fudge depth offsets on Webgl

crates/re_renderer/shader/device_info.wgsl

@@ -0,0 +1,3 @@
+// True if we're running on Gles/WebGL.
+// (patched accordingly if necessary)
+const GLES = false;

crates/re_renderer/shader/utils/depth_offset.wgsl

@@ -1,5 +1,6 @@
 #import <../global_bindings.wgsl>
 #import <../types.wgsl>
+#import <../device_info.wgsl>
 
 fn apply_depth_offset(position: Vec4, offset: f32) -> Vec4 {
     // We're using inverse z, i.e. 0.0 is far, 1.0 is near.
@@ -16,7 +17,17 @@ fn apply_depth_offset(position: Vec4, offset: f32) -> Vec4 {
 
         // 1.0 * eps _should_ be enough, but in practice it causes Z-fighting for unknown reasons.
         // Maybe because of GPU interpolation of vertex coordinates?
-        let eps = 5.0 * f32eps;
+        var eps = 5.0 * f32eps;
+
+        if GLES {
+            // On GLES/WebGL, the NDC clipspace range for depth is from -1 to 1 and y is flipped.
+            // wgpu/Naga counteracts this by patching all vertex shader with:
+            // "gl_Position.yz = vec2(-gl_Position.y, gl_Position.z * 2.0 - gl_Position.w);",
+            // This is great, since it means that we can pretend depth is 0 to 1 as specified by WebGPU.
+            // But it completely messes up depth precision, in particular since we use
+            // an inverse depth projection that tries to make use of the high float precision closer to zero.
+            eps *= 1000.0;
+        }
 
         return Vec4(
             position.xy,

crates/re_renderer/src/context.rs

@@ -201,6 +201,11 @@ impl RenderContext {
                     "@invariant @builtin(position)".to_owned(),
                     "@builtin(position)".to_owned(),
                 ));
+        } else if adapter.get_info().backend == wgpu::Backend::Gl {
+            gpu_resources
+                .shader_modules
+                .shader_text_workaround_replacements
+                .push(("GLES = false".to_owned(), "GLES = true".to_owned()));
         }
 
         RenderContext {

crates/re_renderer/src/workspace_shaders.rs

@@ -43,6 +43,12 @@ pub fn init() {
         fs.create_file(virtpath, content).unwrap();
     }
 
+    {
+        let virtpath = Path::new("shader/device_info.wgsl");
+        let content = include_str!("../shader/device_info.wgsl").into();
+        fs.create_file(virtpath, content).unwrap();
+    }
+
     {
         let virtpath = Path::new("shader/generic_skybox.wgsl");
         let content = include_str!("../shader/generic_skybox.wgsl").into();