One of the most fundamental operations on an image is simply getting basic information about the image. RMagick assigns dozens of attributes to an image. All you have to do is read the image and then call the attribute methods. Here's a Ruby program that takes image filenames from the command line and then prints a variety of information about each image to the terminal.
require 'RMagick' ARGV.each { |file| puts file img = Magick::Image::read(file).first puts " Format: #{img.format}" puts " Geometry: #{img.columns}x#{img.rows}" puts " Class: " + case img.class_type when Magick::DirectClass "DirectClass" when Magick::PseudoClass "PseudoClass" end puts " Depth: #{img.depth} bits-per-pixel" puts " Colors: #{img.number_colors}" puts " Filesize: #{img.filesize}" puts " Resolution: #{img.x_resolution.to_i}x#{img.y_resolution.to_i} "+ "pixels/#{img.units == Magick::PixelsPerInchResolution ? "inch" : "centimeter"}" if img.properties.length > 0 puts " Properties:" img.properties { |name,value| puts %Q| #{name} = "#{value}"| } end }
Converting an image to another format is as simple as writing the image to a file. ×Magick uses the output filename suffix (".jpg" for JPEG, ".gif" for GIF, for example) or prefix ("ps:" for PostScript, for example) to determine the format of the output image.
RMagick gives you four different methods for resizing an
image: resize
,
sample
, scale
, and thumbnail
. All four are
equally easy to use. Specify the number of columns and rows you
want the thumbnail to have, like this:
img = Image.new "bigimage.gif" thumb = img.scale(125, 125) thumb.write "thumb.gif"
Alternatively, just pass a single Float
argument
that represents the change in size. For example, to
proportionally reduce the size of an image to 25% of its original
size, do this:
img = Image.new "bigimage.gif" thumb = img.scale(0.25) thumb.write "thumb.gif"
The resize
method gives you more control by
allowing you to specify a filter to use when scaling the
image. Some filters produce a better-looking thumbnail at the
expense of extra processing time. You can also use a
blur
argument, which specifies how much blurriness
or sharpness the resize method should introduce.
The sample
method, unlike the other two, does not
do any color interpolation when resizing.
The thumbnail
method is faster than
resize
if the thumbnail is less than 10% of the size
of the original image.
Say you need to make all your thumbnails no bigger than 64x64
but with the same aspect ratio as the original. Or, you don't
want to resize the image if it's already smaller than 64x64. The
change_geometry
method can help.
The change_geometry
method accepts an
×Magick geometry string
argument and a block. The geometry string specifies how to change
the image's size: one or two numbers to specify the new size and
optional flags to describe any constraints. The
change_geometry
method parses the geometry string
and computes new width and height values. Then it calls the
block, passing the values it computed.
Within the block you can do whatever you want with the new
values. Typically you'll call one of the resize methods mentioned
in the previous section and make the resized image the return
value from the block. The change_geometry
method
then returns that value to its caller.
Use the quantize
method with
the Magick::GRAYColorspace
argument. If you want real "grayscale," quantize the image to 256
colors. If you want to convert a color image to black-and-white,
use 2 colors. See demo.rb.
Here's one way to make a drop shadow behind text. Make the
shadow first by drawing the text in a light gray color. Position
the text slightly to the right and down from where the real text
will be. Then use the blur_image
method to
make the shadow by blurring the text. Finally, draw the text
again in whatever color you want. (Click the image to see the
Ruby program that created it.)