1import package::{
2    render::{
3        forward_io::VertexOutput,
4        mesh_functions::get_world_from_local,
5        mesh_view_bindings::view,
6        pbr_functions::calculate_tbn_mikktspace,
7        view_transformations::depth_ndc_to_view_z,
8    },
9    prepass::utils::prepass_depth,
10};
11import bevy_render::maths::project_onto;
12
13@group(constants::MATERIAL_BIND_GROUP) @binding(200)
14var<uniform> inv_depth_fade_factor: f32;
15
16struct ForwardDecalInformation {
17    world_position: vec4<f32>,
18    uv: vec2<f32>,
19    alpha: f32,
20}
21
22fn get_forward_decal_info(in: VertexOutput) -> ForwardDecalInformation {
23    let world_from_local = get_world_from_local(in.instance_index);
24    let scale = (world_from_local * vec4(1.0, 1.0, 1.0, 0.0)).xyz;
25    let scaled_tangent = vec4(in.world_tangent.xyz / scale, in.world_tangent.w);
26
27    let V = normalize(view.world_position - in.world_position.xyz);
28
29    // Transform V from fragment to camera in world space to tangent space.
30    let TBN = calculate_tbn_mikktspace(in.world_normal, scaled_tangent);
31    let T = TBN[0];
32    let B = TBN[1];
33    let N = TBN[2];
34    let Vt = vec3(dot(V, T), dot(V, B), dot(V, N));
35
36    let frag_depth = depth_ndc_to_view_z(in.position.z);
37    let depth_pass_depth = depth_ndc_to_view_z(prepass_depth(in.position, 0u));
38    let diff_depth = frag_depth - depth_pass_depth;
39    let diff_depth_abs = abs(diff_depth);
40
41    // Apply UV parallax
42    let contact_on_decal = project_onto(V * diff_depth, in.world_normal);
43    let normal_depth = length(contact_on_decal);
44    let view_steepness = abs(Vt.z);
45    let delta_uv = normal_depth * Vt.xy * vec2(1.0, -1.0) / view_steepness;
46    let uv = in.uv + delta_uv;
47
48    let world_position = vec4(in.world_position.xyz + V * diff_depth_abs, in.world_position.w);
49    let alpha = saturate(1.0 - (normal_depth * inv_depth_fade_factor));
50
51    return ForwardDecalInformation(world_position, uv, alpha);
52}