glow painter improvements (#1406)

* Add viewport info to PaintCallback
* glow: be more explicit with more state
* glow: Refactor VAO

epaint/src/lib.rs

@@ -31,7 +31,10 @@ pub use {
     image::{AlphaImage, ColorImage, ImageData, ImageDelta},
     mesh::{Mesh, Mesh16, Vertex},
     shadow::Shadow,
-    shape::{CircleShape, PaintCallback, PathShape, RectShape, Rounding, Shape, TextShape},
+    shape::{
+        CircleShape, PaintCallback, PaintCallbackInfo, PathShape, RectShape, Rounding, Shape,
+        TextShape,
+    },
     stats::PaintStats,
     stroke::Stroke,
     tessellator::{tessellate_shapes, TessellationOptions, Tessellator},

epaint/src/shape.rs

@@ -642,6 +642,52 @@ fn dashes_from_line(
 
 // ----------------------------------------------------------------------------
 
+/// Information passed along with [`PaintCallback`] ([`Shape::Callback`]).
+pub struct PaintCallbackInfo {
+    /// Viewport in points.
+    pub rect: Rect,
+
+    /// Pixels per point.
+    pub pixels_per_point: f32,
+
+    /// Full size of the screen, in pixels.
+    pub screen_size_px: [u32; 2],
+}
+
+impl PaintCallbackInfo {
+    /// Physical pixel offset for left side of the viewport.
+    #[inline]
+    pub fn viewport_left_px(&self) -> f32 {
+        self.rect.min.x * self.pixels_per_point
+    }
+
+    /// Physical pixel offset for top side of the viewport.
+    #[inline]
+    pub fn viewport_top_px(&self) -> f32 {
+        self.rect.min.y * self.pixels_per_point
+    }
+
+    /// Physical pixel offset for bottom side of the viewport.
+    ///
+    /// This is what `glViewport` etc expects for the y axis.
+    #[inline]
+    pub fn viewport_from_bottom_px(&self) -> f32 {
+        self.screen_size_px[1] as f32 - self.rect.max.y * self.pixels_per_point
+    }
+
+    /// Viewport width in physical pixels.
+    #[inline]
+    pub fn viewport_width_px(&self) -> f32 {
+        self.rect.width() * self.pixels_per_point
+    }
+
+    /// Viewport width in physical pixels.
+    #[inline]
+    pub fn viewport_height_px(&self) -> f32 {
+        self.rect.height() * self.pixels_per_point
+    }
+}
+
 /// If you want to paint some 3D shapes inside an egui region, you can use this.
 ///
 /// This is advanced usage, and is backend specific.
@@ -650,21 +696,22 @@ pub struct PaintCallback {
     /// Where to paint.
     pub rect: Rect,
 
-    /// Paint something custom using.
+    /// Paint something custom (e.g. 3D stuff).
     ///
     /// The argument is the render context, and what it contains depends on the backend.
     /// In `eframe` it will be `egui_glow::Painter`.
     ///
     /// The rendering backend is responsible for first setting the active viewport to [`Self::rect`].
-    /// The rendering backend is also responsible for restoring any state it needs,
+    ///
+    /// The rendering backend is also responsible for restoring any state,
     /// such as the bound shader program and vertex array.
-    pub callback: std::sync::Arc<dyn Fn(&dyn std::any::Any) + Send + Sync>,
+    pub callback: std::sync::Arc<dyn Fn(&PaintCallbackInfo, &dyn std::any::Any) + Send + Sync>,
 }
 
 impl PaintCallback {
     #[inline]
-    pub fn call(&self, render_ctx: &dyn std::any::Any) {
-        (self.callback)(render_ctx);
+    pub fn call(&self, info: &PaintCallbackInfo, render_ctx: &dyn std::any::Any) {
+        (self.callback)(info, render_ctx);
     }
 }