PaintCallback: supply enough context to create a viewport

egui/src/lib.rs

@@ -307,8 +307,8 @@ pub use epaint::{
     color, mutex,
     text::{FontData, FontDefinitions, FontFamily, FontId, FontTweak},
     textures::TexturesDelta,
-    AlphaImage, ClippedPrimitive, Color32, ColorImage, ImageData, Mesh, PaintCallback, Rgba,
-    Rounding, Shape, Stroke, TextureHandle, TextureId,
+    AlphaImage, ClippedPrimitive, Color32, ColorImage, ImageData, Mesh, PaintCallback,
+    PaintCallbackInfo, Rgba, Rounding, Shape, Stroke, TextureHandle, TextureId,
 };
 
 pub mod text {

egui_glow/src/painter.rs

@@ -377,7 +377,13 @@ impl Painter {
                             );
                         }
 
-                        callback.call(self);
+                        let info = egui::PaintCallbackInfo {
+                            rect: callback.rect,
+                            pixels_per_point,
+                            screen_size_px: inner_size,
+                        };
+
+                        callback.call(&info, self);
 
                         // Restore state:
                         unsafe {

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 +642,45 @@ fn dashes_from_line(
 
 // ----------------------------------------------------------------------------
 
+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 {
+    #[inline]
+    pub fn viewport_left_px(&self) -> f32 {
+        self.rect.min.x * self.pixels_per_point
+    }
+
+    #[inline]
+    pub fn viewport_top_px(&self) -> f32 {
+        self.rect.min.y * self.pixels_per_point
+    }
+
+    // This is what `glViewport` etc expects.
+    #[inline]
+    pub fn viewport_from_bottom_px(&self) -> f32 {
+        self.screen_size_px[1] as f32 - self.rect.max.y * self.pixels_per_point
+    }
+
+    #[inline]
+    pub fn viewport_width_px(&self) -> f32 {
+        self.rect.width() * self.pixels_per_point
+    }
+
+    #[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 +689,22 @@ 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);
     }
 }