Enable instancing

gl/glsl_common.cpp

@@ -14,6 +14,7 @@ void gl::glsl_common_setup()
     "#define MAX_LIGHTS " + std::to_string(MAX_LIGHTS) + "U\n" +
     "#define MAX_CASCADES " + std::to_string(MAX_CASCADES) + "U\n" +
     "#define MAX_PARAMS " + std::to_string(MAX_PARAMS) + "U\n" +
+    "#define MAX_INSTANCES_PER_BATCH " + std::to_string(MAX_INSTANCES_PER_BATCH) + "U\n" +
     R"STRING(
     const uint LIGHT_SPOT = 0U;
     const uint LIGHT_POINT = 1U;
@@ -78,5 +79,31 @@ void gl::glsl_common_setup()
 		vec4 wiper_timer_return;
     };
 
+    // Per-instance modelview matrices for GPU-instanced draws.
+    // The UBO itself is declared in every stage so the binding stays valid,
+    // but the helper functions reference gl_InstanceID which is vertex-only,
+    // so they are guarded behind STAGE_VERTEX.
+    layout (std140) uniform instance_ubo
+    {
+        mat4 instance_modelview[MAX_INSTANCES_PER_BATCH];
+    };
+
+    #ifdef STAGE_VERTEX
+    // For non-instanced draws gl_InstanceID == 0 and slot 0 is permanently set
+    // to identity, so effective_modelview() reduces to model_ubo.modelview.
+    // For instanced draws the C++ side uploads camera-space root transforms for
+    // instances 0..N-1 and sets model_ubo.modelview to the submodel-local chain
+    // (computed from identity, NOT from the camera/instance), so the product
+    // gives the correct final transform per instance per submodel.
+    mat4 effective_modelview() {
+        return instance_modelview[gl_InstanceID] * modelview;
+    }
+    mat3 effective_modelviewnormal() {
+        // instance_modelview is rotation+translation only (no scale), so its
+        // upper-3x3 is its own inverse-transpose (rotation matrix).
+        return mat3(instance_modelview[gl_InstanceID]) * modelviewnormal;
+    }
+    #endif
+
     )STRING";
 }