feat(lambda-rs): Update pipelines to disable blending by default

[vmarcella]

Summary

Pipelines previously hardcoded ALPHA_BLENDING for all color targets, adding unnecessary blending overhead for the common case (fully opaque geometry). This PR changes the default to no blending (opaque/replace) and adds an explicit opt-in API for transparency.

Related Issues

• Resolves [Feature] Default pipelines to opaque blending (blend: None) #102

Changes

• Default RenderPipelineBuilder color target blending to None (replace/opaque) instead of always enabling ALPHA_BLENDING.
• Add BlendMode enum + RenderPipelineBuilder::with_blend(BlendMode) to opt into:
  • None (default)
  • AlphaBlending
  • PremultipliedAlpha
  • Additive
  • Custom(wgpu::BlendState) (platform-level advanced use)
• Thread blend configuration through the engine-level builder so end users can opt in without touching wgpu.
• Update the reflective-room demo + tutorial to opt into alpha blending for the translucent floor pass.
• Update docs with guidance that pipelines are opaque by default and blending must be enabled explicitly for transparency.
• Add unit tests verifying the default blend mode is None and that presets map to the expected wgpu blend states.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (updates to docs, specs, tutorials, or comments)
• Refactor (code change that neither fixes a bug nor adds a feature)
• Performance (change that improves performance)
• Test (adding or updating tests)
• Build/CI (changes to build process or CI configuration)

Affected Crates

• lambda-rs
• lambda-rs-platform
• lambda-rs-args
• lambda-rs-logging
• Other: demos, docs

Checklist

• Code follows the repository style guidelines (cargo +nightly fmt --all)
• Code passes clippy (cargo clippy --workspace --all-targets -- -D warnings)
• Tests pass (cargo test --workspace)
• New code includes appropriate documentation
• Public API changes are documented
• Breaking changes are noted in this PR description

Testing

Commands run:

cargo test -p lambda-rs-platform
cargo test -p lambda-rs
cargo fmt

Manual verification steps (if applicable):

1. (Optional) Run demos/render reflective-room and confirm the floor tint remains translucent by enabling .with_blend(BlendMode::AlphaBlending).

Screenshots/Recordings

N/A

Platform Testing

• macOS
• Windows
• Linux

Additional Notes

Migration: any pipeline that relied on implicit alpha blending must now opt in:

use lambda::render::pipeline::BlendMode;

let pipeline = RenderPipelineBuilder::new()
  // ...
  .with_blend(BlendMode::AlphaBlending);

This improves default performance for opaque geometry, especially on tile-based GPUs where blending can increase bandwidth/cost.

[github-actions]

✅ Coverage Report

📊 View Full HTML Report (download artifact)

Overall Coverage

Metric	Value
Total Line Coverage	76.34%
Lines Covered	10330 / 13531

Changed Files in This PR

File	Coverage	Lines
crates/lambda-rs-platform/src/wgpu/pipeline.rs	83.65%	394/471
crates/lambda-rs/src/render/pipeline.rs	90.86%	497/547

PR Files Coverage: 87.52% (891/1018 lines)

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Generated by cargo-llvm-cov · Latest main coverage

_{Last updated: 2026-02-12 00:20:52 UTC · Commit: 3bc0187}

[Copilot]

Pull request overview

This PR changes the default render pipeline behavior to be opaque (no blending) for better performance, and adds an explicit opt-in blending API so transparency is enabled only when needed.

Changes:

• Introduces BlendMode + with_blend(...) on both engine-level and platform-level RenderPipelineBuilder, defaulting to None (opaque/replace).
• Threads the blend configuration through the engine builder into the platform wgpu pipeline creation (replacing the prior hardcoded ALPHA_BLENDING).
• Updates the reflective-room demo/tutorial and rendering docs to explicitly enable alpha blending where translucency is intended, and adds unit tests in the platform layer.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
docs/tutorials/rendering/techniques/reflective-room.md	Updates tutorial code to explicitly opt into alpha blending for the translucent floor pass.
docs/rendering.md	Documents the new default (opaque) and shows how to opt into blending.
demos/render/src/bin/reflective_room.rs	Updates demo pipeline setup to explicitly enable alpha blending for the floor visual pass.
crates/lambda-rs/src/render/pipeline.rs	Adds engine-level BlendMode and threads it into the platform pipeline builder via with_blend.
crates/lambda-rs-platform/src/wgpu/pipeline.rs	Adds platform-level BlendMode (including Custom) and uses it to set ColorTargetState.blend, plus adds unit tests.

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