Add draw opacity

src/draw.rs

@@ -4,6 +4,7 @@ use crate::{Color, Vec2};
 pub struct DrawContext {
     panel: UiPanel,
     offset: Vec2,
+    opacity: Option<f32>,
 }
 
 impl DrawContext {
@@ -11,13 +12,19 @@ impl DrawContext {
         Self {
             panel,
             offset: Vec2::ZERO,
+            opacity: None,
         }
     }
 
-    pub fn draw_triangle(&self, v1: Vec2, v2: Vec2, v3: Vec2, color: Color) {
+    pub fn draw_triangle(&self, v1: Vec2, v2: Vec2, v3: Vec2, mut color: Color) {
         let v1 = v1 + self.offset;
         let v2 = v2 + self.offset;
         let v3 = v3 + self.offset;
+
+        if let Some(opacity) = self.opacity.as_ref() {
+            color.a *= opacity;
+        }
+
         self.panel.draw_triangle(v1, v2, v3, color);
     }
 
@@ -73,8 +80,21 @@ impl DrawContext {
 
     pub fn with_offset(&self, offset: Vec2) -> Self {
         Self {
-            panel: self.panel,
             offset: self.offset + offset,
+            ..*self
+        }
+    }
+
+    pub fn with_opacity(&self, opacity: f32) -> Self {
+        let opacity = if let Some(old) = self.opacity.as_ref() {
+            opacity * old
+        } else {
+            opacity
+        };
+
+        Self {
+            opacity: Some(opacity),
+            ..*self
         }
     }
 }