Rails 5 Carrierwave 和 MiniMagick 在调整缩略图大小时打破了 gif

Question

我正在使用carrierwave 和mini_magick 来调整我的图像大小，生成缩略图。它适用于 jpg 和 png 等静止图像，但当我尝试使用 gif 时，它会扭曲它。您可以在下面看到原始与调整大小的示例。知道如何让它生成 gif thumb 而不会破坏它吗？

原始 GIF :) https://imgur.com/oi1f8XT

生成的缩略图 GIF :( https://imgur.com/a/PwAXv

---

ps 为什么缩略图的尺寸比原来的大？原图为 800*600px，缩略图为 400*300px。无论如何，生成缩略图的重点是文件大小更小。

谢谢！

---

image_uploader.rb

class ImageUploader < CarrierWave::Uploader::Base

  # Include RMagick or MiniMagick support:
  # include CarrierWave::RMagick
  include CarrierWave::MiniMagick

  # Choose what kind of storage to use for this uploader:
  storage :file
  # storage :fog

  # Override the directory where uploaded files will be stored.
  # This is a sensible default for uploaders that are meant to be mounted:
  def store_dir
    "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end

  # Provide a default URL as a default if there hasn't been a file uploaded:
  # def default_url(*args)
  #   # For Rails 3.1+ asset pipeline compatibility:
  #   # ActionController::Base.helpers.asset_path("fallback/" + [version_name, "default.png"].compact.join('_'))
  #
  #   "/images/fallback/" + [version_name, "default.png"].compact.join('_')
  # end
  def default_url(*args)
    ActionController::Base.helpers.asset_path("fallback/" + [version_name, "post.jpg"].compact.join('_'))
  end

  # Process files as they are uploaded:
  # process scale: [800, 600]
  # process :resize_to_fit => [800, 600]
  #
  # def scale(width, height)
  #   # do something
  # end

  # Create different versions of your uploaded files:
  # version :thumb do
  #   process resize_to_fit: [50, 50]
  # end
  version :thumb do
    process resize_to_fit: [400, 300]
  end

  # Add a white list of extensions which are allowed to be uploaded.
  # For images you might use something like this:
  def extension_whitelist
    %w(jpg jpeg gif png)
  end

  # Override the filename of the uploaded files:
  # Avoid using model.id or version_name here, see uploader/store.rb for details.
  # def filename
  #   "something.jpg" if original_filename
  # end

end

Answer 1

您尝试调整大小的 gif 已优化，因此在第一帧之后，每个后续帧仅包含需要更改的像素。当 gif 动画时，帧被堆叠在一起创建完整的动画。

此外，每个帧仅大到足以包含需要更改的像素。所以，每一帧的大小是不同的。当您调整 gif 大小时，Imagemagick 会将每帧的大小调整为 400 x 300 像素，而不管其原始大小如何，这会导致您看到的失真。

您可以应用名为 coalesce 的 Imagemagick 命令（在 Ruby 中使用 Minimagick 绑定）来解决此问题。它对原始图像进行去优化，使每一帧都是画布的完整大小。

调用coalesce 会使文件变大，因此在完成调整大小后有必要重新优化 gif。

这是一个例子：

version :thumb do
  process my_resize: [400, 300]
end

def my_resize(width, height)
  if @file.content_type == "image/gif"
    gif_safe_transform! do |image|
      image.resize "#{width}x#{height}" # Perform any transformations here.
    end
  else
    # Process other filetypes if necessary.
  end
end

def gif_safe_transform!
  MiniMagick::Tool::Convert.new do |image|
    image << @file.path
    image.coalesce # Remove optimizations so each layer shows the full image.

    yield image

    image.layers "Optimize" # Re-optimize the image.
    image << @file.path
  end
end

我用一些例子写了更深入的解释here