1#define_import_path bevy_pbr::parallax_mapping
2
3#import bevy_render::bindless::{bindless_samplers_filtering, bindless_textures_2d}
4
5#import bevy_pbr::{
6    pbr_bindings::{depth_map_texture, depth_map_sampler},
7    mesh_bindings::mesh
8}
9
10#ifdef BINDLESS
11#import bevy_pbr::pbr_bindings::material_indices
12#endif  // BINDLESS
13
14fn sample_depth_map(uv: vec2<f32>, material_bind_group_slot: u32) -> f32 {
15    // We use `textureSampleLevel` over `textureSample` because the wgpu DX12
16    // backend (Fxc) panics when using "gradient instructions" inside a loop.
17    // It results in the whole loop being unrolled by the shader compiler,
18    // which it can't do because the upper limit of the loop in steep parallax
19    // mapping is a variable set by the user.
20    // The "gradient instructions" comes from `textureSample` computing MIP level
21    // based on UV derivative. With `textureSampleLevel`, we provide ourselves
22    // the MIP level, so no gradient instructions are used, and we can use
23    // sample_depth_map in our loop.
24    // See https://stackoverflow.com/questions/56581141/direct3d11-gradient-instruction-used-in-a-loop-with-varying-iteration-forcing
25    return textureSampleLevel(
26#ifdef BINDLESS
27        bindless_textures_2d[material_indices[material_bind_group_slot].depth_map_texture],
28        bindless_samplers_filtering[material_indices[material_bind_group_slot].depth_map_sampler],
29#else   // BINDLESS
30        depth_map_texture,
31        depth_map_sampler,
32#endif  // BINDLESS
33        uv,
34        0.0
35    ).r;
36}
37
38// An implementation of parallax mapping, see https://en.wikipedia.org/wiki/Parallax_mapping
39// Code derived from: https://web.archive.org/web/20150419215321/http://sunandblackcat.com/tipFullView.php?l=eng&topicid=28
40fn parallaxed_uv(
41    depth_scale: f32,
42    max_layer_count: f32,
43    max_steps: u32,
44    // The original interpolated uv
45    original_uv: vec2<f32>,
46    // The vector from the camera to the fragment at the surface in tangent space
47    Vt: vec3<f32>,
48    material_bind_group_slot: u32,
49) -> vec2<f32> {
50    if max_layer_count < 1.0 {
51        return original_uv;
52    }
53    var uv = original_uv;
54
55    // Steep Parallax Mapping
56    // ======================
57    // Split the depth map into `layer_count` layers.
58    // When Vt hits the surface of the mesh (excluding depth displacement),
59    // if the depth is not below or on surface including depth displacement (textureSample), then
60    // look forward (+= delta_uv) on depth texture according to
61    // Vt and distance between hit surface and depth map surface,
62    // repeat until below the surface.
63    //
64    // Where `layer_count` is interpolated between `1.0` and
65    // `max_layer_count` according to the steepness of Vt.
66
67    let view_steepness = abs(Vt.z);
68    // We mix with minimum value 1.0 because otherwise,
69    // with 0.0, we get a division by zero in surfaces parallel to viewport,
70    // resulting in a singularity.
71    let layer_count = mix(max_layer_count, 1.0, view_steepness);
72    let layer_depth = 1.0 / layer_count;
73    var delta_uv = depth_scale * layer_depth * Vt.xy * vec2(1.0, -1.0) / view_steepness;
74
75    var current_layer_depth = 0.0;
76    var texture_depth = sample_depth_map(uv, material_bind_group_slot);
77
78    // texture_depth > current_layer_depth means the depth map depth is deeper
79    // than the depth the ray would be at this UV offset so the ray has not
80    // intersected the surface
81    for (var i: i32 = 0; texture_depth > current_layer_depth && i <= i32(layer_count); i++) {
82        current_layer_depth += layer_depth;
83        uv += delta_uv;
84        texture_depth = sample_depth_map(uv, material_bind_group_slot);
85    }
86
87#ifdef RELIEF_MAPPING
88    // Relief Mapping
89    // ==============
90    // "Refine" the rough result from Steep Parallax Mapping
91    // with a **binary search** between the layer selected by steep parallax
92    // and the next one to find a point closer to the depth map surface.
93    // This reduces the jaggy step artifacts from steep parallax mapping.
94
95    delta_uv *= 0.5;
96    var delta_depth = 0.5 * layer_depth;
97
98    uv -= delta_uv;
99    current_layer_depth -= delta_depth;
100
101    for (var i: u32 = 0u; i < max_steps; i++) {
102        texture_depth = sample_depth_map(uv, material_bind_group_slot);
103
104        // Halve the deltas for the next step
105        delta_uv *= 0.5;
106        delta_depth *= 0.5;
107
108        // Step based on whether the current depth is above or below the depth map
109        if (texture_depth > current_layer_depth) {
110            uv += delta_uv;
111            current_layer_depth += delta_depth;
112        } else {
113            uv -= delta_uv;
114            current_layer_depth -= delta_depth;
115        }
116    }
117#else
118    // Parallax Occlusion mapping
119    // ==========================
120    // "Refine" Steep Parallax Mapping by interpolating between the
121    // previous layer's depth and the computed layer depth.
122    // Only requires a single lookup, unlike Relief Mapping, but
123    // may skip small details and result in writhing material artifacts.
124    let previous_uv = uv - delta_uv;
125    let next_depth = texture_depth - current_layer_depth;
126    let previous_depth = sample_depth_map(previous_uv, material_bind_group_slot) -
127        current_layer_depth + layer_depth;
128
129    let weight = next_depth / (next_depth - previous_depth);
130
131    uv = mix(uv, previous_uv, weight);
132
133    current_layer_depth += mix(next_depth, previous_depth, weight);
134#endif
135
136    // Note: `current_layer_depth` is not returned, but may be useful
137    // for light computation later on in future improvements of the pbr shader.
138    return uv;
139}