Skip CAS HDR conversions for LDR input and use half-precision in WGSL

[mvaligursky]

Skip unnecessary HDR-to-LDR tone mapping in the CAS (Contrast Adaptive Sharpening) shader when the scene texture is already LDR (RGBA8), and use half-precision types for the CAS computation in WGSL.

Changes:

• Add hdrScene property to RenderPassCompose to track whether the scene texture is HDR
• Set hdrScene based on hdrFormat in RenderPassCameraFrame.setupComposePass
• Conditionally define CAS_HDR shader define only when the scene is HDR
• When LDR, toSDR/toHDR become identity functions — the compiler eliminates them, avoiding redundant tone mapping math
• Use half3/half types for CAS intermediate calculations in the WGSL shader

Performance:

• LDR scenes skip the toSDR/toHDR Reinhard tone mapping entirely (6 divides + 4 max operations removed)
• WGSL CAS computation uses half-precision for reduced register pressure and better ALU throughput on capable GPUs

[Copilot]

Pull request overview

This PR optimizes the Contrast Adaptive Sharpening (CAS) post-processing shader in two ways: (1) it skips unnecessary HDR↔LDR Reinhard tone mapping when the scene texture is already in LDR format (RGBA8), and (2) uses half-precision (half3/half) types for the CAS intermediate calculations in the WGSL shader.

Changes:

• Add hdrScene property to RenderPassCompose to control whether the CAS_HDR define is emitted, which selects between real and identity tone-mapping functions
• Set hdrScene from hdrFormat !== PIXELFORMAT_RGBA8 in setupComposePass
• Update GLSL and WGSL CAS shaders to use #ifdef CAS_HDR branching for tone-mapping functions, and convert WGSL intermediate values to half3/half

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/extras/render-passes/render-pass-compose.js	Adds _hdrScene field, getter/setter with shader-dirty invalidation, and emits CAS_HDR define when sharpening is enabled and scene is HDR; updates shader cache key
src/extras/render-passes/render-pass-camera-frame.js	Sets composePass.hdrScene based on whether the format is HDR (not PIXELFORMAT_RGBA8)
src/scene/shader-lib/glsl/chunks/render-pass/frag/compose/compose-cas.js	Wraps HDR tone-mapping functions in #ifdef CAS_HDR; #else branch provides identity functions
src/scene/shader-lib/wgsl/chunks/render-pass/frag/compose/compose-cas.js	Same #ifdef CAS_HDR branching as GLSL; also converts intermediate CAS values to half3/half

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[Copilot]

The comment // reversible LDR <-> HDR tone mapping, as CAS needs LDR input (line 10 in the GLSL file and same in WGSL) now precedes a #ifdef CAS_HDR / #else block. In the #else (LDR) branch, no tone mapping is performed at all — toSDR and toHDR are identity functions. The comment is therefore misleading in the LDR case. It should be updated to clarify that the tone mapping is only applied for HDR scenes, for example: // reversible LDR <-> HDR tone mapping (HDR only), as CAS needs LDR input.

Suggested change

	// reversible LDR <-> HDR tone mapping, as CAS needs LDR input
	// reversible LDR <-> HDR tone mapping (HDR only), as CAS needs LDR input

[Copilot]

The comment // reversible LDR <-> HDR tone mapping, as CAS needs LDR input (line 10) now precedes a #ifdef CAS_HDR / #else block. In the #else (LDR) branch, no tone mapping is performed at all — toSDR and toHDR are identity functions. The comment is therefore misleading in the LDR case. It should be updated to clarify that the tone mapping is only applied for HDR scenes, for example: // reversible LDR <-> HDR tone mapping (HDR only), as CAS needs LDR input.

Suggested change

	// reversible LDR <-> HDR tone mapping, as CAS needs LDR input
	// reversible LDR <-> HDR tone mapping (HDR only), as CAS needs LDR input