ruby-rmagick new PKGBUILD. Patch from https://github.com/bricef/rmagick/commit/689271fbb439dbf735f120811ac3b2dc79ba95a2 It seems to work for simple cases (at least we can compile and use rmagick), no idea how badly HDR images handling is broken.

hdri.patch

From 689271fbb439dbf735f120811ac3b2dc79ba95a2 Mon Sep 17 00:00:00 2001
From: Brice Fernandes <[email protected]>
Date: Mon, 19 Aug 2013 03:02:54 +0100
Subject: [PATCH] Attempt to fix bug #18 in original repo. (to enable Ruby
 bundling on Arch Linux)

---
 ext/RMagick/extconf.rb | 8 --------
 ext/RMagick/rmimage.c  | 2 +-
 2 files changed, 1 insertion(+), 9 deletions(-)

diff --git a/ext/RMagick/extconf.rb b/ext/RMagick/extconf.rb
index d3228dd..b6a1f04 100644
--- a/ext/RMagick/extconf.rb
+++ b/ext/RMagick/extconf.rb
@@ -152,14 +152,6 @@ def check_partial_imagemagick_versions()
 
   $magick_version = `Magick-config --version`.chomp
 
-  # Ensure ImageMagick is not configured for HDRI
-  unless checking_for("HDRI disabled version of ImageMagick") do
-    not (`Magick-config --version`["HDRI"])
-  end
-    exit_failure "\nCan't install RMagick #{RMAGICK_VERS}."+
-           "\nRMagick does not work when ImageMagick is configured for High Dynamic Range Images."+
-           "\nDon't use the --enable-hdri option when configuring ImageMagick.\n"
-  end
 
   # Save flags
   $CFLAGS     = ENV["CFLAGS"].to_s   + " " + `Magick-config --cflags`.chomp
diff --git a/ext/RMagick/rmimage.c b/ext/RMagick/rmimage.c
index 64de23a..4ecd117 100644
--- a/ext/RMagick/rmimage.c
+++ b/ext/RMagick/rmimage.c
@@ -9819,7 +9819,7 @@ VALUE Image_filesize(VALUE self)
 #else
             IndexPacket *indexes = GetIndexes(image);
 #endif
-            old_color = image->colormap[*indexes];
+            old_color = image->colormap[(unsigned long)*indexes];
         }
         if (!image->matte)
         {
-- 
1.8.4

PKGBUILD

# Contributor: Alexsandr Pavlov <kidoz at mail dot ru>
pkgname=ruby-rmagick
_gemname=${pkgname#ruby-}
pkgver=2.13.2
pkgrel=2
pkgdesc="Interface between the Ruby programming language and the ImageMagick"
arch=('i686' 'x86_64')
url="http://rmagick.rubyforge.org"
license=(MIT)
depends=('ruby' 'imagemagick' 'rubygems')
source=(http://rubygems.org/downloads/${_gemname}-${pkgver}.gem
        hdri.patch)
noextract=(${_gemname}-${pkgver}.gem)
sha256sums=('e4f3b4c6ef6d6e89d8183080ee17093f42860c960f854608f4aa44ec8bce85bb'
            'a44d1572249fefb3c0c10f22c1a259e16d502bf5ade7c456e88008acce906cc0')

build() {
  cd "${srcdir}"
  gem unpack --target="${srcdir}" ${_gemname}-${pkgver}.gem
  cd "${srcdir}/${_gemname}-${pkgver}"
  patch -p1 < "${srcdir}/hdri.patch"
  gem build rmagick.gemspec
}

package() {
  cd "${srcdir}"
  export HOME=/tmp
  local _gemdir="$(ruby -rubygems -e'puts Gem.default_dir')"
  gem install --no-user-install --ignore-dependencies -i "${pkgdir}${_gemdir}" \
    "${srcdir}/${_gemname}-${pkgver}/${_gemname}-${pkgver}.gem"
}

I think the checksum for hdri.patch needs to be updated in the PKGBUILD. It's conceivable something is weird on my end but I'm getting: 6a72973fe20d42abca8fde027d29982fe80f6e57b9b5c8d892f88d17078423d2 as the sha256sum