1#import bevy_sprite::mesh2d_vertex_output::VertexOutput
2
3@group(#{MATERIAL_BIND_GROUP}) @binding(0) var<uniform> frame: vec4<f32>;
4// current/previous ring origin X, stable anchor X, and view width.
5@group(#{MATERIAL_BIND_GROUP}) @binding(1) var<uniform> anchor: vec4<f32>;
6
7@fragment
8fn fragment(_in: VertexOutput) -> @location(0) vec4<f32> {
9    // The physical point is anchored in world space. Camera-follow ring origin
10    // changes in anchor.xy are deliberately absent from both positions.
11    let current_world_x = anchor.z + frame.z;
12    let previous_world_x = anchor.z + frame.w;
13    let current_ndc_x = 2.0 * (current_world_x - frame.x) / anchor.w;
14    let previous_ndc_x = 2.0 * (previous_world_x - frame.y) / anchor.w;
15    let current_ndc = vec2<f32>(current_ndc_x, 0.0);
16    let previous_ndc = vec2<f32>(previous_ndc_x, 0.0);
17
18    // Terra and Bevy use current minus previous NDC, transformed to UV space.
19    let raw_motion = (current_ndc - previous_ndc) * vec2<f32>(0.5, -0.5);
20    let motion = select(raw_motion, vec2(0.0), raw_motion == vec2(0.0));
21    return vec4<f32>(motion, 0.0, 1.0);
22}