VK_EXT_robustness2 — safer defaults for real‑world engines

Vulkan lets you run fast and close to the metal. That also means a bad index or out‑of‑range access can produce undefined results. VK_EXT_robustness2 tightens that up so mistakes fail predictably instead of corrupting memory or producing flicker.

What it gives you

  • Robust buffer access 2 — out‑of‑bounds buffer reads return zero; writes are discarded.

  • Robust image access 2 — out‑of‑range image coordinates clamp or return zero per the spec.

  • Null descriptors — a descriptor can be left “null” and the shader sees a defined zero value instead of UB.

These behaviors make the engine more forgiving while students iterate and while textures stream in.

How we use it here

  • We enable the extension and feature structs during device creation when available.

  • Shaders are written assuming legal ranges, but if a streaming texture or optional binding is temporarily missing, sampling a null descriptor is defined and safe.

  • The Forward+/reflection paths avoid mid‑frame descriptor edits; robustness2 then acts as an extra safety net.

When to enable

Always enable when the device supports it for teaching samples and tools. For shipping titles, you can still keep it on; the performance cost is generally negligible on modern drivers, and the safety is worth it.

Where to look in the code

  • Device extension/feature enable:

    • renderer_core.cpp

    • vulkan_device.cpp

  • Bounds checks and defensive indexing in the Ray Query shader:

    • shaders/ray_query.slang (bounds checks for geometryInfoCount / materialCount)

  • Safe descriptor update patterns (so you don’t rely on robustness for correctness):

    • renderer_rendering.cpp (per-frame safe point)

    • Descriptor_Indexing_UpdateAfterBind.adoc

Takeaways

  • Robustness doesn’t replace good synchronization and lifetime rules; it complements them.

  • Null descriptors and “safe zero” reads make streaming and feature toggles less fragile.

Future work ideas

If you want to stress-test robustness (without turning the engine into a debugging tool):

  • Add a development-only “fault injection” toggle that intentionally feeds out-of-range indices in a controlled shader path.

  • Add a small runtime report that prints whether VK_EXT_robustness2 is enabled on the current device.

  • Add a unit-style GPU test scene that exercises missing textures / missing buffers while keeping VVL clean.

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