Add background_opacity option to set terminal transparency (#331)

The option is an Alpha struct that ensures that the contained float is
between 0.0 and 1.0. Background colors are multiplied by the opacity
to properly alpha blend them.

alacritty.yml

@@ -158,6 +158,9 @@ visual_bell:
   animation: EaseOutExpo
   duration: 0
 
+# Background opacity
+background_opacity: 1.0
+
 # Key bindings
 #
 # Each binding is defined as an object with some properties. Most of the

alacritty_macos.yml

@@ -158,6 +158,9 @@ visual_bell:
   animation: EaseOutExpo
   duration: 0
 
+# Background opacity
+background_opacity: 1.0
+
 # Key bindings
 #
 # Each binding is defined as an object with some properties. Most of the

res/text.f.glsl

@@ -21,15 +21,17 @@ flat in int background;
 layout(location = 0, index = 0) out vec4 color;
 layout(location = 0, index = 1) out vec4 alphaMask;
 
+uniform float bgOpacity;
 uniform sampler2D mask;
 
 void main()
 {
     if (background != 0) {
-        alphaMask = vec4(1.0, 1.0, 1.0, 1.0);
-        color = vec4(bg + vb, 1.0);
+        alphaMask = vec4(1.0);
+        color = vec4(bg + vb, 1.0) * bgOpacity;
     } else {
-        alphaMask = vec4(texture(mask, TexCoords).rgb, 1.0);
+        vec3 textColor = texture(mask, TexCoords).rgb;
+        alphaMask = vec4(textColor, textColor.r);
         color = vec4(fg, 1.0);
     }
 }

src/config.rs

@@ -173,6 +173,40 @@ impl<'a> Shell<'a> {
     }
 }
 
+/// Wrapper around f32 that represents an alpha value between 0.0 and 1.0
+#[derive(Clone, Copy, Debug)]
+pub struct Alpha(f32);
+
+impl Alpha {
+    pub fn new(value: f32) -> Self {
+        Alpha(Self::clamp_to_valid_range(value))
+    }
+
+    pub fn set(&mut self, value: f32) {
+        self.0 = Self::clamp_to_valid_range(value);
+    }
+
+    pub fn get(&self) -> f32 {
+        self.0
+    }
+
+    fn clamp_to_valid_range(value: f32) -> f32 {
+        if value < 0.0 {
+            0.0
+        } else if value > 1.0 {
+            1.0
+        } else {
+            value
+        }
+    }
+}
+
+impl Default for Alpha {
+    fn default() -> Self {
+        Alpha(1.0)
+    }
+}
+
 /// Top-level config type
 #[derive(Debug, Deserialize)]
 pub struct Config {
@@ -211,6 +245,10 @@ pub struct Config {
     #[serde(default)]
     colors: Colors,
 
+    /// Background opacity from 0.0 to 1.0
+    #[serde(default)]
+    background_opacity: Alpha,
+
     /// Keybindings
     #[serde(default="default_key_bindings")]
     key_bindings: Vec<KeyBinding>,
@@ -281,6 +319,7 @@ impl Default for Config {
             render_timer: Default::default(),
             custom_cursor_colors: false,
             colors: Default::default(),
+            background_opacity: Default::default(),
             key_bindings: Vec::new(),
             mouse_bindings: Vec::new(),
             selection: Default::default(),
@@ -696,6 +735,16 @@ impl de::Deserialize for RawBinding {
     }
 }
 
+
+impl de::Deserialize for Alpha {
+    fn deserialize<D>(deserializer: D) -> ::std::result::Result<Self, D::Error>
+        where D: de::Deserializer
+    {
+        let value = f32::deserialize(deserializer)?;
+        Ok(Alpha::new(value))
+    }
+}
+
 impl de::Deserialize for MouseBinding {
     fn deserialize<D>(deserializer: D) -> ::std::result::Result<Self, D::Error>
         where D: de::Deserializer
@@ -1038,6 +1087,10 @@ impl Config {
         &self.colors
     }
 
+    pub fn background_opacity(&self) -> Alpha {
+        self.background_opacity
+    }
+
     pub fn key_bindings(&self) -> &[KeyBinding] {
         &self.key_bindings[..]
     }

src/renderer/mod.rs

@@ -119,6 +119,8 @@ pub struct ShaderProgram {
 
     padding_x: f32,
     padding_y: f32,
+
+    u_bg_opacity: GLint,
 }
 
 
@@ -604,6 +606,7 @@ impl QuadRenderer {
             self.program.activate();
             self.program.set_term_uniforms(props);
             self.program.set_visual_bell(visual_bell_intensity as _);
+            self.program.set_bg_opacity(config.background_opacity().get());
 
             gl::BindVertexArray(self.vao);
             gl::BindBuffer(gl::ELEMENT_ARRAY_BUFFER, self.ebo);
@@ -685,12 +688,13 @@ impl QuadRenderer {
 
 impl<'a> RenderApi<'a> {
     pub fn clear(&self, color: Rgb) {
+        let alpha = self.config.background_opacity().get();
         unsafe {
             gl::ClearColor(
-                (self.visual_bell_intensity + color.r as f32 / 255.0).min(1.0),
-                (self.visual_bell_intensity + color.g as f32 / 255.0).min(1.0),
-                (self.visual_bell_intensity + color.b as f32 / 255.0).min(1.0),
-                1.0
+                (self.visual_bell_intensity + color.r as f32 / 255.0).min(1.0) * alpha,
+                (self.visual_bell_intensity + color.g as f32 / 255.0).min(1.0) * alpha,
+                (self.visual_bell_intensity + color.b as f32 / 255.0).min(1.0) * alpha,
+                alpha
                 );
             gl::Clear(gl::COLOR_BUFFER_BIT);
         }
@@ -916,13 +920,14 @@ impl ShaderProgram {
         }
 
         // get uniform locations
-        let (projection, term_dim, cell_dim, visual_bell, background) = unsafe {
+        let (projection, term_dim, cell_dim, visual_bell, background, bg_opacity) = unsafe {
             (
                 gl::GetUniformLocation(program, cptr!(b"projection\0")),
                 gl::GetUniformLocation(program, cptr!(b"termDim\0")),
                 gl::GetUniformLocation(program, cptr!(b"cellDim\0")),
                 gl::GetUniformLocation(program, cptr!(b"visualBell\0")),
                 gl::GetUniformLocation(program, cptr!(b"backgroundPass\0")),
+                gl::GetUniformLocation(program, cptr!(b"bgOpacity\0")),
             )
         };
 
@@ -937,6 +942,7 @@ impl ShaderProgram {
             u_background: background,
             padding_x: config.padding().x.floor(),
             padding_y: config.padding().y.floor(),
+            u_bg_opacity: bg_opacity,
         };
 
         shader.update_projection(*size.width as f32, *size.height as f32);
@@ -997,6 +1003,12 @@ impl ShaderProgram {
         }
     }
 
+    fn set_bg_opacity(&self, bg_opacity: f32) {
+        unsafe {
+            gl::Uniform1f(self.u_bg_opacity, bg_opacity);
+        }
+    }
+
     fn create_program(vertex: GLuint, fragment: GLuint) -> GLuint {
         unsafe {
             let program = gl::CreateProgram();

src/window.rs

@@ -186,7 +186,8 @@ impl Window {
 
         Window::platform_window_init();
         let window = WindowBuilder::new()
-            .with_title(title);
+            .with_title(title)
+            .with_transparency(true);
         let context = ContextBuilder::new()
             .with_vsync(true);
         let window = ::glutin::GlWindow::new(window, context, &event_loop)?;