Simplified draw state

Closes PistonDevelopers#996

- Removed draw_state dependency
- Fixed [PR 30742](rust-lang/rust#30724)
(default generic parameters will become a hard error)

Cargo.toml

@@ -22,7 +22,6 @@ name = "graphics"
 path = "./src/lib.rs"
 
 [dependencies]
-draw_state = "0.3.0"
 interpolation = "0.1.0"
 piston-texture = "0.3.0"
 piston-viewport = "0.2.0"

src/context.rs

@@ -1,7 +1,6 @@
 //! Transformation context
 
 use DrawState;
-use default_draw_state;
 use math::{
     abs_transform,
     identity,
@@ -33,7 +32,7 @@ impl Context {
         Context {
             view: identity(),
             transform: identity(),
-            draw_state: *default_draw_state(),
+            draw_state: Default::default(),
             viewport: None,
         }
     }
@@ -54,7 +53,7 @@ impl Context {
         Context {
             view: mat,
             transform: mat,
-            draw_state: *default_draw_state(),
+            draw_state: Default::default(),
             viewport: Some(viewport),
         }
     }
@@ -75,7 +74,7 @@ impl Context {
         Context {
             view: mat,
             transform: mat,
-            draw_state: *default_draw_state(),
+            draw_state: Default::default(),
             viewport: None,
         }
     }

src/draw_state.rs

@@ -0,0 +1,125 @@
+//! Graphics draw state.
+
+/// Graphics draw state used for blending, clipping and stencil rendering.
+#[derive(Copy, Clone, PartialEq, Debug, PartialOrd)]
+pub struct DrawState {
+    /// Scissor mask to use. If set, no pixel outside of this
+    /// rectangle (in screen space) will be written to as a result of rendering.
+    pub scissor: Option<[u32; 4]>,
+    /// Stencil test to use. If None, no stencil testing is done.
+    pub stencil: Option<Stencil>,
+    /// Blend function to use. If None, blending is disabled.
+    pub blend: Option<Blend>,
+}
+
+impl Default for DrawState {
+    fn default() -> Self {
+        DrawState::new_alpha()
+    }
+}
+
+impl DrawState {
+    /// Uses alpha blending.
+    pub fn new_alpha() -> DrawState {
+        DrawState {
+            blend: Some(Blend::Alpha),
+            stencil: None,
+            scissor: None,
+        }
+    }
+
+    /// Draws to stencil buffer with value 255.
+    /// This can be used for clipping.
+    pub fn new_clip() -> DrawState {
+        DrawState {
+            blend: Some(Blend::Alpha),
+            stencil: Some(Stencil::Clip(255)),
+            scissor: None,
+        }
+    }
+
+    /// Tests against stencil buffer with value 255.
+    /// Draws inside the shape defined by stencil buffer.
+    pub fn new_inside() -> DrawState {
+        DrawState {
+            blend: Some(Blend::Alpha),
+            stencil: Some(Stencil::Inside(255)),
+            scissor: None,
+        }
+    }
+
+    /// Tests against stencil buffer with value 255.
+    /// Draws outside the shape defined by stencil buffer.
+    pub fn new_outside() -> DrawState {
+        DrawState {
+            blend: Some(Blend::Alpha),
+            stencil: Some(Stencil::Outside(255)),
+            scissor: None,
+        }
+    }
+
+    /// Sets blending.
+    pub fn blend(mut self, blend: Blend) -> DrawState {
+        self.blend = Some(blend);
+        self
+    }
+
+    /// Sets scissor `[x, y, w, h]`.
+    pub fn scissor(mut self, scissor: [u32; 4]) -> DrawState {
+        self.scissor = Some(scissor);
+        self
+    }
+}
+
+/// The blend setting to use when drawing.
+///
+/// Using presets since some backends need one pipeline state object instance
+/// per blending technique.
+#[derive(Copy, Clone, Debug, PartialEq, PartialOrd)]
+pub enum Blend {
+    /// Alpha blending (allows semi-transparent pixels).
+    ///
+    /// ```ignore
+    /// dest_color' = src_color * src_alpha + dest_color * (1 - src_alpha)
+    /// dest_alpha' = src_alpha + dest_alpha
+    /// ```
+    Alpha,
+    /// Additive blending.
+    ///
+    /// ```ignore
+    /// dest_color' = src_color + dest_color
+    /// dest_alpha' = src_alpha + dest_alpha
+    /// ```
+    Add,
+    /// Multiply color components.
+    ///
+    /// ```ignore
+    /// dest_color' = src_color * dest_color
+    /// dest_alpha' = src_alpha * dest_alpha
+    /// ```
+    Multiply,
+    /// Invert colors when rendering a white shape.
+    ///
+    /// ```ignore
+    /// dest_color' = ref_color - src_color
+    /// dest_alpha' = dest_alpha
+    /// ```
+    ///
+    /// When combining two fragments, subtract the destination color from a constant color
+    /// using the source color as weight. Has an invert effect with the constant color
+    /// as base and source color controlling displacement from the base color.
+    /// A white source color and a white value results in plain invert.
+    /// The output alpha is same as destination alpha.
+    Invert,
+}
+
+/// Stencil buffer settings.
+#[derive(Copy, Clone, Debug, PartialEq, PartialOrd)]
+pub enum Stencil {
+    /// Draw to stencil buffer.
+    Clip(u8),
+    /// Draw pixels that have stencil value.
+    Inside(u8),
+    /// Draw pixels that does not have stencil value.
+    Outside(u8),
+}