perf(preview): shrink with Imaginary before rotating

[martin-rueegg]

• Depends on perf(preview): only autorotate with Imaginary when needed #65148, which has been merged since opening the PR on its head 6b3aec3. The PR is now rebased onto master.

Summary

imaginary-powered preview generation is applying multiple operations on the full-sized image, where it can be handled more efficiently if - in a first step - the resize to the target scale is performed and other operations like rotation and cropping followed from there. Saves memory pressure and increases speed.

What changes

• Images: fit into a square box of the larger side, without rotating and keeping EXIF →
  autorotate → fit to the requested box, stripping metadata.
• perf(preview): only autorotate with Imaginary when needed #65148's EXIF check in PHP goes again, as it is no longer needed.
• The HEIC case goes.
• PDF, SVG and Illustrator: unchanged.
• Crop: unchanged, autorotate → smartcrop.

Technical details

• Why autorotate first is expensive: bimg's autorotate-only path decodes and re-encodes the
  whole image whatever the orientation (resizer.go:38-45). The 2022 premise, "almost instant
  without rotation", does not hold.
• Why not a single fit:
  • Imaginary's Fit swaps the box for orientation > 4 (image.go:134-147, since 2018), and bimg
    rotates as well.
  • For orientations 5–8 the result is an upright image in a box of the wrong shape.
  • This is very likely what Fix imaginary with rotated exif images #31829 worked around in 2022.
• Why the square box first: with a box that is not square, fitting before rotating fits the
  stored image, not the shown one. Measured: 2048 × 1536 where 2731 × 2048 fits.

Measurements

AIO Imaginary sha256:0e32abf8…, 2500 MB memory limit, box 4096², quality 80.

• Peak anonymous memory:

  Input	today	this PR
  JPEG 200 MP, orientation 1	715 MB	193 MB
  JPEG 200 MP, orientations 2–8	~710 MB	~200 MB
  LZW TIFF 200 MP	~2.0 GB, at the limit	~1.0 GB
  uncompressed TIFF 761 MB	killed, out of memory	passes

  • Times: JPEG about 2.8 s instead of 5–8 s.
  • PNG 100 MP: 24 s instead of 52 s.

• Correctness: every result upright, sized as expected, and without EXIF:

  • JPEG at all eight orientations, at boxes 4096², 4096 × 2048, 2048 × 4096 and 256²;
  • TIFF at orientations 1 and 6, and PNG;
  • 15 iPhone HEICs at orientations 1, 3, 6, 8 and none, checked against heif-convert.

Tests

• ImaginaryTest covers six image types, a non-square box, WebP output and the three convert types.
• manual test:
  • checked HEIC, JPEG, and TIFF
  • EXIF orientation 1,2,3,4,5,6,7,8
  • generated preview all with correct orientation, no metadata

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was mostly generated using AI: the measurements, code and test cases. Manual test verified on a local ddev instance with Nextcloud 35.0.1 - as required.

[martin-rueegg]

Thanks, @CarlSchwan, for the review!

PR has been rebased and force-pushed.

Tests: ImaginaryTest passes on the rebased branch (11 tests, 47 assertions).