Phase 16.1-16.3/16.8: Extract duplicated magic numbers to Constants.h

[Cle2ment]

Extract 4 duplicated magic numbers to Constants.h:

1. BT.709 luminance (0.2126f, 0.7152f, 0.0722f) → kLuminanceR/kLuminanceG/kLuminanceB

   • CUDARenderer.cuh:348
   • CPUBackend.cpp:296
   • PathTracer.ispc:377

2. ONB threshold 0.9999f → kONBThreshold

   • PathTracerCore.h:102
   • CUDARenderer.cuh:97
   • PathTracer.ispc:103

3. MB conversion 1024*1024 → kBytesPerMB

   • CUDARenderer.cu:162,216
   • OptiXDenoiser.cpp:144

4. BVH stack depth 64 → kBVHMaxStackDepth

   • BVH.h:107
   • CUDARenderer.cuh:208
   • PathTracer.ispc:166

Note

ISPC cannot include C++ headers, so PathTracer.ispc keeps its own static const float equivalents — but should use the same recognizable names.

[github-actions]

Here's my analysis of the proposed changes in issue #Phase 16.1-16.3/16.8:

1. BT.709 luminance (0.2126f, 0.7152f, 0.0722f)

Both CUDARenderer.cuh and CPUBackend.cpp already #include "Constants.h", so constexpr float definitions there will work directly. ISPC (PathTracer.ispc:377) needs its own static const float kLuminanceR = 0.2126f etc. — the issue already notes this correctly.

2. ONB threshold (0.9999f)

PathTracerCore.h:102 and CUDARenderer.cuh:97 both include Constants.h, so this is straightforward. ISPC (PathTracer.ispc:103) needs its own static const float.

3. 1024*1024 → kBytesPerMB

• CUDARenderer.cu:162,216 — this file includes CUDARenderer.cuh which includes Constants.h, so transitively available. However, line 162 uses integer 1024 * 1024 (for size_t context), while OptiXDenoiser.cpp:144 uses 1024.0f * 1024.0f (float division). A single constexpr size_t kBytesPerMB = 1024 * 1024 in Constants.h with a cast-to-float at the use site (static_cast<float>(kBytesPerMB)) would unify them.
• Caveat: OptiXDenoiser.cpp does not include Constants.h and has no transitive include path to it. You'll need to #include "Constants.h" in OptiXDenoiser.cpp (or OptiXDenoiser.h).

4. BVH stack depth (64) → kBVHMaxStackDepth

• CUDARenderer.cuh:208 already includes Constants.h — trivial.
• BVH.h:107 does not include Constants.h. You'll need to add #include "Constants.h" to BVH.h.
• PathTracer.ispc:166 — ISPC static const int32_t kBVHMaxStackDepth = 64;.

Summary of includes needed

New constant	Files that need #include "Constants.h" added
kLuminanceR/kLuminanceG/kLuminanceB	(none — already included in both C++ files)
kONBThreshold	(none — already included in both C++ files)
kBytesPerMB	OptiXDenoiser.cpp (or .h)
kBVHMaxStackDepth	BVH.h

All ISPC equivalents go in PathTracer.ispc with static const float / static const int32_t, using matching names.

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