vello/piet-scene/src/scene/builder.rs

// Copyright 2022 The piet-gpu authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// Also licensed under MIT license, at your choice.

use super::{conv, Scene, SceneData, SceneFragment};
use crate::ResourcePatch;
use bytemuck::{Pod, Zeroable};
use peniko::kurbo::{Affine, PathEl, Shape};
use peniko::{BlendMode, Brush, ColorStop, Fill, Stroke};
use smallvec::SmallVec;

/// Builder for constructing a scene or scene fragment.
pub struct SceneBuilder<'a> {
    scene: &'a mut SceneData,
    layers: SmallVec<[(BlendMode, bool); 8]>,
}

impl<'a> SceneBuilder<'a> {
    /// Creates a new builder for filling a scene. Any current content in the scene
    /// will be cleared.
    pub fn for_scene(scene: &'a mut Scene) -> Self {
        Self::new(&mut scene.data, false)
    }

    /// Creates a new builder for filling a scene fragment. Any current content in
    /// the fragment will be cleared.    
    pub fn for_fragment(fragment: &'a mut SceneFragment) -> Self {
        Self::new(&mut fragment.data, true)
    }

    /// Creates a new builder for constructing a scene.
    fn new(scene: &'a mut SceneData, is_fragment: bool) -> Self {
        scene.reset(is_fragment);
        Self {
            scene,
            layers: Default::default(),
        }
    }

    /// Pushes a new layer bound by the specifed shape and composed with
    /// previous layers using the specified blend mode.
    pub fn push_layer(&mut self, blend: BlendMode, transform: Affine, shape: &impl Shape) {
        self.maybe_encode_transform(transform);
        self.linewidth(-1.0);
        if self.encode_path(shape, true) {
            self.begin_clip(blend);
            self.layers.push((blend, true));
        } else {
            // When the layer has an empty path, record an entry to prevent
            // the stack from becoming unbalanced. This is handled in
            // pop_layer.
            self.layers.push((blend, false));
        }
    }

    /// Pops the current layer.
    pub fn pop_layer(&mut self) {
        if let Some((blend, active)) = self.layers.pop() {
            if active {
                self.end_clip(blend);
            }
        }
    }

    /// Fills a shape using the specified style and brush.
    pub fn fill(
        &mut self,
        _style: Fill,
        transform: Affine,
        brush: &Brush,
        brush_transform: Option<Affine>,
        shape: &impl Shape,
    ) {
        self.maybe_encode_transform(transform);
        self.linewidth(-1.0);
        if self.encode_path(shape, true) {
            if let Some(brush_transform) = brush_transform {
                self.encode_transform(transform * brush_transform);
                self.swap_last_tags();
                self.encode_brush(brush);
            } else {
                self.encode_brush(brush);
            }
        }
    }

    /// Strokes a shape using the specified style and brush.
    pub fn stroke(
        &mut self,
        style: &Stroke,
        transform: Affine,
        brush: &Brush,
        brush_transform: Option<Affine>,
        shape: &impl Shape,
    ) {
        self.maybe_encode_transform(transform);
        self.linewidth(style.width);
        if self.encode_path(shape, false) {
            if let Some(brush_transform) = brush_transform {
                self.encode_transform(transform * brush_transform);
                self.swap_last_tags();
                self.encode_brush(brush);
            } else {
                self.encode_brush(brush);
            }
        }
    }

    /// Appends a fragment to the scene.
    pub fn append(&mut self, fragment: &SceneFragment, transform: Option<Affine>) {
        self.scene.append(&fragment.data, &transform);
    }

    /// Completes construction and finalizes the underlying scene.
    pub fn finish(mut self) {
        while !self.layers.is_empty() {
            self.pop_layer();
        }
    }
}

impl<'a> SceneBuilder<'a> {
    fn encode_path(&mut self, shape: &impl Shape, is_fill: bool) -> bool {
        let mut b = PathBuilder::new(
            &mut self.scene.tag_stream,
            &mut self.scene.pathseg_stream,
            is_fill,
        );
        for el in shape.path_elements(0.1) {
            match el {
                PathEl::MoveTo(p0) => b.move_to(p0.x as f32, p0.y as f32),
                PathEl::LineTo(p0) => b.line_to(p0.x as f32, p0.y as f32),
                PathEl::QuadTo(p0, p1) => {
                    b.quad_to(p0.x as f32, p0.y as f32, p1.x as f32, p1.y as f32)
                }
                PathEl::CurveTo(p0, p1, p2) => b.cubic_to(
                    p0.x as f32,
                    p0.y as f32,
                    p1.x as f32,
                    p1.y as f32,
                    p2.x as f32,
                    p2.y as f32,
                ),
                PathEl::ClosePath => b.close_path(),
            }
        }
        b.finish();
        if b.n_pathseg != 0 {
            self.scene.n_path += 1;
            self.scene.n_pathseg += b.n_pathseg;
            true
        } else {
            false
        }
    }

    fn maybe_encode_transform(&mut self, transform: Affine) {
        if self.scene.transform_stream.last() != Some(&conv::affine_to_f32(&transform)) {
            self.encode_transform(transform);
        }
    }

    fn encode_transform(&mut self, transform: Affine) {
        self.scene.tag_stream.push(0x20);
        self.scene
            .transform_stream
            .push(conv::affine_to_f32(&transform));
    }

    // Swap the last two tags in the tag stream; used for transformed
    // gradients.
    fn swap_last_tags(&mut self) {
        let len = self.scene.tag_stream.len();
        self.scene.tag_stream.swap(len - 1, len - 2);
    }

    // -1.0 means "fill"
    fn linewidth(&mut self, linewidth: f32) {
        if self.scene.linewidth_stream.last() != Some(&linewidth) {
            self.scene.tag_stream.push(0x40);
            self.scene.linewidth_stream.push(linewidth);
        }
    }

    fn encode_brush(&mut self, brush: &Brush) {
        match brush {
            Brush::Solid(color) => {
                self.scene.drawtag_stream.push(DRAWTAG_FILLCOLOR);
                let rgba_color = color.to_premul_u32();
                self.scene
                    .drawdata_stream
                    .extend(bytemuck::bytes_of(&FillColor { rgba_color }));
            }
            Brush::LinearGradient(gradient) => {
                let index = self.add_ramp(&gradient.stops);
                self.scene.drawtag_stream.push(DRAWTAG_FILLLINGRADIENT);
                self.scene
                    .drawdata_stream
                    .extend(bytemuck::bytes_of(&FillLinGradient {
                        index,
                        p0: conv::point_to_f32(gradient.start),
                        p1: conv::point_to_f32(gradient.end),
                    }));
            }
            Brush::RadialGradient(gradient) => {
                let index = self.add_ramp(&gradient.stops);
                self.scene.drawtag_stream.push(DRAWTAG_FILLRADGRADIENT);
                self.scene
                    .drawdata_stream
                    .extend(bytemuck::bytes_of(&FillRadGradient {
                        index,
                        p0: conv::point_to_f32(gradient.start_center),
                        p1: conv::point_to_f32(gradient.end_center),
                        r0: gradient.start_radius,
                        r1: gradient.end_radius,
                    }));
            }
            Brush::SweepGradient(_gradient) => todo!("sweep gradients aren't done yet!"),
        }
    }

    fn add_ramp(&mut self, stops: &[ColorStop]) -> u32 {
        let offset = self.scene.drawdata_stream.len();
        let resources = &mut self.scene.resources;
        let stops_start = resources.stops.len();
        resources.stops.extend_from_slice(stops);
        resources.patches.push(ResourcePatch::Ramp {
            offset,
            stops: stops_start..stops_start + stops.len(),
        });
        0
    }

    /// Start a clip.
    fn begin_clip(&mut self, blend: BlendMode) {
        self.scene.drawtag_stream.push(DRAWTAG_BEGINCLIP);
        let element = Clip {
            blend: encode_blend_mode(blend),
        };
        self.scene
            .drawdata_stream
            .extend(bytemuck::bytes_of(&element));
        self.scene.n_clip += 1;
    }

    fn end_clip(&mut self, blend: BlendMode) {
        self.scene.drawtag_stream.push(DRAWTAG_ENDCLIP);
        let element = Clip {
            blend: encode_blend_mode(blend),
        };
        self.scene
            .drawdata_stream
            .extend(bytemuck::bytes_of(&element));
        // This is a dummy path, and will go away with the new clip impl.
        self.scene.tag_stream.push(0x10);
        self.scene.n_path += 1;
        self.scene.n_clip += 1;
    }
}

fn encode_blend_mode(mode: BlendMode) -> u32 {
    (mode.mix as u32) << 8 | mode.compose as u32
}

// Tags for draw objects. See shader/drawtag.h for the authoritative source.
const DRAWTAG_FILLCOLOR: u32 = 0x44;
const DRAWTAG_FILLLINGRADIENT: u32 = 0x114;
const DRAWTAG_FILLRADGRADIENT: u32 = 0x2dc;
const DRAWTAG_BEGINCLIP: u32 = 0x05;
const DRAWTAG_ENDCLIP: u32 = 0x25;

#[repr(C)]
#[derive(Clone, Copy, Debug, Default, Zeroable, Pod)]
pub struct FillColor {
    rgba_color: u32,
}

#[repr(C)]
#[derive(Clone, Copy, Debug, Default, Zeroable, Pod)]
pub struct FillLinGradient {
    index: u32,
    p0: [f32; 2],
    p1: [f32; 2],
}

#[repr(C)]
#[derive(Clone, Copy, Debug, Default, Zeroable, Pod)]
pub struct FillRadGradient {
    index: u32,
    p0: [f32; 2],
    p1: [f32; 2],
    r0: f32,
    r1: f32,
}

#[allow(unused)]
#[repr(C)]
#[derive(Clone, Copy, Debug, Default, Zeroable, Pod)]
pub struct FillImage {
    index: u32,
    // [i16; 2]
    offset: u32,
}

#[repr(C)]
#[derive(Clone, Copy, Debug, Default, Zeroable, Pod)]
pub struct Clip {
    blend: u32,
}

struct PathBuilder<'a> {
    tag_stream: &'a mut Vec<u8>,
    // If we're never going to use the i16 encoding, it might be
    // slightly faster to store this as Vec<u32>, we'd get aligned
    // stores on ARM etc.
    pathseg_stream: &'a mut Vec<u8>,
    first_pt: [f32; 2],
    state: PathState,
    n_pathseg: u32,
    is_fill: bool,
}

#[derive(PartialEq)]
enum PathState {
    Start,
    MoveTo,
    NonemptySubpath,
}

impl<'a> PathBuilder<'a> {
    pub fn new(tags: &'a mut Vec<u8>, pathsegs: &'a mut Vec<u8>, is_fill: bool) -> PathBuilder<'a> {
        PathBuilder {
            tag_stream: tags,
            pathseg_stream: pathsegs,
            first_pt: [0.0, 0.0],
            state: PathState::Start,
            n_pathseg: 0,
            is_fill,
        }
    }

    pub fn move_to(&mut self, x: f32, y: f32) {
        if self.is_fill {
            self.close_path();
        }
        let buf = [x, y];
        let bytes = bytemuck::bytes_of(&buf);
        self.first_pt = buf;
        if self.state == PathState::MoveTo {
            let new_len = self.pathseg_stream.len() - 8;
            self.pathseg_stream.truncate(new_len);
        } else if self.state == PathState::NonemptySubpath {
            if let Some(tag) = self.tag_stream.last_mut() {
                *tag |= 4;
            }
        }
        self.pathseg_stream.extend_from_slice(bytes);
        self.state = PathState::MoveTo;
    }

    pub fn line_to(&mut self, x: f32, y: f32) {
        if self.state == PathState::Start {
            self.move_to(self.first_pt[0], self.first_pt[1]);
        }
        let buf = [x, y];
        let bytes = bytemuck::bytes_of(&buf);
        self.pathseg_stream.extend_from_slice(bytes);
        self.tag_stream.push(9);
        self.state = PathState::NonemptySubpath;
        self.n_pathseg += 1;
    }

    pub fn quad_to(&mut self, x1: f32, y1: f32, x2: f32, y2: f32) {
        if self.state == PathState::Start {
            self.move_to(self.first_pt[0], self.first_pt[1]);
        }
        let buf = [x1, y1, x2, y2];
        let bytes = bytemuck::bytes_of(&buf);
        self.pathseg_stream.extend_from_slice(bytes);
        self.tag_stream.push(10);
        self.state = PathState::NonemptySubpath;
        self.n_pathseg += 1;
    }

    pub fn cubic_to(&mut self, x1: f32, y1: f32, x2: f32, y2: f32, x3: f32, y3: f32) {
        if self.state == PathState::Start {
            self.move_to(self.first_pt[0], self.first_pt[1]);
        }
        let buf = [x1, y1, x2, y2, x3, y3];
        let bytes = bytemuck::bytes_of(&buf);
        self.pathseg_stream.extend_from_slice(bytes);
        self.tag_stream.push(11);
        self.state = PathState::NonemptySubpath;
        self.n_pathseg += 1;
    }

    pub fn close_path(&mut self) {
        match self.state {
            PathState::Start => return,
            PathState::MoveTo => {
                let new_len = self.pathseg_stream.len() - 8;
                self.pathseg_stream.truncate(new_len);
                self.state = PathState::Start;
                return;
            }
            PathState::NonemptySubpath => (),
        }
        let len = self.pathseg_stream.len();
        if len < 8 {
            // can't happen
            return;
        }
        let first_bytes = bytemuck::bytes_of(&self.first_pt);
        if &self.pathseg_stream[len - 8..len] != first_bytes {
            self.pathseg_stream.extend_from_slice(first_bytes);
            self.tag_stream.push(13);
            self.n_pathseg += 1;
        } else {
            if let Some(tag) = self.tag_stream.last_mut() {
                *tag |= 4;
            }
        }
        self.state = PathState::Start;
    }

    pub fn finish(&mut self) {
        if self.is_fill {
            self.close_path();
        }
        if self.state == PathState::MoveTo {
            let new_len = self.pathseg_stream.len() - 8;
            self.pathseg_stream.truncate(new_len);
        }
        if self.n_pathseg != 0 {
            if let Some(tag) = self.tag_stream.last_mut() {
                *tag |= 4;
            }
            self.tag_stream.push(0x10);
        }
    }
}