Add glow::Context to epi::Frame (#1425)

This can be used, for instance, to:

* Render things to offscreen buffers.
* Read the pixel buffer from the previous frame (glow::Context::read_pixels).
* Render things behind the egui windows.

eframe/CHANGELOG.md

@@ -18,6 +18,7 @@ NOTE: [`egui_web`](../egui_web/CHANGELOG.md), [`egui-winit`](../egui-winit/CHANG
   * You can now load/save state in `App::update`
   * Changed `App::update` to take `&mut Frame` instead of `&Frame`.
   * `Frame` is no longer `Clone` or `Sync`.
+* Add `glow` (OpenGL) context to `Frame` ([#1425](https://github.com/emilk/egui/pull/1425)).
 
 
 ## 0.17.0 - 2022-02-22

egui-winit/src/epi.rs

@@ -149,6 +149,7 @@ pub struct EpiIntegration {
 impl EpiIntegration {
     pub fn new(
         integration_name: &'static str,
+        gl: std::rc::Rc<glow::Context>,
         max_texture_side: usize,
         window: &winit::window::Window,
         storage: Option<Box<dyn epi::Storage>>,
@@ -169,6 +170,7 @@ impl EpiIntegration {
             },
             output: Default::default(),
             storage,
+            gl,
         };
 
         if prefer_dark_mode == Some(true) {

egui/src/ui.rs

@@ -792,7 +792,7 @@ impl Ui {
     /// So one can think of `cursor` as a constraint on the available region.
     ///
     /// If something has already been added, this will point to `style.spacing.item_spacing` beyond the latest child.
-    /// The cursor can thus be `style.spacing.item_spacing` pixels outside of the min_rect.
+    /// The cursor can thus be `style.spacing.item_spacing` pixels outside of the `min_rect`.
     pub fn cursor(&self) -> Rect {
         self.placer.cursor()
     }

egui_demo_lib/src/backend_panel.rs

@@ -12,7 +12,7 @@ enum RunMode {
     /// For instance, a GUI for a thermostat need to repaint each time the temperature changes.
     /// To ensure the UI is up to date you need to call `egui::Context::request_repaint()` each
     /// time such an event happens. You can also chose to call `request_repaint()` once every second
-    /// or after every single frame - this is called `Continuous` mode,
+    /// or after every single frame - this is called "Continuous" mode,
     /// and for games and interactive tools that need repainting every frame anyway, this should be the default.
     Reactive,
 

egui_glow/src/epi_backend.rs

@@ -1,4 +1,3 @@
-use crate::*;
 use egui_winit::winit;
 
 struct RequestRepaintEvent;
@@ -50,6 +49,7 @@ pub fn run(app_name: &str, native_options: &epi::NativeOptions, app_creator: epi
 
     let mut integration = egui_winit::epi::EpiIntegration::new(
         "egui_glow",
+        gl.clone(),
         painter.max_texture_side(),
         gl_window.window(),
         storage,
@@ -86,6 +86,10 @@ pub fn run(app_name: &str, native_options: &epi::NativeOptions, app_creator: epi
                 std::thread::sleep(std::time::Duration::from_millis(10));
             }
 
+            let screen_size_in_pixels: [u32; 2] = gl_window.window().inner_size().into();
+
+            crate::painter::clear(&gl, screen_size_in_pixels, app.clear_color());
+
             let egui::FullOutput {
                 platform_output,
                 needs_repaint,
@@ -97,24 +101,14 @@ pub fn run(app_name: &str, native_options: &epi::NativeOptions, app_creator: epi
 
             let clipped_primitives = integration.egui_ctx.tessellate(shapes);
 
-            // paint:
-            {
-                let color = app.clear_color();
-                unsafe {
-                    use glow::HasContext as _;
-                    gl.disable(glow::SCISSOR_TEST);
-                    gl.clear_color(color[0], color[1], color[2], color[3]);
-                    gl.clear(glow::COLOR_BUFFER_BIT);
-                }
-                painter.paint_and_update_textures(
-                    gl_window.window().inner_size().into(),
-                    integration.egui_ctx.pixels_per_point(),
-                    &clipped_primitives,
-                    &textures_delta,
-                );
-
-                gl_window.swap_buffers().unwrap();
-            }
+            painter.paint_and_update_textures(
+                screen_size_in_pixels,
+                integration.egui_ctx.pixels_per_point(),
+                &clipped_primitives,
+                &textures_delta,
+            );
+
+            gl_window.swap_buffers().unwrap();
 
             {
                 *control_flow = if integration.should_quit() {

egui_glow/src/painter.rs

@@ -627,11 +627,16 @@ impl Painter {
     }
 }
 
-pub fn clear(gl: &glow::Context, dimension: [u32; 2], clear_color: egui::Rgba) {
+pub fn clear(gl: &glow::Context, screen_size_in_pixels: [u32; 2], clear_color: egui::Rgba) {
     unsafe {
         gl.disable(glow::SCISSOR_TEST);
 
-        gl.viewport(0, 0, dimension[0] as i32, dimension[1] as i32);
+        gl.viewport(
+            0,
+            0,
+            screen_size_in_pixels[0] as i32,
+            screen_size_in_pixels[1] as i32,
+        );
 
         let clear_color: Color32 = clear_color.into();
         gl.clear_color(

egui_web/src/backend.rs

@@ -155,6 +155,7 @@ impl AppRunner {
             },
             output: Default::default(),
             storage: Some(Box::new(LocalStorage::default())),
+            gl: painter.gl().clone(),
         };
 
         let needs_repaint: std::sync::Arc<NeedRepaint> = Default::default();
@@ -274,12 +275,14 @@ impl AppRunner {
         Ok((needs_repaint, clipped_primitives))
     }
 
+    pub fn clear_color_buffer(&self) {
+        self.painter.clear(self.app.clear_color());
+    }
+
     /// Paint the results of the last call to [`Self::logic`].
     pub fn paint(&mut self, clipped_primitives: &[egui::ClippedPrimitive]) -> Result<(), JsValue> {
         let textures_delta = std::mem::take(&mut self.textures_delta);
 
-        self.painter.clear(self.app.clear_color());
-
         self.painter.paint_and_update_textures(
             clipped_primitives,
             self.egui_ctx.pixels_per_point(),

egui_web/src/glow_wrapping.rs

@@ -32,6 +32,10 @@ impl WrappedGlowPainter {
 }
 
 impl WrappedGlowPainter {
+    pub fn gl(&self) -> &std::rc::Rc<glow::Context> {
+        self.painter.gl()
+    }
+
     pub fn max_texture_side(&self) -> usize {
         self.painter.max_texture_side()
     }
@@ -48,7 +52,7 @@ impl WrappedGlowPainter {
         self.painter.free_texture(tex_id);
     }
 
-    pub fn clear(&mut self, clear_color: Rgba) {
+    pub fn clear(&self, clear_color: Rgba) {
         let canvas_dimension = [self.canvas.width(), self.canvas.height()];
         egui_glow::painter::clear(self.painter.gl(), canvas_dimension, clear_color);
     }

egui_web/src/lib.rs

@@ -339,6 +339,7 @@ fn paint_and_schedule(runner_ref: &AppRunnerRef, panicked: Arc<AtomicBool>) -> R
     fn paint_if_needed(runner_ref: &AppRunnerRef) -> Result<(), JsValue> {
         let mut runner_lock = runner_ref.lock();
         if runner_lock.needs_repaint.fetch_and_clear() {
+            runner_lock.clear_color_buffer();
             let (needs_repaint, clipped_primitives) = runner_lock.logic()?;
             runner_lock.paint(&clipped_primitives)?;
             if needs_repaint {

epi/src/lib.rs

@@ -248,8 +248,6 @@ pub struct IconData {
 ///
 /// It provides methods to inspect the surroundings (are we on the web?),
 /// allocate textures, and change settings (e.g. window size).
-///
-/// [`Frame`] is cheap to clone and is safe to pass to other threads.
 pub struct Frame {
     /// Information about the integration.
     #[doc(hidden)]
@@ -262,6 +260,10 @@ pub struct Frame {
     /// A place where you can store custom data in a way that persists when you restart the app.
     #[doc(hidden)]
     pub storage: Option<Box<dyn Storage>>,
+
+    /// A reference to the underlying [`glow`] (OpenGL) context.
+    #[doc(hidden)]
+    pub gl: std::rc::Rc<glow::Context>,
 }
 
 impl Frame {
@@ -285,6 +287,19 @@ impl Frame {
         self.storage.as_deref_mut()
     }
 
+    /// A reference to the underlying [`glow`] (OpenGL) context.
+    ///
+    /// This can be used, for instance, to:
+    /// * Render things to offscreen buffers.
+    /// * Read the pixel buffer from the previous frame (`glow::Context::read_pixels`).
+    /// * Render things behind the egui windows.
+    ///
+    /// Note that all egui painting is deferred to after the call to [`App::update`]
+    /// ([`egui`] only collects [`egui::Shape`]s and then eframe paints them all in one go later on).
+    pub fn gl(&self) -> &std::rc::Rc<glow::Context> {
+        &self.gl
+    }
+
     /// Signal the app to stop/exit/quit the app (only works for native apps, not web apps).
     /// The framework will not quit immediately, but at the end of the this frame.
     pub fn quit(&mut self) {