Add JPEG XL converter example

example/convert_to_jxl.rb

@@ -0,0 +1,70 @@
+#!/usr/bin/env ruby
+# Convert an image to JPEG XL using ruby-vips when its libvips was built
+# with libjxl, falling back to cjxl (the only backend that can transcode
+# JPEG losslessly, bit-exact), ffmpeg/libjxl, then ImageMagick.
+# The result is only reported as success if it starts with a JPEG XL
+# codestream (FF 0A) or container signature.
+#
+#   ruby convert_to_jxl.rb [--lossless] [--distance 1.0] [--effort 7] in.png [out.jxl]
+require "open3"
+
+CODESTREAM = "\xFF\x0A".b
+CONTAINER = "\x00\x00\x00\x0CJXL \x0D\x0A\x87\x0A".b
+
+def jpeg_xl?(path)
+  return false unless File.file?(path)
+
+  head = File.binread(path, 12)
+  head.start_with?(CODESTREAM) || head.start_with?(CONTAINER)
+rescue SystemCallError
+  false
+end
+
+def convert(src, dst, lossless:, distance:, effort:)
+  begin
+    require "vips"
+    quality = lossless ? 100 : [[100 - (distance * 10).round, 10].max, 100].min
+    Vips::Image.new_from_file(src, access: :sequential)
+               .jxlsave(dst, "lossless" => lossless, "Q" => quality, "effort" => effort)
+    return dst if jpeg_xl?(dst)
+  rescue LoadError, StandardError
+    File.delete(dst) if File.exist?(dst)
+  end
+
+  cjxl = ["cjxl", src, dst, "--effort=#{effort}",
+          lossless ? "--lossless=1" : "--distance=#{distance}"]
+  ffmpeg = ["ffmpeg", "-y", "-i", src, "-c:v", "libjxl", "-effort", effort.to_s,
+            *(lossless ? ["-lossless", "1"] : ["-distance", distance.to_s]), dst]
+  [cjxl, ffmpeg, ["magick", src, dst]].each do |cmd|
+    ok = begin
+      _o, _e, s = Open3.capture3(*cmd)
+      s.success?
+    rescue Errno::ENOENT
+      false
+    end
+    return dst if ok && jpeg_xl?(dst)
+
+    File.delete(dst) if File.exist?(dst)
+  end
+  raise "no working JPEG XL backend (install libvips with libjxl, cjxl, " \
+        "ffmpeg --enable-libjxl, or ImageMagick with jxl delegate)"
+end
+
+if __FILE__ == $PROGRAM_NAME
+  lossless, distance, effort = false, 1.0, 7
+  positional = []
+  args = ARGV.dup
+  until args.empty?
+    case (a = args.shift)
+    when "--lossless" then lossless = true
+    when "--distance" then distance = Float(args.shift)
+    when "--effort" then effort = Integer(args.shift)
+    else positional << a
+    end
+  end
+  abort "usage: convert_to_jxl.rb [--lossless] [--distance 1.0] [--effort 7] in [out.jxl]" if positional.empty? || positional.size > 2
+
+  src = positional[0]
+  dst = positional[1] || src.sub(/\.[^.]+\z/, ".jxl")
+  puts "#{src} -> #{convert(src, dst, lossless: lossless, distance: distance, effort: effort)}"
+end