SetPositionBoxModifier (#551)

Create a `SetPositionBoxModifier` to initialize (or move) the particles randomly inside or at the surface of a cube.

Author: hukasu

CHANGELOG.md

@@ -5,6 +5,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Added
+- Added `SetPositionBoxModifier`
+
 ### Changed
 
 - Batch same-effect instances. This greatly improves performance when using many instances

examples/init.rs

@@ -157,6 +157,22 @@ fn setup(
         cube.clone(),
         mat.clone(),
     );
+
+    spawn_effect(
+        &mut commands,
+        "SetPositionBoxModifier".to_string(),
+        3.,
+        Transform::from_translation(Vec3::new(0., 10., 0.)),
+        effects.add(base_effect("SetPositionBoxModifier", |writer| {
+            SetPositionBoxModifier {
+                center: writer.lit(Vec3::ZERO).expr(),
+                extent: writer.lit(Vec3::splat(5.)).expr(),
+                dimension: ShapeDimension::Volume,
+            }
+        })),
+        cube.clone(),
+        mat.clone(),
+    );
 }
 
 fn rotate_effect(time: Res<Time>, mut query: Query<(&RotateSpeed, &mut Transform)>) {

src/modifier/position.rs

@@ -343,3 +343,134 @@ impl Modifier for SetPositionCone3dModifier {
         Ok(())
     }
 }
+
+/// A modifier to set the position of particles on a box.
+///
+/// The box is axis aligned.
+///
+/// Particles are moved somewhere inside the volume or on the surface of a
+/// box defined by its extent.
+///
+/// # Attributes
+///
+/// This modifier requires the following particle attributes:
+/// - [`Attribute::POSITION`]
+#[derive(Clone, Copy, Hash, Reflect)]
+pub struct SetPositionBoxModifier {
+    /// The box center, relative to the emitter position.
+    ///
+    /// Expression type: `Vec3`
+    pub center: ExprHandle,
+    /// The extent of the box.
+    ///
+    /// Expression type: `Vec3`
+    pub extent: ExprHandle,
+    /// The shape dimension to set the position to.
+    pub dimension: ShapeDimension,
+}
+
+impl SetPositionBoxModifier {
+    fn eval(
+        &self,
+        module: &mut Module,
+        context: &mut dyn EvalContext,
+    ) -> Result<String, ExprError> {
+        let func_id = calc_func_id(self);
+        let func_name = format!("set_position_box_{0:016X}", func_id);
+
+        context.make_fn(
+            &func_name,
+            "particle: ptr<function, Particle>",
+            module,
+            &mut |m: &mut Module, ctx: &mut dyn EvalContext| -> Result<String, ExprError> {
+                let center = ctx.eval(m, self.center)?;
+                let extent = ctx.eval(m, self.extent)?;
+
+                let code = match self.dimension {
+                    ShapeDimension::Surface => {
+                        format!(
+                            r#"    let center = {};
+    let extent = {};
+
+    let face = frand();
+    let rand1 = frand() - 0.5;
+    let rand2 = frand() - 0.5;
+    let fixed = 0.5;
+
+    var x: f32;
+    var y: f32;
+    var z: f32;
+
+    if face < (1. / 6.) {{
+        x = rand1 * extent.x;
+        y = fixed * extent.y;
+        z = rand2 * extent.z;
+    }} else if face < (2. / 6.) {{
+        x = rand1 * extent.x;
+        y = -fixed * extent.y;
+        z = rand2 * extent.z;
+    }} else if face < (3. / 6.) {{
+        x = fixed * extent.x;
+        y = rand1 * extent.y;
+        z = rand2 * extent.z;
+    }} else if face < (4. / 6.) {{
+        x = -fixed * extent.x;
+        y = rand1 * extent.y;
+        z = rand2 * extent.z;
+    }} else if face < (5. / 6.) {{
+        x = rand1 * extent.x;
+        y = rand2 * extent.y;
+        z = fixed * extent.z;
+    }} else {{
+        x = rand1 * extent.x;
+        y = rand2 * extent.y;
+        z = -fixed * extent.z;
+    }}
+    (*particle).{} = center + vec3(x, y, z);
+"#,
+                            center,
+                            extent,
+                            Attribute::POSITION.name()
+                        )
+                    }
+                    ShapeDimension::Volume => format!(
+                        r#"    let center = {};
+    let extent = {};
+    let mult = frand3() - 0.5;
+    (*particle).{} = center + extent * mult;
+"#,
+                        center,
+                        extent,
+                        Attribute::POSITION.name()
+                    ),
+                };
+
+                Ok(code)
+            },
+        )?;
+
+        let code = format!("{}(&particle);\n", func_name);
+
+        Ok(code)
+    }
+}
+
+impl Modifier for SetPositionBoxModifier {
+    fn context(&self) -> ModifierContext {
+        ModifierContext::Init | ModifierContext::Update
+    }
+
+    fn attributes(&self) -> &[Attribute] {
+        &[Attribute::POSITION]
+    }
+
+    fn boxed_clone(&self) -> BoxedModifier {
+        Box::new(*self)
+    }
+
+    fn apply(&self, module: &mut Module, context: &mut ShaderWriter) -> Result<(), ExprError> {
+        let code = self.eval(module, context)?;
+        context.main_code += &code;
+        Ok(())
+    }
+}