ff_render/compositor/

mod.rs

#[cfg(feature = "wgpu")]
mod compositor_inner;

use ff_format::VideoFrame;

use crate::nodes::BlendMode;

// ── LayerTransform ────────────────────────────────────────────────────────────

/// 2D affine transform parameters for a compositor layer.
///
/// All values use UV-space coordinates where 0.0 is no change. The default
/// (identity) transform leaves the layer centred and unscaled.
#[derive(Debug, Clone)]
pub struct LayerTransform {
    /// Horizontal UV-space offset (positive = shift right). Default: `0.0`.
    pub x: f32,
    /// Vertical UV-space offset (positive = shift down). Default: `0.0`.
    pub y: f32,
    /// Horizontal scale factor (`1.0` = no change). Default: `1.0`.
    pub scale_x: f32,
    /// Vertical scale factor (`1.0` = no change). Default: `1.0`.
    pub scale_y: f32,
    /// Counter-clockwise rotation in radians. Default: `0.0`.
    pub rotation: f32,
}

impl Default for LayerTransform {
    fn default() -> Self {
        Self {
            x: 0.0,
            y: 0.0,
            scale_x: 1.0,
            scale_y: 1.0,
            rotation: 0.0,
        }
    }
}

impl LayerTransform {
    /// Returns `true` when this transform is the identity (no visual change).
    #[must_use]
    pub fn is_identity(&self) -> bool {
        self.x.abs() < 1e-6
            && self.y.abs() < 1e-6
            && (self.scale_x - 1.0).abs() < 1e-6
            && (self.scale_y - 1.0).abs() < 1e-6
            && self.rotation.abs() < 1e-6
    }
}

// ── FrameLayer ────────────────────────────────────────────────────────────────

/// A single layer in the composition stack.
pub struct FrameLayer {
    /// Source video frame (uploaded to GPU by [`Compositor`]).
    pub frame: VideoFrame,
    /// 2D affine transform applied before compositing.
    pub transform: LayerTransform,
    /// Blend mode used when compositing this layer over layers below.
    pub blend_mode: BlendMode,
    /// Layer opacity (`0.0` = transparent, `1.0` = fully opaque).
    pub opacity: f32,
    /// Z-order — lower values are further back. Layers are sorted ascending
    /// by this field before compositing.
    pub z_order: i32,
}

// ── Compositor ────────────────────────────────────────────────────────────────

/// Stateful high-level multi-layer GPU compositor.
///
/// Accepts a list of [`FrameLayer`]s, sorts them by [`FrameLayer::z_order`],
/// uploads each frame to the GPU, applies per-layer transforms and blend modes,
/// and returns the composited [`wgpu::Texture`].
///
/// The wgpu render pipeline is built on the first call to
/// [`composite`](Self::composite) and reused across frames. It is rebuilt only
/// when the number of layers changes.
///
/// # Thread safety
///
/// `Compositor` is [`Send`] and can be moved to a background thread. When
/// multiple threads need to share a compositor, wrap it in
/// `Arc<Mutex<Compositor>>`.
///
/// Requires the `wgpu` feature.
#[cfg(feature = "wgpu")]
pub struct Compositor {
    ctx: std::sync::Arc<crate::context::RenderContext>,
    width: u32,
    height: u32,
    graph: Option<compositor_inner::CompositorGraph>,
    last_layer_count: usize,
}

#[cfg(feature = "wgpu")]
impl Compositor {
    /// Create a compositor targeting the given output resolution.
    #[must_use]
    pub fn new(
        ctx: std::sync::Arc<crate::context::RenderContext>,
        width: u32,
        height: u32,
    ) -> Self {
        Self {
            ctx,
            width,
            height,
            graph: None,
            last_layer_count: 0,
        }
    }

    /// Composite `layers` into a single [`wgpu::Texture`].
    ///
    /// Layers are sorted by [`FrameLayer::z_order`] before compositing
    /// (ascending — lowest `z_order` is the bottom layer).
    ///
    /// The wgpu pipeline is built on the first call and cached; it is rebuilt
    /// only when `layers.len()` changes between calls.
    ///
    /// # Errors
    ///
    /// Returns [`RenderError`](crate::error::RenderError) on GPU texture
    /// creation failure, unsupported pixel format, or render failure.
    pub fn composite(
        &mut self,
        layers: &mut [FrameLayer],
    ) -> Result<wgpu::Texture, crate::error::RenderError> {
        layers.sort_unstable_by_key(|l| l.z_order);

        let need_rebuild = self.graph.is_none() || self.last_layer_count != layers.len();
        if need_rebuild {
            self.graph = Some(compositor_inner::CompositorGraph::build(
                &self.ctx,
                layers.len(),
                self.width,
                self.height,
            ));
            self.last_layer_count = layers.len();
        }

        let Some(graph) = self.graph.as_mut() else {
            return Err(crate::error::RenderError::Composite {
                message: "compositor graph not initialized".to_string(),
            });
        };
        graph.composite(&self.ctx, layers, self.width, self.height)
    }
}

// ── Tests ─────────────────────────────────────────────────────────────────────

#[cfg(test)]
mod tests {
    use super::*;
    use ff_format::{PixelFormat, VideoFrame};

    fn make_frame() -> VideoFrame {
        VideoFrame::empty(2, 2, PixelFormat::Rgba).expect("test frame")
    }

    #[test]
    fn layer_transform_default_should_be_identity() {
        let t = LayerTransform::default();
        assert!(
            t.is_identity(),
            "default LayerTransform must be the identity"
        );
    }

    #[test]
    fn layer_transform_nonzero_x_should_not_be_identity() {
        let t = LayerTransform {
            x: 0.1,
            ..Default::default()
        };
        assert!(
            !t.is_identity(),
            "LayerTransform with non-zero x must not be identity"
        );
    }

    #[test]
    fn frame_layer_should_construct_with_defaults() {
        let layer = FrameLayer {
            frame: make_frame(),
            transform: LayerTransform::default(),
            blend_mode: BlendMode::Normal,
            opacity: 1.0,
            z_order: 0,
        };
        assert_eq!(layer.z_order, 0);
        assert!((layer.opacity - 1.0).abs() < 1e-6);
    }

    #[test]
    fn compositor_layers_should_sort_by_z_order() {
        let mut layers = vec![
            FrameLayer {
                frame: make_frame(),
                transform: LayerTransform::default(),
                blend_mode: BlendMode::Normal,
                opacity: 1.0,
                z_order: 3,
            },
            FrameLayer {
                frame: make_frame(),
                transform: LayerTransform::default(),
                blend_mode: BlendMode::Normal,
                opacity: 1.0,
                z_order: 1,
            },
            FrameLayer {
                frame: make_frame(),
                transform: LayerTransform::default(),
                blend_mode: BlendMode::Normal,
                opacity: 1.0,
                z_order: 2,
            },
        ];
        layers.sort_unstable_by_key(|l| l.z_order);
        let z_orders: Vec<i32> = layers.iter().map(|l| l.z_order).collect();
        assert_eq!(
            z_orders,
            vec![1, 2, 3],
            "layers must sort ascending by z_order"
        );
    }

    #[cfg(feature = "wgpu")]
    #[test]
    fn compositor_should_be_send() {
        fn assert_send<T: Send>() {}
        assert_send::<Compositor>();
    }
}