Improve text contrast in bright mode (#1412)

* Rename AlphaImage to FontImage to discourage any other use for it
* Encode FontImage as f32 and postpone the alpha correction
* Interpret alpha coverage in a new, making dark text darker, improving contrast in bright mode

CHANGELOG.md

@@ -16,10 +16,12 @@ NOTE: [`epaint`](epaint/CHANGELOG.md), [`eframe`](eframe/CHANGELOG.md), [`egui_w
 ### Changed 🔧
 * `ClippedMesh` has been replaced with `ClippedPrimitive` ([#1351](https://github.com/emilk/egui/pull/1351)).
 * Renamed `Frame::margin` to `Frame::inner_margin`.
+* Renamed `AlphaImage` to `FontImage` to discourage any other use for it ([#1412](https://github.com/emilk/egui/pull/1412)).
 
 ### Fixed 🐛
 * Fixed ComboBoxes always being rendered left-aligned ([#1304](https://github.com/emilk/egui/pull/1304)).
 * Fixed ui code that could lead to a deadlock ([#1380](https://github.com/emilk/egui/pull/1380)).
+* Text is darker and more readable in bright mode ([#1412](https://github.com/emilk/egui/pull/1412)).
 
 ### Removed 🔥
 * Removed the `single_threaded/multi_threaded` flags - egui is now always thread-safe ([#1390](https://github.com/emilk/egui/pull/1390)).

egui/src/context.rs

@@ -17,7 +17,7 @@ impl Default for WrappedTextureManager {
         // Will be filled in later
         let font_id = tex_mngr.alloc(
             "egui_font_texture".into(),
-            epaint::AlphaImage::new([0, 0]).into(),
+            epaint::FontImage::new([0, 0]).into(),
         );
         assert_eq!(font_id, TextureId::default());
 

egui/src/lib.rs

@@ -307,7 +307,7 @@ pub use epaint::{
     color, mutex,
     text::{FontData, FontDefinitions, FontFamily, FontId, FontTweak},
     textures::TexturesDelta,
-    AlphaImage, ClippedPrimitive, Color32, ColorImage, ImageData, Mesh, PaintCallback,
+    ClippedPrimitive, Color32, ColorImage, FontImage, ImageData, Mesh, PaintCallback,
     PaintCallbackInfo, Rgba, Rounding, Shape, Stroke, TextureHandle, TextureId,
 };
 

egui_glium/src/painter.rs

@@ -235,7 +235,7 @@ impl Painter {
                 );
                 image.pixels.iter().map(|color| color.to_tuple()).collect()
             }
-            egui::ImageData::Alpha(image) => {
+            egui::ImageData::Font(image) => {
                 let gamma = 1.0;
                 image
                     .srgba_pixels(gamma)

egui_glow/src/painter.rs

@@ -478,7 +478,7 @@ impl Painter {
 
                 self.upload_texture_srgb(delta.pos, image.size, data);
             }
-            egui::ImageData::Alpha(image) => {
+            egui::ImageData::Font(image) => {
                 assert_eq!(
                     image.width() * image.height(),
                     image.pixels.len(),

epaint/CHANGELOG.md

@@ -7,6 +7,8 @@ All notable changes to the epaint crate will be documented in this file.
 * Removed the `single_threaded/multi_threaded` flags - epaint is now always thread-safe ([#1390](https://github.com/emilk/egui/pull/1390)).
 * `Tessellator::from_options` is now `Tessellator::new` ([#1408](https://github.com/emilk/egui/pull/1408)).
 * Renamed `TessellationOptions::anti_alias` to `feathering` ([#1408](https://github.com/emilk/egui/pull/1408)).
+* Renamed `AlphaImage` to `FontImage` to discourage any other use for it ([#1412](https://github.com/emilk/egui/pull/1412)).
+* Dark text is darker and more readable on bright backgrounds ([#1412](https://github.com/emilk/egui/pull/1412)).
 
 
 ## 0.17.0 - 2022-02-22

epaint/src/image.rs

@@ -6,21 +6,21 @@ use crate::Color32;
 ///
 /// In order to paint the image on screen, you first need to convert it to
 ///
-/// See also: [`ColorImage`], [`AlphaImage`].
+/// See also: [`ColorImage`], [`FontImage`].
 #[derive(Clone, PartialEq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub enum ImageData {
     /// RGBA image.
     Color(ColorImage),
     /// Used for the font texture.
-    Alpha(AlphaImage),
+    Font(FontImage),
 }
 
 impl ImageData {
     pub fn size(&self) -> [usize; 2] {
         match self {
             Self::Color(image) => image.size,
-            Self::Alpha(image) => image.size,
+            Self::Font(image) => image.size,
         }
     }
 
@@ -35,7 +35,7 @@ impl ImageData {
     pub fn bytes_per_pixel(&self) -> usize {
         match self {
             Self::Color(_) => 4,
-            Self::Alpha(_) => 1,
+            Self::Font(_) => 4,
         }
     }
 }
@@ -157,25 +157,28 @@ impl From<ColorImage> for ImageData {
 
 // ----------------------------------------------------------------------------
 
-/// An 8-bit image, representing difference levels of transparent white.
+/// A single-channel image designed for the font texture.
 ///
-/// Used for the font texture
-#[derive(Clone, Default, Eq, Hash, PartialEq)]
+/// Each value represents "coverage", i.e. how much a texel is covered by a character.
+///
+/// This is roughly interpreted as the opacity of a white image.
+#[derive(Clone, Default, PartialEq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
-pub struct AlphaImage {
+pub struct FontImage {
     /// width, height
     pub size: [usize; 2],
-    /// The alpha (linear space 0-255) of something white.
+
+    /// The coverage value.
     ///
-    /// One byte per pixel. Often you want to use [`Self::srgba_pixels`] instead.
-    pub pixels: Vec<u8>,
+    /// Often you want to use [`Self::srgba_pixels`] instead.
+    pub pixels: Vec<f32>,
 }
 
-impl AlphaImage {
+impl FontImage {
     pub fn new(size: [usize; 2]) -> Self {
         Self {
             size,
-            pixels: vec![0; size[0] * size[1]],
+            pixels: vec![0.0; size[0] * size[1]],
         }
     }
 
@@ -194,24 +197,19 @@ impl AlphaImage {
     /// `gamma` should normally be set to 1.0.
     /// If you are having problems with text looking skinny and pixelated, try
     /// setting a lower gamma, e.g. `0.5`.
-    pub fn srgba_pixels(
-        &'_ self,
-        gamma: f32,
-    ) -> impl ExactSizeIterator<Item = super::Color32> + '_ {
-        let srgba_from_alpha_lut: Vec<Color32> = (0..=255)
-            .map(|a| {
-                let a = super::color::linear_f32_from_linear_u8(a).powf(gamma);
-                super::Rgba::from_white_alpha(a).into()
-            })
-            .collect();
-
-        self.pixels
-            .iter()
-            .map(move |&a| srgba_from_alpha_lut[a as usize])
+    pub fn srgba_pixels(&'_ self, gamma: f32) -> impl ExactSizeIterator<Item = Color32> + '_ {
+        self.pixels.iter().map(move |coverage| {
+            // This is arbitrarily chosen to make text look as good as possible.
+            // In particular, it looks good with gamma=1 and the default eframe backend,
+            // which uses linear blending.
+            // See https://github.com/emilk/egui/issues/1410
+            let a = fast_round(coverage.powf(gamma / 2.2) * 255.0);
+            Color32::from_rgba_premultiplied(a, a, a, a) // this makes no sense, but works
+        })
     }
 
-    /// Clone a sub-region as a new image
-    pub fn region(&self, [x, y]: [usize; 2], [w, h]: [usize; 2]) -> AlphaImage {
+    /// Clone a sub-region as a new image.
+    pub fn region(&self, [x, y]: [usize; 2], [w, h]: [usize; 2]) -> FontImage {
         assert!(x + w <= self.width());
         assert!(y + h <= self.height());
 
@@ -221,40 +219,44 @@ impl AlphaImage {
             pixels.extend(&self.pixels[offset..(offset + w)]);
         }
         assert_eq!(pixels.len(), w * h);
-        AlphaImage {
+        FontImage {
             size: [w, h],
             pixels,
         }
     }
 }
 
-impl std::ops::Index<(usize, usize)> for AlphaImage {
-    type Output = u8;
+impl std::ops::Index<(usize, usize)> for FontImage {
+    type Output = f32;
 
     #[inline]
-    fn index(&self, (x, y): (usize, usize)) -> &u8 {
+    fn index(&self, (x, y): (usize, usize)) -> &f32 {
         let [w, h] = self.size;
         assert!(x < w && y < h);
         &self.pixels[y * w + x]
     }
 }
 
-impl std::ops::IndexMut<(usize, usize)> for AlphaImage {
+impl std::ops::IndexMut<(usize, usize)> for FontImage {
     #[inline]
-    fn index_mut(&mut self, (x, y): (usize, usize)) -> &mut u8 {
+    fn index_mut(&mut self, (x, y): (usize, usize)) -> &mut f32 {
         let [w, h] = self.size;
         assert!(x < w && y < h);
         &mut self.pixels[y * w + x]
     }
 }
 
-impl From<AlphaImage> for ImageData {
+impl From<FontImage> for ImageData {
     #[inline(always)]
-    fn from(image: AlphaImage) -> Self {
-        Self::Alpha(image)
+    fn from(image: FontImage) -> Self {
+        Self::Font(image)
     }
 }
 
+fn fast_round(r: f32) -> u8 {
+    (r + 0.5).floor() as _ // rust does a saturating cast since 1.45
+}
+
 // ----------------------------------------------------------------------------
 
 /// A change to an image.

epaint/src/lib.rs

@@ -28,7 +28,7 @@ pub mod util;
 pub use {
     bezier::{CubicBezierShape, QuadraticBezierShape},
     color::{Color32, Rgba},
-    image::{AlphaImage, ColorImage, ImageData, ImageDelta},
+    image::{ColorImage, FontImage, ImageData, ImageDelta},
     mesh::{Mesh, Mesh16, Vertex},
     shadow::Shadow,
     shape::{

epaint/src/text/font.rs

@@ -377,7 +377,7 @@ fn allocate_glyph(
                 if v > 0.0 {
                     let px = glyph_pos.0 + x as usize;
                     let py = glyph_pos.1 + y as usize;
-                    image[(px, py)] = fast_round(v * 255.0);
+                    image[(px, py)] = v;
                 }
             });
 
@@ -405,7 +405,3 @@ fn allocate_glyph(
         uv_rect,
     }
 }
-
-fn fast_round(r: f32) -> u8 {
-    (r + 0.5).floor() as _ // rust does a saturating cast since 1.45
-}

epaint/src/text/fonts.rs

@@ -539,7 +539,7 @@ impl FontsImpl {
             // Make the top left pixel fully white:
             let (pos, image) = atlas.allocate((1, 1));
             assert_eq!(pos, (0, 0));
-            image[pos] = 255;
+            image[pos] = 1.0;
         }
 
         let atlas = Arc::new(Mutex::new(atlas));

epaint/src/texture_atlas.rs

@@ -1,4 +1,4 @@
-use crate::{AlphaImage, ImageDelta};
+use crate::{FontImage, ImageDelta};
 
 #[derive(Clone, Copy, Eq, PartialEq)]
 struct Rectu {
@@ -32,7 +32,7 @@ impl Rectu {
 /// More characters can be added, possibly expanding the texture.
 #[derive(Clone)]
 pub struct TextureAtlas {
-    image: AlphaImage,
+    image: FontImage,
     /// What part of the image that is dirty
     dirty: Rectu,
 
@@ -48,7 +48,7 @@ impl TextureAtlas {
     pub fn new(size: [usize; 2]) -> Self {
         assert!(size[0] >= 1024, "Tiny texture atlas");
         Self {
-            image: AlphaImage::new(size),
+            image: FontImage::new(size),
             dirty: Rectu::EVERYTHING,
             cursor: (0, 0),
             row_height: 0,
@@ -91,7 +91,7 @@ impl TextureAtlas {
 
     /// Returns the coordinates of where the rect ended up,
     /// and invalidates the region.
-    pub fn allocate(&mut self, (w, h): (usize, usize)) -> ((usize, usize), &mut AlphaImage) {
+    pub fn allocate(&mut self, (w, h): (usize, usize)) -> ((usize, usize), &mut FontImage) {
         /// On some low-precision GPUs (my old iPad) characters get muddled up
         /// if we don't add some empty pixels between the characters.
         /// On modern high-precision GPUs this is not needed.
@@ -138,13 +138,13 @@ impl TextureAtlas {
     }
 }
 
-fn resize_to_min_height(image: &mut AlphaImage, required_height: usize) -> bool {
+fn resize_to_min_height(image: &mut FontImage, required_height: usize) -> bool {
     while required_height >= image.height() {
         image.size[1] *= 2; // double the height
     }
 
     if image.width() * image.height() > image.pixels.len() {
-        image.pixels.resize(image.width() * image.height(), 0);
+        image.pixels.resize(image.width() * image.height(), 0.0);
         true
     } else {
         false