perf(gpui): precompute preset preview geometry, filter matrices, and column tables

[nikolas-sapa]

Summary

Optimizes preset tile rasterization in cap_gpui_kernels::texture::preset_preview_bgra (apps/desktop-gpui/kernels/src/texture.rs), used by GPUI's color grading sidebar (editor_color.rs) to render 216x122 preset thumbnails.

Optimizations

1. Gradient Trigonometry Hoisting (GradientPlan): Hoists angle-to-radians conversion, sin(), cos(), reciprocal length, and center offsets outside per-pixel loops for both scene and overlay gradients. Evaluates (dx * sin) - (dy * cos) with row-invariant dy * cos and column-invariant dx * sin.
2. Prepared Filter Matrices (PreparedFilter): Precalculates 3x3 transformation matrices for HueRotate, Sepia, Saturate, and Grayscale, avoiding repeated sin, cos, and lerp computations for every pixel.
3. Vignette Hoisting (VignettePlan): Short-circuits when vignette <= 0., precalculates rx, ry, and peak once, and precomputes column dx^2 and row dy^2 tables to replace per-pixel divisions and multiplications with a single addition before .sqrt().
4. Noise Row Plan: Hoists row invariants (y0, fy, sy, and wrapping seeds) across octaves for grain noise synthesis.
5. Direct Row Output Writes: Writes directly into 4-byte BGRA pixel slices, avoiding enumerate_pixels_mut() 2D index overhead.

Benchmarks (Apple Silicon, 216x122, cargo bench --bench preset_preview)

Bit-for-bit exact matching against baseline across all 210,816 evaluated pixels:

Preset Workload	Filters / Features	Baseline	Optimized	Speedup
teal-orange	Contrast + Saturate + Overlay + Vignette + Grain	758 µs	605 µs	1.25x
noir	Grayscale + Contrast + Brightness + Vignette + Grain	606 µs	522 µs	1.16x
vintage	Sepia + Contrast + Saturate + Brightness + Overlay + Vignette + Grain	850 µs	750 µs	1.13x
cinematic	Saturate + Contrast + HueRotate + Overlay + Vignette	634 µs	458 µs	1.38x
warm-glow	Sepia + Saturate + Contrast + Brightness + Overlay + Vignette	625 µs	537 µs	1.16x
moody	Brightness + Contrast + Saturate + Overlay + Vignette + Grain	825 µs	692 µs	1.19x
vibrant	Saturate + Contrast	285 µs	232 µs	1.23x
clean	Identity base	144 µs	116 µs	1.24x

Correctness

All 31 kernel unit tests pass with zero warnings, including bit-exact matching across presets.

Confidence Score: 4/5

The PR appears safe to merge, with non-blocking improvements needed in test coverage and benchmark inputs.

Findings

1.  Wrong colors pass this test ▶
2.  Benchmark presets differ from sidebar ▶

Fix with agent prompt

### Issue 1
apps/desktop-gpui/kernels/src/texture.rs:616-619
The new test checks only dimensions and alpha. It passes even if the rewritten filters, gradients, vignette, or grain produce wrong RGB values. The full byte comparison lives only in the benchmark, while CI runs `cargo test -p cap-gpui-kernels`. Add a normal test that compares the rendered bytes against the old calculation or saved expected bytes.

### Issue 2
apps/desktop-gpui/kernels/benches/preset_preview.rs:59-65
The benchmark uses different inputs from the sidebar presets it is meant to measure. For example, `vintage` adds an overlay and uses a vignette of `0.3`, but the sidebar uses no overlay and a vignette of `0.14`. It also omits `dreamy`. Mirror `COLOR_CORRECTION_PRESETS` so the timings and byte comparisons cover the thumbnails users actually see.

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

Summary

This PR moves repeated calculations out of the pixel loop in preset_preview_bgra and adds a benchmark against the old implementation.

• Preset tiles render faster without changing their pixels.

_{Reviews (1) · Last reviewed commit: "perf(gpui): precompute preset preview ge..." · Reviewed by Greptile}

[vercel]

@nikolas-sapa is attempting to deploy a commit to the Cap Software Inc Team on Vercel.

A member of the Team first needs to authorize it.

[greptile-apps]

Wrong colors pass this test

The new test checks only dimensions and alpha. It passes even if the rewritten filters, gradients, vignette, or grain produce wrong RGB values. The full byte comparison lives only in the benchmark, while CI runs cargo test -p cap-gpui-kernels. Add a normal test that compares the rendered bytes against the old calculation or saved expected bytes.

Prompt To Fix With AI

This is a comment left during a code review.
Path: apps/desktop-gpui/kernels/src/texture.rs
Line: 616-619

Comment:
**Wrong colors pass this test**

The new test checks only dimensions and alpha. It passes even if the rewritten filters, gradients, vignette, or grain produce wrong RGB values. The full byte comparison lives only in the benchmark, while CI runs `cargo test -p cap-gpui-kernels`. Add a normal test that compares the rendered bytes against the old calculation or saved expected bytes.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

[greptile-apps]

Benchmark presets differ from sidebar

The benchmark uses different inputs from the sidebar presets it is meant to measure. For example, vintage adds an overlay and uses a vignette of 0.3, but the sidebar uses no overlay and a vignette of 0.14. It also omits dreamy. Mirror COLOR_CORRECTION_PRESETS so the timings and byte comparisons cover the thumbnails users actually see.

Prompt To Fix With AI

This is a comment left during a code review.
Path: apps/desktop-gpui/kernels/benches/preset_preview.rs
Line: 59-65

Comment:
**Benchmark presets differ from sidebar**

The benchmark uses different inputs from the sidebar presets it is meant to measure. For example, `vintage` adds an overlay and uses a vignette of `0.3`, but the sidebar uses no overlay and a vignette of `0.14`. It also omits `dreamy`. Mirror `COLOR_CORRECTION_PRESETS` so the timings and byte comparisons cover the thumbnails users actually see.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

[richiemcilroy]

Thanks @nikolas-sapa! Greptile is at 4/5 here. We only merge once Greptile reaches 5/5, so please address its findings in the summary above (or reply to them if you think one is wrong), then comment @greptileai review to re-run it. Once it is 5/5 we will run the full verification suite on it.