start a way to upload once, draw many times

ezgui/src/drawing.rs

@@ -163,6 +163,11 @@ impl<'a> GfxCtx<'a> {
     }
 
     pub fn draw_polygon_batch(&mut self, list: Vec<(Color, &Polygon)>) {
+        let obj = self.prerender(list);
+        self.draw(&obj);
+    }
+
+    pub fn prerender(&mut self, list: Vec<(Color, &Polygon)>) -> Drawable {
         let mut vertices: Vec<Vertex> = Vec::new();
         let mut indices: Vec<u32> = Vec::new();
 
@@ -189,10 +194,15 @@ impl<'a> GfxCtx<'a> {
         .unwrap();
         self.num_new_uploads += 1;
 
+        Drawable {
+            vertex_buffer, index_buffer, }
+    }
+
+    pub fn draw(&mut self, obj: &Drawable) {
         self.target
             .draw(
-                &vertex_buffer,
-                &index_buffer,
+                &obj.vertex_buffer,
+                &obj.index_buffer,
                 &self.program,
                 &self.uniforms,
                 &self.params,
@@ -201,3 +211,9 @@ impl<'a> GfxCtx<'a> {
         self.num_draw_calls += 1;
     }
 }
+
+// Something that's been sent to the GPU already.
+pub struct Drawable {
+    vertex_buffer: glium::VertexBuffer<Vertex>,
+    index_buffer: glium::IndexBuffer<u32>,
+}

ezgui/src/lib.rs

@@ -15,7 +15,7 @@ mod wizard;
 
 pub use crate::canvas::{Canvas, HorizontalAlignment, VerticalAlignment, BOTTOM_LEFT, CENTERED};
 pub use crate::color::Color;
-pub use crate::drawing::GfxCtx;
+pub use crate::drawing::{Drawable, GfxCtx};
 pub use crate::event::{Event, Key};
 pub use crate::input::{ModalMenu, UserInput};
 pub use crate::log_scroller::LogScroller;