# Image Compression

## Objective

Shrink uploaded images before they slow down every page that shows them.

Automatic resizing and re-encoding of uploaded images, with orientation, transparency, and metadata handled correctly.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Decide the maximum stored dimensions and target quality once, and apply them to every upload path in the app.
2. Read orientation metadata and rotate the pixels before resizing, then drop the orientation tag. Resizing first bakes in a sideways image.
3. Choose the output format by content: keep transparency where the source has it, and do not convert a transparent PNG to JPEG.
4. Compress from the original bytes every time. Never re-compress an already-compressed derivative — each pass loses quality permanently.
5. Strip EXIF by default, including GPS coordinates. A holiday photo uploaded as an avatar should not publish someone's home address.
6. Do NOT compress on the client only. A client-side pass is a bandwidth optimisation; the server must still enforce the limits because the client can be bypassed.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- Animated GIFs and animated WebP must keep their frames or be rejected explicitly — a silent first-frame-only conversion looks like a bug.
- A file claiming to be an image must be validated by its actual content, not its extension or declared type.
- Enormous dimensions can exhaust memory during decode even when the file is small. Cap decoded pixel count, not just file size.
- An image already smaller than the target must be left alone rather than re-encoded larger.
- Colour profiles matter: dropping an embedded profile shifts colours visibly on wide-gamut screens.
- Some uploads must keep their originals — proof-of-purchase photos, design source files. Define which, and store the original alongside.
- A failed compression must not lose the upload; fall back to storing the original and log it.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Uploaded images are stored within the defined dimension and size limits.
- [ ] Rotated photos display upright without client-side correction.
- [ ] Transparency is preserved where the source had it.
- [ ] Re-uploading a processed image does not degrade it further.
- [ ] GPS and other EXIF metadata is removed from images served publicly.
- [ ] Server-side limits hold even when the client-side pass is skipped.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.