tar zxvf jpegsrc.v6b.tar.gz
cd jpeg-6b
cp /usr/share/libtool/config.sub .
cp /usr/share/libtool/config.guess .
./configure --enable-shared --enable-static
make
sudo make install
sudo ranlib /usr/local/lib/libjpeg.a

require 'RMagick'

maxwidth = 120
maxheight = 160
aspectratio = maxwidth.to_f / maxheight.to_f
imgfile = 'world'

pic = Magick::Image.read(imgfile + '.jpg').first
imgwidth = pic.columns
imgheight = pic.rows
imgratio = imgwidth.to_f / imgheight.to_f
imgratio > aspectratio ? scaleratio = maxwidth.to_f / imgwidth : scaleratio = maxheight.to_f / imgheight
thumb = pic.resize(scaleratio)

white_bg = Magick::Image.new(maxwidth, thumb.height)
pic = white_bg.composite(thumb, Magick::CenterGravity, Magick::OverCompositeOp)
pic.write(imgfile + '.thumb.jpg')