Add a `display_list::background` module

components/layout_2020/display_list/background.rs

@@ -0,0 +1,238 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use crate::replaced::IntrinsicSizes;
+use euclid::{Size2D, Vector2D};
+use style::computed_values::background_clip::single_value::T as Clip;
+use style::computed_values::background_origin::single_value::T as Origin;
+use style::values::computed::background::BackgroundSize as Size;
+use style::values::computed::{Length, LengthPercentage};
+use style::values::specified::background::BackgroundRepeat as RepeatXY;
+use style::values::specified::background::BackgroundRepeatKeyword as Repeat;
+use webrender_api::{self as wr, units};
+
+pub(super) struct Layer {
+    pub common: wr::CommonItemProperties,
+    pub bounds: units::LayoutRect,
+    pub tile_size: units::LayoutSize,
+    pub tile_spacing: units::LayoutSize,
+    pub repeat: bool,
+}
+
+struct Layout1DResult {
+    repeat: bool,
+    bounds_origin: f32,
+    bounds_size: f32,
+}
+
+fn get_cyclic<T>(values: &[T], layer_index: usize) -> &T {
+    &values[layer_index % values.len()]
+}
+
+pub(super) fn layout_layer(
+    fragment_builder: &mut super::BuilderForBoxFragment,
+    builder: &mut super::DisplayListBuilder,
+    layer_index: usize,
+    intrinsic: IntrinsicSizes,
+) -> Option<Layer> {
+    let b = fragment_builder.fragment.style.get_background();
+
+    let clipping_area = match get_cyclic(&b.background_clip.0, layer_index) {
+        Clip::ContentBox => fragment_builder.content_rect(),
+        Clip::PaddingBox => fragment_builder.padding_rect(),
+        Clip::BorderBox => &fragment_builder.border_rect,
+    };
+    let positioning_area = match get_cyclic(&b.background_origin.0, layer_index) {
+        Origin::ContentBox => fragment_builder.content_rect(),
+        Origin::PaddingBox => fragment_builder.padding_rect(),
+        Origin::BorderBox => &fragment_builder.border_rect,
+    };
+
+    // https://drafts.csswg.org/css-backgrounds/#background-size
+    enum ContainOrCover {
+        Contain,
+        Cover,
+    }
+    let size_contain_or_cover = |background_size| {
+        let mut tile_size = positioning_area.size;
+        if let Some(intrinsic_ratio) = intrinsic.ratio {
+            let positioning_ratio = positioning_area.size.width / positioning_area.size.height;
+            // Whether the tile width (as opposed to height)
+            // is scaled to that of the positioning area
+            let fit_width = match background_size {
+                ContainOrCover::Contain => positioning_ratio <= intrinsic_ratio,
+                ContainOrCover::Cover => positioning_ratio > intrinsic_ratio,
+            };
+            // The other dimension needs to be adjusted
+            if fit_width {
+                tile_size.height = tile_size.width / intrinsic_ratio
+            } else {
+                tile_size.width = tile_size.height * intrinsic_ratio
+            }
+        }
+        tile_size
+    };
+    let mut tile_size = match get_cyclic(&b.background_size.0, layer_index) {
+        Size::Contain => size_contain_or_cover(ContainOrCover::Contain),
+        Size::Cover => size_contain_or_cover(ContainOrCover::Cover),
+        Size::ExplicitSize { width, height } => {
+            let mut width = width.non_auto().map(|lp| {
+                lp.0.percentage_relative_to(Length::new(positioning_area.size.width))
+            });
+            let mut height = height.non_auto().map(|lp| {
+                lp.0.percentage_relative_to(Length::new(positioning_area.size.height))
+            });
+
+            if width.is_none() && height.is_none() {
+                // Both computed values are 'auto':
+                // use intrinsic sizes, treating missing width or height as 'auto'
+                width = intrinsic.width;
+                height = intrinsic.height;
+            }
+
+            match (width, height) {
+                (Some(w), Some(h)) => units::LayoutSize::new(w.px(), h.px()),
+                (Some(w), None) => {
+                    let h = if let Some(intrinsic_ratio) = intrinsic.ratio {
+                        w / intrinsic_ratio
+                    } else if let Some(intrinsic_height) = intrinsic.height {
+                        intrinsic_height
+                    } else {
+                        // Treated as 100%
+                        Length::new(positioning_area.size.height)
+                    };
+                    units::LayoutSize::new(w.px(), h.px())
+                },
+                (None, Some(h)) => {
+                    let w = if let Some(intrinsic_ratio) = intrinsic.ratio {
+                        h * intrinsic_ratio
+                    } else if let Some(intrinsic_width) = intrinsic.width {
+                        intrinsic_width
+                    } else {
+                        // Treated as 100%
+                        Length::new(positioning_area.size.width)
+                    };
+                    units::LayoutSize::new(w.px(), h.px())
+                },
+                // Both comptued values were 'auto', and neither intrinsic size is present
+                (None, None) => size_contain_or_cover(ContainOrCover::Contain),
+            }
+        },
+    };
+
+    if tile_size.width == 0.0 || tile_size.height == 0.0 {
+        return None;
+    }
+
+    let mut tile_spacing = units::LayoutSize::zero();
+    let RepeatXY(repeat_x, repeat_y) = *get_cyclic(&b.background_repeat.0, layer_index);
+    let result_x = layout_1d(
+        &mut tile_size.width,
+        &mut tile_spacing.width,
+        repeat_x,
+        get_cyclic(&b.background_position_x.0, layer_index),
+        clipping_area.origin.x - positioning_area.origin.x,
+        clipping_area.size.width,
+        positioning_area.size.width,
+    );
+    let result_y = layout_1d(
+        &mut tile_size.height,
+        &mut tile_spacing.height,
+        repeat_y,
+        get_cyclic(&b.background_position_y.0, layer_index),
+        clipping_area.origin.y - positioning_area.origin.y,
+        clipping_area.size.height,
+        positioning_area.size.height,
+    );
+    let bounds = units::LayoutRect::new(
+        positioning_area.origin + Vector2D::new(result_x.bounds_origin, result_y.bounds_origin),
+        Size2D::new(result_x.bounds_size, result_y.bounds_size),
+    );
+
+    // The 'backgound-clip' property maps directly to `clip_rect` in `CommonItemProperties`:
+    let mut common = builder.common_properties(*clipping_area);
+    fragment_builder.with_border_edge_clip(builder, &mut common);
+
+    Some(Layer {
+        common,
+        bounds,
+        tile_size,
+        tile_spacing,
+        repeat: result_x.repeat || result_y.repeat,
+    })
+}
+
+/// Abstract over the horizontal or vertical dimension
+/// Coordinates (0, 0) for the purpose of this function are the positioning area’s origin.
+fn layout_1d(
+    tile_size: &mut f32,
+    tile_spacing: &mut f32,
+    mut repeat: Repeat,
+    position: &LengthPercentage,
+    clipping_area_origin: f32,
+    clipping_area_size: f32,
+    positioning_area_size: f32,
+) -> Layout1DResult {
+    // https://drafts.csswg.org/css-backgrounds/#background-repeat
+    if let Repeat::Round = repeat {
+        *tile_size = positioning_area_size / (positioning_area_size / *tile_size).round();
+    }
+    // https://drafts.csswg.org/css-backgrounds/#background-position
+    let mut position = position
+        .percentage_relative_to(Length::new(positioning_area_size - *tile_size))
+        .px();
+    // https://drafts.csswg.org/css-backgrounds/#background-repeat
+    if let Repeat::Space = repeat {
+        // The most entire tiles we can fit
+        let tile_count = (positioning_area_size / *tile_size).floor();
+        if tile_count >= 2.0 {
+            position = 0.0;
+            // Make the outsides of the first and last of that many tiles
+            // touch the edges of the positioning area:
+            let total_space = positioning_area_size - *tile_size * tile_count;
+            let spaces_count = tile_count - 1.0;
+            *tile_spacing = total_space / spaces_count;
+        } else {
+            repeat = Repeat::NoRepeat
+        }
+    }
+    match repeat {
+        Repeat::Repeat | Repeat::Round | Repeat::Space => {
+            // WebRender’s `RepeatingImageDisplayItem` contains a `bounds` rectangle and:
+            //
+            // * The tiling is clipped to the intersection of `clip_rect` and `bounds`
+            // * The origin (top-left corner) of `bounds` is the position
+            //   of the “first” (top-left-most) tile.
+            //
+            // In the general case that first tile is not the one that is positioned by
+            // `background-position`.
+            // We want it to be the top-left-most tile that intersects with `clip_rect`.
+            // We find it by offsetting by a whole number of strides,
+            // then compute `bounds` such that:
+            //
+            // * Its bottom-right is the bottom-right of `clip_rect`
+            // * Its top-left is the top-left of first tile.
+            let tile_stride = *tile_size + *tile_spacing;
+            let offset = position - clipping_area_origin;
+            let bounds_origin = position - tile_stride * (offset / tile_stride).ceil();
+            let bounds_size = clipping_area_size - bounds_origin - clipping_area_origin;
+            Layout1DResult {
+                repeat: true,
+                bounds_origin,
+                bounds_size,
+            }
+        },
+        Repeat::NoRepeat => {
+            // `RepeatingImageDisplayItem` always repeats in both dimension.
+            // When we want only one of the dimensions to repeat,
+            // we use the `bounds` rectangle to clip the tiling to one tile
+            // in that dimension.
+            Layout1DResult {
+                repeat: false,
+                bounds_origin: position,
+                bounds_size: *tile_size,
+            }
+        },
+    }
+}