Fix the WebGL 2 backend by giving the visibility_ranges array a fixed length. (#13210)

WebGL 2 doesn't support variable-length uniform buffer arrays. So we
arbitrarily set the length of the visibility ranges field to 64 on that
platform.

---------

Co-authored-by: IceSentry <c.giguere42@gmail.com>

crates/bevy_pbr/src/render/mesh_functions.wgsl

@@ -1,7 +1,11 @@
 #define_import_path bevy_pbr::mesh_functions
 
 #import bevy_pbr::{
-    mesh_view_bindings::{view, visibility_ranges},
+    mesh_view_bindings::{
+        view,
+        visibility_ranges,
+        VISIBILITY_RANGE_UNIFORM_BUFFER_SIZE
+    },
     mesh_bindings::mesh,
     mesh_types::MESH_FLAGS_SIGN_DETERMINANT_MODEL_3X3_BIT,
     view_transformations::position_world_to_clip,
@@ -90,8 +94,16 @@ fn mesh_tangent_local_to_world(model: mat4x4<f32>, vertex_tangent: vec4<f32>, in
 // camera distance to determine the dithering level.
 #ifdef VISIBILITY_RANGE_DITHER
 fn get_visibility_range_dither_level(instance_index: u32, world_position: vec4<f32>) -> i32 {
+#if AVAILABLE_STORAGE_BUFFER_BINDINGS >= 6
+    // If we're using a storage buffer, then the length is variable.
+    let visibility_buffer_array_len = arrayLength(&visibility_ranges);
+#else   // AVAILABLE_STORAGE_BUFFER_BINDINGS >= 6
+    // If we're using a uniform buffer, then the length is constant
+    let visibility_buffer_array_len = VISIBILITY_RANGE_UNIFORM_BUFFER_SIZE;
+#endif  // AVAILABLE_STORAGE_BUFFER_BINDINGS >= 6
+
     let visibility_buffer_index = mesh[instance_index].flags & 0xffffu;
-    if (visibility_buffer_index > arrayLength(&visibility_ranges)) {
+    if (visibility_buffer_index > visibility_buffer_array_len) {
         return -16;
     }
 

crates/bevy_pbr/src/render/mesh_view_bindings.wgsl

@@ -35,10 +35,11 @@
 @group(0) @binding(10) var<uniform> fog: types::Fog;
 @group(0) @binding(11) var<uniform> light_probes: types::LightProbes;
 
+const VISIBILITY_RANGE_UNIFORM_BUFFER_SIZE: u32 = 64u;
 #if AVAILABLE_STORAGE_BUFFER_BINDINGS >= 6
 @group(0) @binding(12) var<storage> visibility_ranges: array<vec4<f32>>;
 #else
-@group(0) @binding(12) var<uniform> visibility_ranges: array<vec4<f32>>;
+@group(0) @binding(12) var<uniform> visibility_ranges: array<vec4<f32>, VISIBILITY_RANGE_UNIFORM_BUFFER_SIZE>;
 #endif
 
 @group(0) @binding(13) var screen_space_ambient_occlusion_texture: texture_2d<f32>;

crates/bevy_render/src/view/visibility/range.rs

@@ -19,7 +19,7 @@ use bevy_reflect::Reflect;
 use bevy_transform::components::GlobalTransform;
 use bevy_utils::{prelude::default, EntityHashMap, HashMap};
 use nonmax::NonMaxU16;
-use wgpu::BufferUsages;
+use wgpu::{BufferBindingType, BufferUsages};
 
 use crate::{
     camera::Camera,
@@ -38,6 +38,11 @@ use super::{check_visibility, VisibilitySystems, WithMesh};
 /// buffer slot.
 pub const VISIBILITY_RANGES_STORAGE_BUFFER_COUNT: u32 = 4;
 
+/// The size of the visibility ranges buffer in elements (not bytes) when fewer
+/// than 6 storage buffers are available and we're forced to use a uniform
+/// buffer instead (most notably, on WebGL 2).
+const VISIBILITY_RANGE_UNIFORM_BUFFER_SIZE: usize = 64;
+
 /// A plugin that enables [`VisibilityRange`]s, which allow entities to be
 /// hidden or shown based on distance to the camera.
 pub struct VisibilityRangePlugin;
@@ -424,9 +429,33 @@ pub fn write_render_visibility_ranges(
         return;
     }
 
-    // If the buffer is empty, push *something* so that we allocate it.
-    if render_visibility_ranges.buffer.is_empty() {
-        render_visibility_ranges.buffer.push(default());
+    // Mess with the length of the buffer to meet API requirements if necessary.
+    match render_device.get_supported_read_only_binding_type(VISIBILITY_RANGES_STORAGE_BUFFER_COUNT)
+    {
+        // If we're using a uniform buffer, we must have *exactly*
+        // `VISIBILITY_RANGE_UNIFORM_BUFFER_SIZE` elements.
+        BufferBindingType::Uniform
+            if render_visibility_ranges.buffer.len() > VISIBILITY_RANGE_UNIFORM_BUFFER_SIZE =>
+        {
+            render_visibility_ranges
+                .buffer
+                .truncate(VISIBILITY_RANGE_UNIFORM_BUFFER_SIZE);
+        }
+        BufferBindingType::Uniform
+            if render_visibility_ranges.buffer.len() < VISIBILITY_RANGE_UNIFORM_BUFFER_SIZE =>
+        {
+            while render_visibility_ranges.buffer.len() < VISIBILITY_RANGE_UNIFORM_BUFFER_SIZE {
+                render_visibility_ranges.buffer.push(default());
+            }
+        }
+
+        // Otherwise, if we're using a storage buffer, just ensure there's
+        // something in the buffer, or else it won't get allocated.
+        BufferBindingType::Storage { .. } if render_visibility_ranges.buffer.is_empty() => {
+            render_visibility_ranges.buffer.push(default());
+        }
+
+        _ => {}
     }
 
     // Schedule the write.