feat: add Diff.compare for standalone image comparison

[pftg]

Summary

Browser-independent API for comparing any two images:

result = Capybara::Screenshot::Diff.compare("baseline.png", "current.png")
result.quick_equal?  # byte-identical?
result.different?    # visually different?

Supports all options: driver, tolerance, perceptual_threshold, color_distance_limit.

Based on real usage from corsis/pet which uses the gem for PDF visual regression.

Test plan

• 5 tests: return type, identical, different, driver option, tolerance
• Full suite with CI=true (196 runs, 0 failures)
• Pre-PR: brutal review (0.5, clean) + codex review (arg order fix applied)

🤖 Generated with Claude Code

Summary by Sourcery

Add a public Diff.compare API for standalone image comparison without Capybara or a browser.

New Features:

• Introduce Diff.compare helper for comparing any two image files using existing diff logic and options.

Enhancements:

• Document standalone image comparison usage and options in the README.

Tests:

• Add unit tests covering Diff.compare return type, identical and different images, and option handling for driver and tolerance.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 7b94868b-9ad6-40cc-b25e-c183e74baf13

📥 Commits

Reviewing files that changed from the base of the PR and between 076cbac and a8716a3.

📒 Files selected for processing (3)

• README.md
• lib/capybara_screenshot_diff.rb
• test/unit/compare_api_test.rb

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/compare-api

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[sourcery-ai]

Reviewer's Guide

Adds a new public Diff.compare API for standalone, browser-independent image comparison and documents it, with tests covering behavior and options support.

Sequence diagram for the new Diff.compare standalone image comparison API

sequenceDiagram
  actor Developer
  participant Diff
  participant ImageCompare

  Developer->>Diff: compare(base_image_path, new_image_path, options)
  activate Diff
  Diff->>Diff: default_options()
  Diff-->>Diff: base_options
  Diff->>ImageCompare: new(new_image_path, base_image_path, base_options + options)
  deactivate Diff
  activate ImageCompare
  ImageCompare-->>Developer: image_compare_result
  deactivate ImageCompare

  Developer->>image_compare_result: quick_equal()
  Developer->>image_compare_result: different()

Loading

Updated class diagram for Diff and ImageCompare standalone comparison

classDiagram
  class Diff {
    +compare(base_image_path, new_image_path, options)
    +default_options()
  }

  class ImageCompare {
    +ImageCompare(new_image_path, base_image_path, options)
    +quick_equal()
    +different()
  }

  Diff ..> ImageCompare : creates

Loading

File-Level Changes

Change	Details	Files
Expose a standalone image comparison API via Diff.compare that returns ImageCompare and passes through configuration options.	Add Diff.compare class method that constructs ImageCompare with new and base image paths Merge global default_options with per-call options to configure the comparison Preserve argument order for compatibility with existing ImageCompare usage	lib/capybara_screenshot_diff.rb
Document the new standalone image comparison API and its options in the README.	Add README section describing Diff.compare usage without Capybara/browser Show basic equality/difference checks via quick_equal? and different? Provide example including driver, tolerance, and perceptual color comparison options	README.md
Add unit tests to validate the new Diff.compare API behavior and option handling.	Test that Diff.compare returns an ImageCompare instance Test identical and different images using quick_equal? and different? results Test handling of driver and tolerance options, skipping VIPS-specific tests when unavailable	test/unit/compare_api_test.rb

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[sourcery-ai]

Hey - I've left some high level feedback:

• The Diff.compare signature uses (base_image_path, new_image_path) but then passes (new_image_path, base_image_path) into ImageCompare.new, which works but is easy to misread; consider either aligning the parameter order with ImageCompare or renaming the parameters (baseline_path, current_path) and adding a short code comment to make the reversal explicit.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `Diff.compare` signature uses `(base_image_path, new_image_path)` but then passes `(new_image_path, base_image_path)` into `ImageCompare.new`, which works but is easy to misread; consider either aligning the parameter order with `ImageCompare` or renaming the parameters (`baseline_path`, `current_path`) and adding a short code comment to make the reversal explicit.

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