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Thumbnail gif file with `crop: :centre` always creates 1 scene gif

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
42/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
ruby
Domain
backend

Research direction

Start by running the provided Ruby reproduction with the linked GIF and compare output with and without crop: :centre. Trace the thumbnail and GIF-writing path used by Vips::Image.thumbnail; done means the cropped output remains animated and contains all scenes.

Written by the indexing model from the issue text.

Description

blocked-upstream bug

Describe the bug

Vips::Image.thumbnail("filename.gif[n=-1]", 200, height: 200, crop: :centre) with animated GIF file saves static file with 1 scene only.

Without crop: :centre it saves animated file correctly

I used file https://i.pinimg.com/originals/96/50/f0/9650f01cd3334952b075f8cf55bedd60.gif for testing

To Reproduce
Steps to reproduce the behavior:
Run ruby script

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "ruby-vips"
  gem 'down'
end

GLib::logger.level = Logger::DEBUG

tempfile = Down.download("https://i.pinimg.com/originals/96/50/f0/9650f01cd3334952b075f8cf55bedd60.gif")

resized_image = Vips::Image.thumbnail("#{tempfile.path}[n=-1]", 400, height: 400, crop: :centre)
resized_image.write_to_file "/tmp/rabbit-with-crop.gif"
`browse /tmp/rabbit-with-crop.gif`

Expected behavior
File must be animated with all scenes cropped

Additional context
I have libvips 8.13rc1 compiled with cgif

Dominant language
Ruby
Stars
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Forks
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PR merge metrics
No merged PRs in 30d

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