Docs

wgpu-hal/src/gles/adapter.rs

@@ -552,6 +552,12 @@ impl super::Adapter {
                 extensions.contains("GL_KHR_texture_compression_astc_hdr"),
             );
         }
+        if let Some(full_ver) = full_ver {
+            // Desktop 4.2 and greater specify the first instance parameter.
+            //
+            // For all other versions, the behavior is undefined.
+            features.set(wgt::Features::INDIRECT_FIRST_INSTANCE, full_ver >= (4, 2));
+        }
 
         // We *might* be able to emulate bgra8unorm-storage but currently don't attempt to.
 

wgpu-hal/src/gles/mod.rs

@@ -54,6 +54,31 @@ To address this, we invalidate the vertex buffers based on:
   - whether or not `start_instance` is used
   - stride has changed
 
+## Handling of BaseVertex, BaseInstance, and FirstVertex
+
+Between indirect, the lack of "base instance" semantics, and the availablity of gl_BaseInstance
+in shaders, getting buffers and builtins to work correctly is a bit tricky.
+
+We never emulate `base_vertex` and gl_VertexID behaves as `@builtin(vertex_index)` does, so we
+never need to do anything about that.
+
+We always advertise support for `VERTEX_AND_INSTANCE_INDEX_RESPECTS_RESPECTIVE_INDIRECT_FIRST`.
+
+### GL 4.2+ with ARB_shader_draw_parameters
+
+- `@builtin(instance_index)` translates to `gl_InstanceID + gl_BaseInstance`
+- We bind instance buffers without any offset emulation.
+- We advertise support for the `INDIRECT_FIRST_INSTANCE` feature.
+
+While we can theoretically have a card with 4.2+ support but without ARB_shader_draw_parameters,
+we don't bother with that combination.
+
+### GLES & GL 4.1
+
+- `@builtin(instance_index)` translates to `gl_InstanceID + naga_vs_base_instance`
+- We bind instance buffers with offset emulation.
+- We _do not_ advertise support for `INDIRECT_FIRST_INSTANCE` and cpu-side pretend the base instance is 0 on indirect calls.
+
 */
 
 ///cbindgen:ignore

wgpu-types/src/lib.rs

@@ -1390,7 +1390,16 @@ bitflags::bitflags! {
         ///
         /// DX11 on FL10 level hardware, WebGL2, and GLES 3.0 devices do not support indirect.
         const INDIRECT_EXECUTION = 1 << 2;
-        /// Supports non-zero `base_vertex` parameter to indexed draw calls.
+        /// Supports non-zero `base_vertex` parameter to direct indexed draw calls.
+        /// 
+        /// Indirect calls, if supported, always support non-zero `base_vertex`.
+        /// 
+        /// Supported by:
+        /// - Vulkan
+        /// - DX12
+        /// - Metal
+        /// - OpenGL 3.2+
+        /// - OpenGL ES 3.2
         const BASE_VERTEX = 1 << 3;
         /// Supports reading from a depth/stencil texture while using it as a read-only
         /// depth/stencil attachment.