perf(desktop-gpui): IDCT-scale JPEG library thumbnails

[kyzer1023]

Summary

Library card thumbnails (thumbnail_rgba in apps/desktop-gpui/src/library.rs) decoded JPEGs at full resolution and then resampled to the 392×224 cover size. JPEGs now use jpeg_decoder::Decoder::scale to IDCT-scale toward the cover size first, then resize to the exact size. A 1920×1080 or 3840×2160 frame decodes at 480×270.

The sidebar's existing IDCT decode moved into a shared library/jpeg_thumbnail.rs, so decode_jpeg_thumbnail is the only IDCT decoder. The sidebar still asks for a square max; library cards pass the cover size. PNG is unchanged, and CMYK / 16-bit gray JPEGs still fall back to a full decode.

The pixels differ slightly from the old full-resolution resample, so the cache prefix now ends in -idct. Old cache files are left on disk.

One commit, 4 files, +153/-30. Based on main at 85219015.

Correctness

jpeg_thumbnail tests pass (square downsample, historical cover sizes, 1080p to 480×270, CMYK returns None), run through a standalone crate that path-includes the module. The full desktop-gpui app and the cache-prefix tests in library.rs were not built here because gpui is patched to a local zed-cap checkout.

Benchmarks

Release, Linux VM, median of 7 samples after one warmup. Before is the old image full decode plus cover resample.

Source	Before	After	Speedup	Mean channel diff
Synthetic 1920×1080	7.12 ms	5.00 ms	1.42×	4.73
Synthetic 3840×2160	23.08 ms	12.73 ms	1.81×	9.96
sequoia-dark.jpg 4000×2500	36.07 ms	20.05 ms	1.80×	1.40
tahoe-dark.jpg 3840×2160	68.03 ms	54.30 ms	1.25×	1.58

A second run put synthetic 1080p at 1.67× and 4K at 1.87×.

Overlap

Does not touch clip thumbnails (#2707) or recording recovery in library.rs (#2630).

Confidence Score: 4/5

No blocking behavior defect was found, but the explicit comments policy must be satisfied before merging.

Findings

1.  CMYK test misses its branch ▶
2.  Unsupported JPEGs decode twice ▶
3.  Comment breaks the guide ▶

Fix with agent prompt

### Issue 1
apps/desktop-gpui/src/jpeg_thumbnail.rs:101-105
`cmyk_frame_returns_none` uses a truncated JPEG header with no scan data. The test passes when decoding fails, even if CMYK rejection is removed. Use a complete CMYK JPEG so the test guards the unsupported-format branch.

### Issue 2
apps/desktop-gpui/src/jpeg_thumbnail.rs:17-18
The shared helper decodes pixels before rejecting unsupported formats. Library thumbnails now pay for that discarded decode and then decode again through the fallback. Check `info.pixel_format` before calling `decoder.decode()` so CMYK and 16-bit gray files go straight to the fallback.

```suggestion
    if !matches!(
        info.pixel_format,
        jpeg_decoder::PixelFormat::RGB24 | jpeg_decoder::PixelFormat::L8
    ) {
        return None;
    }
    let pixels = decoder.decode().ok()?;
    let decoded = match info.pixel_format {
```

### Issue 3
apps/desktop-gpui/src/jpeg_thumbnail.rs:34
The new `cover_target` comment only describes what the function does. The repository guide bans comments that narrate code and requires non-obvious context instead. Remove this comment or express the guarantee through a test. This repository requirement must be satisfied before merging.

```suggestion

```

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

Library JPEG thumbnails now decode at a smaller size before the final resize.

• Library cards decode JPEG thumbnails at a smaller size first.

_{Reviews (1) · Last reviewed commit: "perf(desktop-gpui): IDCT-scale JPEG libr..." · Reviewed by Greptile}

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[greptile-apps]

CMYK test misses its branch

cmyk_frame_returns_none uses a truncated JPEG header with no scan data. The test passes when decoding fails, even if CMYK rejection is removed. Use a complete CMYK JPEG so the test guards the unsupported-format branch.

Prompt To Fix With AI

This is a comment left during a code review.
Path: apps/desktop-gpui/src/jpeg_thumbnail.rs
Line: 101-105

Comment:
**CMYK test misses its branch**

`cmyk_frame_returns_none` uses a truncated JPEG header with no scan data. The test passes when decoding fails, even if CMYK rejection is removed. Use a complete CMYK JPEG so the test guards the unsupported-format branch.

---

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

[greptile-apps]

Unsupported JPEGs decode twice

The shared helper decodes pixels before rejecting unsupported formats. Library thumbnails now pay for that discarded decode and then decode again through the fallback. Check info.pixel_format before calling decoder.decode() so CMYK and 16-bit gray files go straight to the fallback.

Suggested change

	let pixels = decoder.decode().ok()?;
	let decoded = match info.pixel_format {
	if !matches!(
	info.pixel_format,
	jpeg_decoder::PixelFormat::RGB24 | jpeg_decoder::PixelFormat::L8
	) {
	return None;
	}
	let pixels = decoder.decode().ok()?;
	let decoded = match info.pixel_format {

Prompt To Fix With AI

This is a comment left during a code review.
Path: apps/desktop-gpui/src/jpeg_thumbnail.rs
Line: 17-18

Comment:
**Unsupported JPEGs decode twice**

The shared helper decodes pixels before rejecting unsupported formats. Library thumbnails now pay for that discarded decode and then decode again through the fallback. Check `info.pixel_format` before calling `decoder.decode()` so CMYK and 16-bit gray files go straight to the fallback.

```suggestion
    if !matches!(
        info.pixel_format,
        jpeg_decoder::PixelFormat::RGB24 | jpeg_decoder::PixelFormat::L8
    ) {
        return None;
    }
    let pixels = decoder.decode().ok()?;
    let decoded = match info.pixel_format {
```

---

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

[greptile-apps]

Comment breaks the guide

The new cover_target comment only describes what the function does. The repository guide bans comments that narrate code and requires non-obvious context instead. Remove this comment or express the guarantee through a test. This repository requirement must be satisfied before merging.

Suggested change

/// Cover size for a card. Never upscales.

Context Used: AGENTS.md (source)

Prompt To Fix With AI

This is a comment left during a code review.
Path: apps/desktop-gpui/src/jpeg_thumbnail.rs
Line: 34

Comment:
**Comment breaks the guide**

The new `cover_target` comment only describes what the function does. The repository guide bans comments that narrate code and requires non-obvious context instead. Remove this comment or express the guarantee through a test. This repository requirement must be satisfied before merging.

```suggestion

```

**Context Used:** AGENTS.md ([source](https://github.com/capsoftware/cap/blob/main/AGENTS.md))

---

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!