ImageMagic resize over background image

Question

I need to resize images in PaperClip which uses ImageMagick over a background image. I made custom PaperClip processors (https://gist.github.com/xxx/1051938 and https://github.com/thoughtbot/paperclip#custom-attachment-processors) and they are working but I dont know which commands to use to compose the images.

This is process I would like:

1. User uploads image X

2. Processor runs and resizes the image X to given dimensions with tranparent fill - Y

3. Next the background image is cropped to given dimensions - Z

4. Y is overlayed over the Z centered.

Thanks!

Answer 1

Well after few days of struggle the answer is here:

For IM command line version step by step (a.png is input, tiled.png is source of background)

convert a.png -thumbnail 100x100 -background none -gravity center -extent 100x100 resized.png
convert tiled.png -crop 100x100+0+0 +repage cropped.png
convert cropped.png out.png -composite result.png

For IM command line combined code

convert tiled.png -crop 1000x1000+0+0 +repage a.png -thumbnail 1000x1000 -background none -gravity center -extent 1000x1000 -composite result.png

For use in image magick copy the default Thumbnail processor and make changes to the make method (or just create your own processor)

def make
  src = @file
  dst = Tempfile.new([@basename, @format ? ".#{@format}" : ''])
  dst.binmode

  begin
    tgeom = @target_geometry.width.to_s + 'x' + @target_geometry.height.to_s
    tiled_path = Rails.root.join('public','system','tiled.png')
    parameters = []
    parameters << tiled_path
    parameters << '-crop'
    parameters << tgeom + '+0+0 +repage'
    parameters << ':source'
    parameters << '-thumbnail ' + tgeom
    parameters << '-background none'
    parameters << '-gravity center'
    parameters << '-extent ' + tgeom
    parameters << '-composite'
    parameters << ':dest'
    parameters = parameters.flatten.compact.join(' ').strip.squeeze(' ')

    success = convert(parameters, :source => "#{File.expand_path(src.path)}#{'[0]' unless animated?}", :dest => File.expand_path(dst.path))
  rescue Cocaine::ExitStatusError => e
    raise Paperclip::Error, "There was an error processing the thumbnail for #{@basename}" if @whiny
  rescue Cocaine::CommandNotFoundError => e
    raise Paperclip::Errors::CommandNotFoundError.new("Could not run the `convert` command. Please install ImageMagick.")
  end

  dst
end