Converting One Image Format to Another

Table of contents:

Credit: Antonio Cangiano

Problem

You want to convert an image to a different format.

Solution

With RMagick, you can just read in the file and write it out with a different extension. This code converts a PNG file to JPEG format:

	require 
ubygems
	require RMagick

	img = Magick::Image.read(myimage.png).first
	img.write(myimage.jpg)

Discussion

As seen in the previous two recipes, Magick::Image.read receives the PNG image and returns an array of Image objects, from which we select the first and only image.

RMagick lets us convert the file into a JPEG by simply changing the filenames extension when we call the write method.

The underlying C library, ImageMagick or GraphicsMagick, has three ways of determining the format of image files:

Checking an explicitly specified format prefix: for example, "GIF:myimage.jpg" indicates that the file myimage contains a GIF image, even though the file extension says otherwise.
Looking inside the file for a "magic number", a set of bytes that indicates the format.
Checking the file extension: for example, "myphoto.gif" is presumably a GIF file.

Although the format prefix takes precedence over the magic number, RMagick won be fooled by an incorrect prefix. Eventually it will have to parse the image file, and the format mismatch will be revealed:

	Magick::Image.read("JPG:myimage.png")
	# Magick::ImageMagickError: Not a JPEG file: starts with 0x89 0x50 `myimage.png:

When you write an image to an output file, you can choose the output format by specifying a file extension or a prefix.

	img = Magick::Image.read("myimage.png").first
	img.write("myimage.jpg") # Writes a JPEG
	img.write("myimage.gif") # Writes a GIF
	img.write("JPG:myimage") # Writes a JPEG
	img.write("JPG:myimage.gif") # Writes a JPEG

You can also get or set the file format of an image by calling the Image#format or Image#format= methods:

	img.format # => "PNG"
	img.format = "GIF"
	img.format # => "GIF"

Of course, RMagick can read to and write from every graphical file format in existence. How can you tell whether your version of RMagick knows how to write a particular file format?

You can query RMagicks capabilities by calling Magick. formats. This method returns a hash that maps an image format to a four-character code:

	Magick. 
formats["GIF"] # => "*rw+"
	Magick.formats["JPG"] # => "*rw-"
	Magick.formats["AVI"] # => "*r--"
	Magick.formats["PS"] # => " rw+"

The code represents the things that RMagick can do with that file format:

The first character is an asterisk if RMagick has native blob support for that format. If not, the first character is a space. RMagick can convert most image formats into a generic string format (with Image#to_blob)that can be stored in the database as a BLOB and converted back into an Image object with Image.from_blob.
The second character is "r" if RMagick knows how to read files in that format. Otherwise, its a minus sign.
The third character is "w" if RMagick knows how to write files in that format. Otherwise, its a minus sign.
The final character is "+" if RMagick knows how to cram multiple images into a single file (as in an animated GIF).

Heres a little bit of metaprogramming that adds four predicate methods to Magick, one for each element of the four-character code. You can use these methods instead of parsing the code string:

	module Magick
	 [["native_blob?", ?*], ["readable?", ?r],
	 ["writable?", ?w], ["multi_image?", ?+]].each_with_index do |m, i|
	 define_method(m[0]) do |format|
	 code = 
formats[format]
	 return code && code[i] == m[1]
	 end
	 module_function(m[0])
	 end
	end

This code demonstrates that the GIF file format supports multi-image files, but the JPG format doesn :

	Magick.multi_image? GIF # => true
	Magick.multi_image? JPG # => false

ImageMagick and GraphicsMagick support the most common image formats (over 90 in total). However, they delegate support for many of these formats to external libraries or programs, which you may need to install separately. For instance, to read or write Postscript files, youll need to have the Ghostscript program installed.

Problem

Solution

Discussion

See Also