition_type,
+            corner,
             transition_duration,
             easing,
         } => {
@@ -294,6 +299,7 @@ pub fn render_frame_task_scaled(
                 scaled_h,
                 progress,
                 transition_type,
+                *corner,
             );
             apply_post_effects(
                 &mut composited,
@@ -348,6 +354,7 @@ pub fn render_frame_task_scaled(
             view_b_idx,
             frame_in_transition,
             transition_type,
+            corner,
             transition_duration,
             easing: _,
         } => {
@@ -377,6 +384,7 @@ pub fn render_frame_task_scaled(
                 scaled_h,
                 progress,
                 transition_type,
+                *corner,
             );
             // By symmetry with `SlideTransition` (which applies scene_b's
             // effects to the blended result, "the transition is the entry
@@ -548,6 +556,7 @@ pub fn build_frame_tasks(scenario: &Scenario) -> Vec<FrameTask> {
                         view_b_idx: view_idx,
                         frame_in_transition: f,
                         transition_type: transition.transition_type.clone(),
+                        corner: transition.corner,
                         transition_duration: transition.duration,
                         easing: transition.easing.clone(),
                     });
@@ -676,6 +685,7 @@ fn build_slide_view_tasks(
                     scene_a_total_frames: scene_frames,
                     scene_b_total_frames: scene_b_frames,
                     transition_type: transition.transition_type.clone(),
+                    corner: transition.corner,
                     transition_duration: outgoing_effective_duration,
                     easing: easing.clone(),
                 });
@@ -867,6 +877,7 @@ pub(super) fn build_slot_frame_tasks(
                         view_b_idx: *view_idx,
                         frame_in_transition: f,
                         transition_type: transition.transition_type.clone(),
+                        corner: transition.corner,
                         transition_duration: transition.duration,
                         easing: transition.easing.clone(),
                     });
@@ -936,6 +947,7 @@ pub(super) fn build_scene_frame_tasks_in_view(
                 scene_a_total_frames: scene_frames,
                 scene_b_total_frames: scene_b_frames,
                 transition_type: transition.transition_type.clone(),
+                corner: transition.corner,
                 transition_duration: outgoing_effective_duration,
                 easing: easing.clone(),
             });