# gpui: Add dithering to linear gradient shader \(\#51211\) · gitcafe/zed

[View on GitCafe](https://git.cafe/gitcafe/zed/commit/a46858ac21ec43b1af94d8b2b9a5a8fde4ac4f01)

Repository: [gitcafe/zed](https://git.cafe/gitcafe/zed)

Visibility: public

Requested revision: a46858ac21ec43b1af94d8b2b9a5a8fde4ac4f01

Requested commit: a46858ac21ec43b1af94d8b2b9a5a8fde4ac4f01

Commit: a46858ac21ec43b1af94d8b2b9a5a8fde4ac4f01

Tree: 43b483c8c3e16e75bf3bd79c476c71b178c66876

Author: iam-liam

Committer: GitHub

## Message

```
gpui: Add dithering to linear gradient shader (#51211)

Linear gradients in dark color ranges (5-15% lightness) show visible
banding due to 8-bit quantization — only ~7 distinct values exist in
that range, producing hard steps instead of smooth transitions. This
affects every dark theme in Zed.

## What this does

Adds triangular-distributed dithering after gradient interpolation in
both the Metal and HLSL fragment shaders. The noise breaks up
quantization steps at the sub-pixel level, producing perceptually smooth
gradients.

## How it works

Two hash-based pseudo-random values (seeded from fragment position x
golden ratio) are summed to produce a triangular probability
distribution. This is added to the RGB channels at +/-1/255 amplitude.

- **Triangular PDF** — mean-zero, so no brightness shift across the
gradient
- **+/-1/255 amplitude** — below perceptual threshold, invisible on
bright gradients where 8-bit precision is already sufficient
- **Deterministic per-pixel** — seeded from position, no temporal
flickering
- **Zero-cost** — a couple of `fract`/`sin` per fragment, negligible vs.
the existing gradient math

### Channel-specific amplitudes

  | Channel | Amplitude | Rationale |

|---------|-----------|----------------------------------------------------------------------------------------------------|
| RGB | ±2/255 | Breaks dark-on-dark banding where adjacent 8-bit values
are perceptually close |
| Alpha | ±3/255 | Alpha gradients over dark backgrounds need stronger
noise — α × dark color = tiny composited steps |

The higher alpha amplitude is necessary because when a semi-transparent
gradient (e.g., 0.4 → 0.0 alpha) composites over a dark background, the
effective visible difference per quantization step is smaller than the
RGB case. ±3/255 is still well below the perceptual threshold on
bright/opaque elements.

## Scope

Two files changed, purely additive:

| File | Change |
|------|--------|
| `crates/gpui_macos/src/shaders.metal` | 13 lines after `mix()` in
`fill_color()` |
| `crates/gpui_windows/src/shaders.hlsl` | 13 lines after `lerp()` in
`gradient_color()` |

No changes to Rust code, no API changes, no new dependencies.

## Screenshots

<img width="1886" height="1003" alt="gradient_dithering_before"
src="https://github.com/user-attachments/assets/f75ae93b-b142-4d0e-9b61-e08f30fe1758"
/>

_Before_

<img width="1902" height="1052" alt="gradient_dithering_after"
src="https://github.com/user-attachments/assets/7aee9a36-f578-4e08-a846-44d092bcf043"
/>

_After_

## Test plan

This is a shader-level fix; no Rust test harness exists for visual
output. Manual testing is appropriate here. Visual regression tests
cover UI layout, not sub-pixel rendering quality.

**Manual (macOS):**

- [x] Dark gradients (5-13% lightness range) — banding eliminated
- [x] Bright gradients — no visible difference (dither amplitude below
precision threshold)
- [x] Oklab and sRGB color spaces — both paths dithered
- [x] Solid colours, pattern fills, checkerboard — unaffected (dither
only applies to LinearGradient case)
- [x] Alpha gradients (semi-transparent over dark bg) — banding
eliminated with alpha dithering
- [x] Path gradients (paint_path) — same fill_colour() function,
dithering applies

**Windows:** HLSL change is identical logic with HLSL built-ins
(`frac`/`lerp` vs `fract`/`mix`) — not tested locally.

Release Notes:

- Improved linear gradient rendering by adding dithering to eliminate
visible banding in dark color ranges

Liam
```

## Parents

- [1b9f3833f00f34b424b3e84ce1d95bf765dd7f67](https://git.cafe/gitcafe/zed/commit/1b9f3833f00f34b424b3e84ce1d95bf765dd7f67?format=markdown)

[Source at this commit](https://git.cafe/gitcafe/zed/tree/a46858ac21ec43b1af94d8b2b9a5a8fde4ac4f01?format=markdown)