Push Constants — per‑object material properties

Push constants are tiny pieces of data you can send to shaders without creating or updating buffers. They’re perfect for per‑draw material knobs.

In this engine we use push constants for values that change per draw call (material factors and “is there a texture?” flags). Anything that changes less frequently (camera data, light lists, big arrays) stays in UBOs/SSBOs.

How we use them

We pack the common PBR controls (base color factor, metallic/roughness, texture presence flags, emissive strength, transmission, IOR) into a single push constant block and update it before each draw.

Where to look

  • C++ push constant struct and update call:

    • renderer.h (MaterialProperties)

    • renderer_rendering.cpp (where we push per-draw material properties)

  • Shader push constant block:

    • shaders/pbr.slang ( block)

  • PBR helper functions used by the shader:

    • shaders/pbr_utils.slang

    • shaders/lighting_utils.slang

Guidelines

  • Keep the block small (Vulkan guarantees at least 128 bytes). This sample fits comfortably.

  • Use push constants for values that change every draw call. Use UBO/SSBO for larger, less‑frequent data.

Future work ideas

If you want to extend this pattern:

  • Split “rarely changing per-material data” into a GPU material buffer and use push constants only for the per-draw index.

  • Add a second push constant block for per-draw debug visualizations (development-only) to keep it out of hot UBO paths.

  • Add a material override system (force roughness/metallic for entire scene) by layering a global UBO on top.

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