Add control of line height and letter spacing (#3302)

* Add `TextFormat::extra_letter_spacing`

* Add control of line height

* Add to text layout demo

* Move the text layout demo to its own window in the demo app

* Fix doclink

* Better document points vs pixels

* Better documentation and code cleanup

crates/egui/src/lib.rs

@@ -92,14 +92,16 @@
 //! # });
 //! ```
 //!
-//! ## Conventions
+//! ## Coordinate system
+//! The left-top corner of the screen is `(0.0, 0.0)`,
+//! with X increasing to the right and Y increasing downwards.
 //!
-//! Conventions unless otherwise specified:
+//! `egui` uses logical _points_ as its coordinate system.
+//! Those related to physical _pixels_ by the `pixels_per_point` scale factor.
+//! For example, a high-dpi screeen can have `pixels_per_point = 2.0`,
+//! meaning there are two physical screen pixels for each logical point.
 //!
-//! * angles are in radians
-//! * `Vec2::X` is right and `Vec2::Y` is down.
-//! * `Pos2::ZERO` is left top.
-//! * Positions and sizes are measured in _points_. Each point may consist of many physical pixels.
+//! Angles are in radians, and are measured clockwise from the X-axis, which has angle=0.
 //!
 //! # Integrating with egui
 //!
@@ -352,7 +354,7 @@ pub mod text {
     pub use crate::text_edit::CCursorRange;
     pub use epaint::text::{
         cursor::CCursor, FontData, FontDefinitions, FontFamily, Fonts, Galley, LayoutJob,
-        LayoutSection, TextFormat, TAB_SIZE,
+        LayoutSection, TextFormat, TextWrapping, TAB_SIZE,
     };
 }
 

crates/egui/src/widget_text.rs

@@ -1,5 +1,4 @@
-use std::borrow::Cow;
-use std::sync::Arc;
+use std::{borrow::Cow, sync::Arc};
 
 use crate::{
     style::WidgetVisuals, text::LayoutJob, Align, Color32, FontFamily, FontSelection, Galley, Pos2,
@@ -25,6 +24,8 @@ use crate::{
 pub struct RichText {
     text: String,
     size: Option<f32>,
+    extra_letter_spacing: f32,
+    line_height: Option<f32>,
     family: Option<FontFamily>,
     text_style: Option<TextStyle>,
     background_color: Color32,
@@ -100,6 +101,32 @@ impl RichText {
         self
     }
 
+    /// Extra spacing between letters, in points.
+    ///
+    /// Default: 0.0.
+    ///
+    /// For even text it is recommended you round this to an even number of _pixels_,
+    /// e.g. using [`crate::Painter::round_to_pixel`].
+    #[inline]
+    pub fn extra_letter_spacing(mut self, extra_letter_spacing: f32) -> Self {
+        self.extra_letter_spacing = extra_letter_spacing;
+        self
+    }
+
+    /// Explicit line height of the text in points.
+    ///
+    /// This is the distance between the bottom row of two subsequent lines of text.
+    ///
+    /// If `None` (the default), the line height is determined by the font.
+    ///
+    /// For even text it is recommended you round this to an even number of _pixels_,
+    /// e.g. using [`crate::Painter::round_to_pixel`].
+    #[inline]
+    pub fn line_height(mut self, line_height: Option<f32>) -> Self {
+        self.line_height = line_height;
+        self
+    }
+
     /// Select the font family.
     ///
     /// This overrides the value from [`Self::text_style`].
@@ -253,6 +280,8 @@ impl RichText {
         let Self {
             text,
             size,
+            extra_letter_spacing,
+            line_height,
             family,
             text_style,
             background_color,
@@ -309,6 +338,8 @@ impl RichText {
 
         let text_format = crate::text::TextFormat {
             font_id,
+            extra_letter_spacing,
+            line_height,
             color: text_color,
             background: background_color,
             italics,