[WIP] Add pixel ratio config, document Style/Layout as logical/physical pixels

Per #460 (comment)

See details in doc-comment changes.

Author: SimonSapin

RELEASES.md

@@ -4,29 +4,31 @@
 
 ### Breaking
 
-Many APIs have been renamed to replace `points` or `Points` with `length` or `Length`.
-This new name better describes one-dimentional measure of space in some unspecified unit
-which is often unrelated to the PostScript point or the CSS `pt` unit.
+- Previously, absolute length values in `Style` and `Layout` were measured in some unspecified abstract unit whose meaning was chosen by Taffy users. Instead, they are now redefinied so that `Style` uses "logical pixel" which correspond to CSS `px`, while `Layout` contains "output pixels" which are recommended to use as device pixels. A new pixel ratio configuration controls the relationship between these two units. See `Taffy::set_pixel_ratio` documentation for details.
 
-This also removes a misleading similarity with the 2D `Point`,
-whose components can have any unit and are not even necessarily absolute lengths.
+- Many APIs have been renamed to replace `points` or `Points` with `length` or `Length`.
+  The previous name suggested the PostScript point or the CSS `pt` unit,
+  but per the above these values are now measured in CSS `px`.
 
-Example usage change:
+  This also removes a misleading similarity with the 2D `Point` struct,
+  whose components can have any unit and are not even necessarily absolute lengths.
 
-```diff
- use taffy::prelude::*;
+  Example usage change:
 
- // …
+  ```diff
+  use taffy::prelude::*;
 
- let header_node = taffy
-     .new_leaf(
-         Style {
--            size: Size { width: points(800.0), height: points(100.0) },
-+            size: Size { width: length(800.0), height: length(100.0) },
-             ..Default::default()
-         },
-     ).unwrap();
-```
+  // …
+
+  let header_node = taffy
+      .new_leaf(
+          Style {
+  -            size: Size { width: points(800.0), height: points(100.0) },
+  +            size: Size { width: length(800.0), height: length(100.0) },
+              ..Default::default()
+          },
+      ).unwrap();
+  ```
 
 ### Removed
 

src/style/dimension.rs

@@ -1,6 +1,8 @@
 //! Style types for representing lengths / sizes
 
 use crate::geometry::{Rect, Size};
+#[cfg(doc)]
+use crate::prelude::Taffy;
 use crate::style_helpers::{FromLength, FromPercent, TaffyAuto, TaffyMaxContent, TaffyMinContent, TaffyZero};
 use crate::util::sys::abs;
 
@@ -10,8 +12,9 @@ use crate::util::sys::abs;
 #[derive(Copy, Clone, PartialEq, Debug)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub enum LengthPercentage {
-    /// An absolute length in some abstract units. Users of Taffy may define what they correspond
-    /// to in their application (pixels, logical pixels, mm, etc) as they see fit.
+    /// An absolute length in CSS `px` units a.k.a. logical pixels.
+    ///
+    /// See [`Taffy::set_pixel_ratio`].
     Length(f32),
     /// The dimension is stored in percentage relative to the parent item.
     Percent(f32),
@@ -36,8 +39,9 @@ impl FromPercent for LengthPercentage {
 #[derive(Copy, Clone, PartialEq, Debug)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub enum LengthPercentageAuto {
-    /// An absolute length in some abstract units. Users of Taffy may define what they correspond
-    /// to in their application (pixels, logical pixels, mm, etc) as they see fit.
+    /// An absolute length in CSS `px` units a.k.a. logical pixels.
+    ///
+    /// See [`Taffy::set_pixel_ratio`].
     Length(f32),
     /// The dimension is stored in percentage relative to the parent item.
     Percent(f32),
@@ -90,8 +94,9 @@ impl LengthPercentageAuto {
 #[derive(Copy, Clone, PartialEq, Debug)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub enum Dimension {
-    /// An absolute length in some abstract units. Users of Taffy may define what they correspond
-    /// to in their application (pixels, logical pixels, mm, etc) as they see fit.
+    /// An absolute length in CSS `px` units a.k.a. logical pixels.
+    ///
+    /// See [`Taffy::set_pixel_ratio`].
     Length(f32),
     /// The dimension is stored in percentage relative to the parent item.
     Percent(f32),

src/tree/layout.rs

@@ -1,6 +1,8 @@
 //! Final data structures that represent the high-level UI layout
 
 use crate::geometry::{Point, Size};
+#[cfg(doc)]
+use crate::prelude::Taffy;
 
 /// Whether we are performing a full layout, or we merely need to size the node
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
@@ -31,6 +33,7 @@ pub enum SizingMode {
 pub struct SizeAndBaselines {
     /// The size of the node
     pub size: Size<f32>,
+
     /// The first baseline of the node in each dimension, if any
     pub first_baselines: Point<Option<f32>>,
 }
@@ -49,9 +52,15 @@ pub struct Layout {
     /// Nodes with a higher order should be rendered on top of those with a lower order.
     /// This is effectively a topological sort of each tree.
     pub order: u32,
-    /// The width and height of the node
+
+    /// The width and height of the node, in output pixels.
+    ///
+    /// See [`Taffy::set_pixel_ratio`].
     pub size: Size<f32>,
-    /// The bottom-left corner of the node
+
+    /// The bottom-left corner of the node, in output pixels.
+    ///
+    /// See [`Taffy::set_pixel_ratio`].
     pub location: Point<f32>,
 }
 

src/tree/taffy_tree/tree.rs

@@ -16,11 +16,15 @@ use super::{TaffyError, TaffyResult};
 pub(crate) struct TaffyConfig {
     /// Whether to round layout values
     pub(crate) use_rounding: bool,
+
+    /// Number of output length units per input length units.
+    /// See `pixel_ratio` and `set_pixel_ratio` methods.
+    pub(crate) pixel_ratio: f32,
 }
 
 impl Default for TaffyConfig {
     fn default() -> Self {
-        Self { use_rounding: true }
+        Self { use_rounding: true, pixel_ratio: 1.0 }
     }
 }
 
@@ -144,16 +148,40 @@ impl Taffy {
         }
     }
 
-    /// Enable rounding of layout values. Rounding is enabled by default.
+    /// Enable rounding of sizes and locations in [`Layout`].
+    /// Rounding is enabled by default.
     pub fn enable_rounding(&mut self) {
         self.config.use_rounding = true;
     }
 
-    /// Disable rounding of layout values. Rounding is enabled by default.
+    /// Disable rounding of sizes and locations in [`Layout`].
+    /// Rounding is enabled by default.
     pub fn disable_rounding(&mut self) {
         self.config.use_rounding = false;
     }
 
+    /// Returns the current pixel ratio. See [`set_pixel_ratio`][Self::set_pixel_ratio].
+    pub fn pixel_ratio(&self) -> f32 {
+        self.config.pixel_ratio
+    }
+
+    /// Sets the pixel ratio: the number of output pixels per logical pixels.
+    ///
+    /// Absolute length values in [`style`][crate::style] are measured in **logical pixels**
+    /// and correspond to CSS the `px` unit.
+    ///
+    /// Sizes and locations in [`Layout`] are measured in output pixels.
+    /// When rendering to screen media, it is recommended to use them as **physical pixels**
+    /// a.k.a. [device pixels](https://drafts.csswg.org/css-values/#device-pixel)
+    /// and set the pixel ratio to the whole number that makes a logical pixel best approximate
+    /// the [reference pixel](https://drafts.csswg.org/css-values/#reference-pixel),
+    /// (the visual angle of one pixel on a 96dpi device held at arm’s length).
+    ///
+    /// The default is 1.0.
+    pub fn set_pixel_ratio(&mut self, new_pixel_ratio: f32) {
+        self.config.pixel_ratio = new_pixel_ratio
+    }
+
     /// Creates and adds a new unattached leaf node to the tree, and returns the node of the new node
     pub fn new_leaf(&mut self, layout: Style) -> TaffyResult<NodeId> {
         let id = self.nodes.insert(NodeData::new(layout));