×Magick supports over 89 major image formats.
×Magick determines the format (GIF, PNG, JPEG, etc.) of
  an image file either from its magic number, the filename suffix
  (.gif, .png, .jpg) or from a prefix attached to the filename. For
  example, ps:mydoc indicates that mydoc
  is a Postscript file. The magic number
  takes precedence over the filename suffix and the prefix takes
  precedence over the magic number and the suffix in input files.
  The prefix takes precedence over the filename suffix in output
  files.
This makes it easy to convert an image file to another format. Simply write the image file using a name that has either a prefix or a suffix corresponding to the format you want.
Note: Keep in mind that files in some formats may only be read by ×Magick, not written.
Many RMagick methods expect color name arguments or return color names. A color name can be
/usr/X11R6/lib/X11/rgb.txt.)| #RGB | 4 bits for each channel | 
| #RRGGBB | 8 bits for each channel | 
| #RRRGGGBBB | 12 bits for each channel | 
| #RRRRGGGGBBBB | 16 bits for each channel | 
| #RGBA | 4 bits for each channel, plus the alpha channel | 
| #RRGGBBAA | 8 bits for each channel, plus the alpha channel | 
| #RRRGGGBBBAAA | 12 bits for each channel, plus the alpha channel | 
| #RRRRGGGGBBBBAAAA | 16 bits for each channel, plus the alpha channel | 
| rgb(r,g,b) | SVG functional notation. r, g, and b are either three integers 0-255 or 3 percentanges 0%-100%. | 
| rgb(r,g,b,a) | SVG functional notation plus the alpha channel. | 
The alpha channel is the opacity of the image, which can range from 0 (Magick::OpaqueOpacity) to MaxRGB (Magick::TransparentOpacity).
A Pixel object contains the numeric
  representation of a color. The Pixel.from_color method
  converts a color name to a pixel. There are two methods to
  convert a pixel to a color name. The Pixel#to_color method requires
  that you specify whether the alpha (opacity) channel is used, the
  depth (8 or 16) and the color standard to use. The Image#to_color method uses the image's
  depth and matte attributes.
  If matte is false the opacity value is ignored.
Hint: The name of the transparent color is "transparent", or "#000000ff".
This is ImageMagick's page about color names.
RMagick methods frequently require a geometry
  string argument. This string generally specifies width and height
  values as well as x and y offset values. The values are usually
  specified in pixels (but see the % flag, below).
This is the format of the geometry string. Any of the values may be omitted, depending on the context:
<width>x<height>+-<x>+-<y>{%@!<>}
  This is the ImageMagick description of the geometry string:
By default, the width and height are maximum values. That is, the image is expanded or contracted to fit the width and height value while maintaining the aspect ratio of the image. Append an exclamation point to the geometry to force the image size to exactly the size you specify. For example, if you specify 640x480! the image width is set to 640 pixels and height to 480.
If only the width is specified, the width assumes the value and the height is chosen to maintain the aspect ratio of the image. Similarly, if only the height is specified (e.g., "x256"), the width is chosen to maintain the aspect ratio. To specify a percentage width or height instead, append %. The image size is multiplied by the width and height percentages to obtain the final image dimensions. To increase the size of an image, use a value greater than 100 (e.g. 125%). To decrease an image's size, use a percentage less than 100.
Use @ to specify the maximum area in pixels of an image.
Use > to change the dimensions of the image only if its width or height exceeds the geometry specification. < resizes the image only if both of its dimensions are less than the geometry specification. For example, if you specify '640x480>' and the image size is 256x256, the image size does not change. However, if the image is 512x512 or 1024x1024, it is resized to 480x480.
The x and y offsets, if present, can be preceeded with either a + or - sign. The + causes x and y to be measured from the left or top edges, respectively. Conversely, - measures from the right or bottom edges. Offsets are always measured in pixels.
Any method that accepts a geometry string will also accept a
  Geometry
  object.
Some RMagick methods interpret the geometry string values differently. Where this is the case the documentation for the method will explain the differences.
×Magick classifies all images into two classes, PseudoClass and DirectClass.
DirectClass images are continuous-tone images stored as RGB
    (red, green, blue), RGBA (red, green, blue, alpha), or CMYK
    (cyan, yellow, magenta, black) intensity values as defined by
    the colorspace [attribute].
PseudoClass images are colormapped RGB images. The colormap
    is stored as a series of red, green, and blue pixel values,
    each value being a byte in size. If the image depth is 16, each
    colormap entry consumes two bytes with the most significant
    byte being first. The number of colormap entries is defined by
    the colors [attribute].
GIF format images are PseudoClass. JPEG format images are
  DirectClass. You can change the class of a image with the
  class_type=
  method.
Some of the image formats that ×Magick supports are special-purpose formats that are built-in to ×Magick itself. That is, even though you can "read" images in these formats, they do not correspond to any real image files.
These are the built-in formats that I know something about.
  (There are more but I've never used them.) When the format is
  marked with an *, you must supply the desired size of
  the image in order to "read" it. Specify the size by assigning a
  string in the form "WxH" to the size attribute in
  the read method's additional parms block. For
  example, to create a image in the gradient format that is 100
  pixels wide and 200 pixels high, use:
i = Image.read("gradient:red-blue") { self.size = "100x200" }
  See demo.rb for more examples of reading built-in formats.
Gradient filenames have the form
    "gradient:color1-color2". These images are created
    by gradually changing from color1 at the top edge
    to color2 at the bottom. Don't confuse this image
    format with the GradientFill class, which is
    part of RMagick.
A mottled gray image suitable for use as a tiled background
    texture. Ex: "granite:".
The ×Magick logo. Ex: "logo:".
The 216-color "Web safe" cube. Ex:
    "netscape:".
An empty image. Ex: "null:".
ImageMagick 5.5.7-3 introduced the pattern:
    format. This format supplies a number of built-in patterns that
    may be referenced by specifying the pattern name. For example,
    pattern:checkerboard. For a list of acceptable
    patterns, see this
    page.
Creates a swirly, psychedelic image. Specify a pair of
    colors in the filename ("plasma:red-blue") or
    specify the filename "plasma:fractal" for best
    results.
A small picture of a rose. Ex: "rose:".
Specify a color name after the
    xc: prefix. For example, "xc:red". This format is
    simply an image of the specified color. You can get exactly the
    same results by specifying the background color when creating
    an image.