Gizmos Billboards

crates/bevy_gizmos/src/billboard/billboards.wgsl

@@ -0,0 +1,89 @@
+// TODO use common view binding
+#import bevy_render::view::View
+
+@group(0) @binding(0) var<uniform> view: View;
+
+
+struct BillboardGizmoUniform {
+    size: f32,
+    depth_bias: f32,
+#ifdef SIXTEEN_BYTE_ALIGNMENT
+    // WebGL2 structs must be 16 byte aligned.
+    _padding: vec2<f32>,
+#endif
+}
+
+@group(1) @binding(0) var<uniform> billboard_gizmo: BillboardGizmoUniform;
+
+struct VertexInput {
+    @location(0) position: vec3<f32>,
+    @location(1) color: vec4<f32>,
+    @builtin(vertex_index) index: u32,
+};
+
+struct VertexOutput {
+    @builtin(position) clip_position: vec4<f32>,
+    @location(0) color: vec4<f32>,
+};
+
+const EPSILON: f32 = 4.88e-04;
+
+@vertex
+fn vertex(vertex: VertexInput) -> VertexOutput {
+    var positions = array<vec2<f32>, 6>(
+        vec2(-0.5, -0.5),
+        vec2(-0.5, 0.5),
+        vec2(0.5, 0.5),
+        vec2(-0.5, -0.5),
+        vec2(0.5, 0.5),
+        vec2(0.5, -0.5)
+    );
+    var offset = positions[vertex.index];
+
+	let center_clip = view.clip_from_world * vec4(vertex.position, 1.0);
+
+    let resolution = view.viewport.zw;
+    let screen_center = resolution * (0.5 * center_clip.xy / center_clip.w + 0.5);
+
+    var color = vertex.color;
+    var billboard_size = billboard_gizmo.size;
+#ifdef PERSPECTIVE
+    billboard_size /= center_clip.w;
+#endif
+    if billboard_size < 1. {
+        color.a *= billboard_size;
+        billboard_size = 1.;
+    }
+
+    let screen = screen_center + offset * billboard_size;
+
+    var depth: f32;
+    if billboard_gizmo.depth_bias >= 0. {
+        depth = center_clip.z * (1. - billboard_gizmo.depth_bias);
+    } else {
+        // depth * (center_clip.w / depth)^-depth_bias. So that when -depth_bias is 1.0, this is equal to center_clip.w
+        // and when equal to 0.0, it is exactly equal to depth.
+        // the epsilon is here to prevent the depth from exceeding center_clip.w when -depth_bias = 1.0
+        // center_clip.w represents the near plane in homogeneous clip space in bevy, having a depth
+        // of this value means nothing can be in front of this
+        // The reason this uses an exponential function is that it makes it much easier for the
+        // user to chose a value that is convenient for them
+        depth = center_clip.z * exp2(-billboard_gizmo.depth_bias * log2(center_clip.w / center_clip.z - EPSILON));
+    }
+
+    var clip_position = vec4(center_clip.w * ((2. * screen) / resolution - 1.), depth, center_clip.w);
+
+    var result: VertexOutput;
+	result.clip_position = clip_position;
+	result.color = color;
+	return result;
+}
+
+struct FragmentOutput {
+    @location(0) color: vec4<f32>,
+};
+
+@fragment
+fn fragment(in: VertexOutput) -> FragmentOutput {
+    return FragmentOutput(in.color);
+}